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INTRODUCTION
A weed can simply be defined as any plant growing out of place or growing where it is not wanted. For example, tall fescue may be considered a weed when grown in a stand of bermudagrass or Kentucky bluegrass, but it is desirable when grown as a monoculture.
Reasons for classifying a plant as a weed are numerous. In addition to being unsightly, weeds compete with turfgrasses for sunlight, soil nutrients, soil moisture, and growing space. Weeds also act as hosts for pests such as plant pathogens, nematodes and insects. Certain weeds cause allergic reactions in humans due to their pollen or their volatile chemicals.
Probably the most undesirable characteristic of weeds in turf is the disruption of the uniformity of a turf. Different leaf width and/or shape, different growth habit, and/or different color contribute to unsightliness. For example, many broadleaf weeds such as dandelion, plantains, and pennywort have leaf widths different from turf. Goosegrass, smutgrass, and dallisgrass tend to form clumps or patches which disrupt turf uniformity. In addition, large clumps are difficult to mow effectively which increases maintenance problems. Turf uniformity also is disrupted by the presence of weed seedheads. Annual bluegrass, for example, is largely unnoticed in cool-season turfgrasses until numerous seedheads appear in late winter and spring.
Plant color is a another factor in determining the potential of a weed
problem in turf. The lighter green color typically associated with annual
sedge often distracts from the playing surface. Bahiagrass often has different
color when grown in combination with other turf species.
Weeds often are the result of a weakened turf, not the cause of it.
Knowing turf weaknesses often help to understand the reason for weed encroachment
within a turf area. Lack of turf density and areas void of growth promote
weed encroachment . Weak turf and bare areas exist because of: (a) improper
selection of a turf species for a given area; (b) damage from pests such
as diseases, insects, nematodes and animals; (c) environmental stresses
such as excessive shade, drought, heat, cold and poor drainage; (d) improper
turf management practices such as misuse of fertilizer and chemicals, improper
mowing height or frequency and lack of proper soil aeration; and (e) physical
damage and compaction from concentrated or constant traffic. Unless all
the factors contributing to turf decline are corrected, continued weed
encroachment should be expected.
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| 1. | Prevention |
| 2. | Cultural |
| 3. | Mechanical (Physical) |
| 4. | Biological |
| 5. | Chemical |
Prevention
Weeds are prolific seed producers. For example, annual bluegrass can produce 2,000 seeds per plant. A single dandelion plant can produce up to 12,000 seed. Weed seed can easily be transported to nearby turf areas by wind, water, equipment and animals.
Prevention is avoiding introduction of weeds within an area. There are often national, state and local prevention efforts against the introduction and spread of weeds. However, a local facility-based preventive program is one of the best methods of introducing future weed problems. Many of these methods are common sense approaches that ensure sanitary conditions and minimize weed introduction. Some of these methods include use of weed-free turf seeds, stolons, sprigs, plugs or sod. The use of clean mulch, topdressing material and the avoidance of manure also are examples of preventive methods.
Clean seed. Only cleaned turfgrass seed that has most weed seeds removed should be planted. The Federal Seed Act of 1939 regulates the seed trade to protect consumers from mislabeled or contaminated crop seed. The following information is required by this act to be listed on seed labels in interstate commerce.
1. Percentage of pure seed of the named crop.
2. Percentage of other crop seed.
3. Percentage of weed seed.
4. The name and occurrence of noxious weed seed.
The following is an example of a typical turf seed label:
Brand name: Sports Turfgrass
Seed
98.75% Pure Seed
Other Ingredients:
0.00% Other Crop Seed
1.00% Inert Matter
0.25% Weed Seed
85.00% Germination
Tested: 1/93
Lot 0001-A
Net Wt. 50 Lbs.
Noxious weed seed are prohibited from entering the United States by the Federal Seed Act. Also prohibited from entering the country are crop seeds containing in excess of 2 percent weed seeds of all kinds.
Vegetative materials. It is important to purchase weed-free vegetative materials such as sod and sprigs. Weeds are not always evident in a commercial sod production field but become more obvious when cultural practices change after planting. Purchasing sod from reputable producers is an excellent way to avoid "buying a weed problem." Additionally, many states have certification programs that restrict the amount and species of weeds in sod.
Once established, certain weeds cannot selectively be controlled with herbicides. For example, common bermudagrass can be the primary weed problem in hybrid bermudagrass sod production. This is the result of unavailable selective herbicides and unsatisfactory control from non-selective herbicides. Inspection of planting material for the presence of weeds weed should occur before purchase.
Sanitation practices. Sanitation practices include the prevention of weed seed movement by mechanical or human means. Mechanical devices, such as mowers or cultivators, pick-up weed seeds and transport them to adjacent areas. Mowers should be rinsed before being transported from weed-infested to weed-free areas.
Weeds can also be introduced into turf through the addition of contaminated topsoil. Most weed seed are located near the soil surface and can remain viable for many years. When contaminated topsoil soil is used for topdressing, germination and subsequent weed emergence will result. All materials used for soil modification or topdressing should be free of viable weed seeds or vegetative propagules through fumigation or heat treatment prior to use.
Irrigation systems fed by open ditches, canals, and ponds also contribute to the spread of weeds. One research study found that more than 130 weed species in irrigation water. Therefore, sports turf managers should keep irrigation sources and embankment areas weed free. Turfgrass areas periodically flooded by streams, canals or rivers also are subject to the introduction of weed seeds.
Another source of weed seed is from unmowed adjacent areas such as fence rows, roadsides and open fields. Mowing these areas, or the use of herbicides, should be a regular part of a maintenance program.
Cultural
Cultural practices that promote vigorous, dense turf are among the most important and least recognized method of controlling weeds. Certain weed species such as crabgrass and goosegrass require light for germination. Cultural practices that increase turf density will prevent light from reaching the soil surface and help to reduce the germination of these annual grasses. Exclusion of light from the soil surface also delays germination of weed seeds in spring since the soil surface is better insulated and remains cooler.
An example of the impact that mowing height and fertilization has on weed occurrence was documented in a red fescue lawn. Red fescue mowed at 2.2 inches had better turf coverage and less crabgrass occurrence over four year time period than turf mowed at 1.25 inches. The yearly addition of nitrogen fertilization at 1.0 or 2.0 lbs. N per acre increased turf vigor, and also reduced crabgrass occurrence. Heavy crabgrass infestation and poor turf quality resulted at a low fertility rate and excessively low (0.75 inches) mowing height.
Soil nutrient levels also may favor one plant species over another. For example, excessive phosphorus (P) applications are known to increase annual bluegrass populations in bentgrass. Bentgrass apparently is better able to absorb P in low P-containing acidic soils and can out-compete annual bluegrass under these conditions. The application of sulfur to acidify soil, or limiting P application, is suggested to decrease annual bluegrass density
A soil pH range between 5.8 and 7.2 is recommended for the proper growth and development of most turfgrass species. Maintaining turfgrasses at the recommended soil pH will improve nutrient utilization and help the turfgrass compete with weeds.
Maintaining proper soil moisture through irrigation and soil drainage also promotes vigorous turf growth. Over-irrigation and/or poor surface and subsurface soil drainage results in low soil oxygen levels. Soil compaction also reduces oxygen diffusion and restricts rooting. Turf density decreases with compaction, but weeds such as annual bluegrass, goosegrass, prostrate knotweed and various sedges will thrive under compacted conditions.
Pest damage also decreases turf density and allows weed encroachment. Insects, diseases, and nematodes are common pests that affect turf density and should be controlled when possible. If the presence of these pests is not limited, weeds will easily infest damaged areas.
Certain weed species may indicate a pest problem . For example, weeds commonly associated with nematode-thinned turf include prostrate spurge, prostrate knotweed and Florida pusley.
Mechanical (Physical) Control
Mechanical control of weeds involves hand pulling, mowing or various types of tractor-powered tillage operations. The most commonly used mechanical weed control methods in turf are tillage (preplant), hand pulling, hoeing and mowing.
Tillage. Tillage, or cultivation, usually is practiced before turfgrass establishment. Weeds are destroyed by breaking them apart, removing them from the soil, disturbing their root systems, causing desiccation and smothering or burying tender tissue. Depleting stored food reserves and reducing soil reserves of vegetative propagules also destroy weeds.
Other potential benefits from proper tillage are increased soil aeration and water penetration, breakage of surface crusts and soil clods, surface smoothing for planting, and incorporation of surface applied fertilizer, liming material and soil amendments. Tillage should be conducted when the soil surface is dry so the disturbed weeds are subjected to desiccation. When soils are too wet or water is applied shortly after tillage, the disturbed plants are more likely to survive. Tillage of wet soils also increases the incidence of clods, crusts, and compaction layers which may interfere with subsequent planting and turf seed germination.
Repeat tillage usually is necessary for control of perennial weeds and the continued emergence of annuals. Intensive tillage during one to two growing season are normally required to deplete the vegetative reproductive structures of most perennial weeds. Tillage should be repeated on a two to four week intervals and continued through the end of the growing season in order to deplete the underground carbohydrate food reserves of tubers, rhizomes and bulbs. Tillage should be performed when the effects of water and wind on soil erosion can be avoided.
Disadvantages of tillage include exposed soil being subjected to wind and water erosion. Also, if weeds are mature when tilled, their seeds are buried and become a future source of weeds. Other possible disadvantages of tillage are scheduling to coincide with proper soil moisture, and the cost of tractors, tillage implements, labor and fuel.
Hand pulling, hoeing, and rouging. Manual weed control often is performed by hand pulling, hoeing and rouging. Manual weed control is not widely practiced and is generally impractical on large sport turfgrass areas. However, because of herbicide-sensitive constraints on certain small turf and ornamental areas, manual weed control may be necessary.
Hand pulling and hoeing effectively controls annual and biennial seedling weeds. These practices are less effective on established perennial weeds, because underground reproductive parts are not eliminated by this method.
Rouging often is used in combination with hand pulling and involves the use of a special implement that has a hooked and sharpened metal blade-end. The implement is pushed into the soil to sever the roots and the plant then is pulled from the soil. Rouging still is widely used to remove weeds such as goosegrass from creeping bentgrass golf greens because the turfgrass species has low tolerance to postemergence herbicides .
Mowing. Proper mowing practices are a valuable weed control method. When frequently repeated at the recommended height, mowing depletes underground food reserves, prevents seed maturation and favors the growth of turfgrass species. Tall-growing, annual broadleaf weeds are easily controlled by mowing. Repeated clipping essentially causes the plant to become weakened through starvation of these below-ground root reserves. Common lambsquarters, pigweed spp., and common ragweed are common weeds in newly established turfs that can be controlled by mowing. Weeds should be mowed in the bud stage or earlier to prevent seed development. Prostrate or rosette type broadleaf weed species, as well as grasses, have their primary growing point, or crown, located at or just below the soil surface and are tolerant of mowing heights used on turfgrasses. New growth resumes because the growing point is unaffected. Examples of weeds tolerant to mowing include goosegrass, annual bluegrass, common purslane, spotted and prostrate spurge, Virginia buttonweed and prostrate knotweed.
Biological
Biological weed control methods uses a weed's natural antagonists as the control agent. For example, certain species of weevils can be used to reduce the population of musk thistle in pastures and non cropland areas. The objective of biological control in most situations is not weed eradication, but to reduce the population below a level of economic or aesthetic injury. Unfortunately for turf managers, a high percentage of control of a weed species is necessary to satisfy their clientele.
Disease-causing pathogens rarely have been used for weed control because they often are pathogenic to the turf. However, research is being conducted to identify pathogens that could be used to selectively control weeds in turfgrasses.
Although some outstanding successes have been achieved with biological agents in other commodities, additional research is needed to identify biological weed control agents for turfgrasses. Through wide publicity, the general public has incorrectly viewed this as a viable and immediately available alternative for the control of all types of turfgrass pests.
Chemical Weed Control
A major contributing factor to the advancement of man's way of life during the 20th century has been the development of chemical pest control. Attempts in developing selective weed controlling chemicals during the period from 1900 to 1915 emphasized solutions of copper nitrate, ammonium salts, sulfuric acid, iron sulfate and potassium salts for selective weed control in cereal crops. Compounds developed between 1900 and 1940 included the arsenicals, chlorates, borates, ammonium sulfamates and the dinitrophenols . The organic arsenicals (MSMA, DSMA, AMA, CMA) were developed during this time and still are used for selective grass control in turf. However, the trigger for development of modern herbicide technology did not occur until World War II.
The discovery of the herbicidal properties of 2,4-D during World War II began the era of selective weed control. The first use of 2,4-D was for dandelion control in Kentucky bluegrass. The compound proved to be economical, reasonably predictable and consistent, and highly effective. Most importantly, it was non-injurious to cool-season turfgrasses. It remains one of the most widely used herbicides.
Herbicides may be classified by several criteria. These include selectivity and movement in plants, chemistry, application method, application timing, persistence and mode of action. Selectivity and movement in plants, as well as method and timing, tend to be the most important classification criteria for turf managers to follow.
Selectivity and movement
Selective. A selective herbicide controls or suppresses certain plant species without seriously affecting the growth of another plant species. Selectivity may be due to differential absorption, translocation, metabolism, morphological and/or physiological differences between turfgrasses and weeds. The majority of turfgrass herbicides are selective. For example, 2,4-D (numerous trade names) is used for selective control of many broadleaf weeds, such as dandelion, without significant injury to most turfgrasses.
Nonselective. Nonselective herbicides control all plants. These generally are used in the renovation or establishment of a new turf area, as "spot treatments" of isolated weeds, or for trimming. Roundup Pro (glyphosate) and Reward LS (diquat) are nonselective herbicides. Herbicides such as Aatrex (atrazine) or MSMA (numerous trade names) can be nonselective at rates higher than those used for selective control.
Systemic. Systemic herbicides are translocated in the plant's vascular system. The vascular system translocates the nutrients, water and organic materials necessary for normal growth and development. In contrast to the quick-kill accomplished with contact herbicides, systemic herbicides require several days or even a few weeks to be fully translocated through the plant's vascular system before the plant dies. Systemic herbicides also are classified as selective or nonselective. Roundup Pro is a nonselective, systemic herbicide while 2,4-D, Banvel (dicamba), Image (imazaquin), and Vantage (sethoxydim) are examples of selective, systemic herbicides.
Contact. Contact herbicides affect only the portion of green plant tissue contacted by the herbicide. These herbicides are essentially not translocated in the vascular system of plants, and do not control underground plant parts such as rhizomes or tubers. Repeat applications will be needed with contact herbicides to kill regrowth from underground plant parts of perennial. Thorough spray coverage of the weed foliage is necessary for effective control. Contact herbicides kill plants quickly, often within a few hours of application. Contact herbicides may be classified as selective or nonselective. The contact herbicides Buctril (bromoxynil) and Basagran T/O (bentazon) are classified as selective, contact herbicides. Diquat is a nonselective contact herbicide.
Timing of Herbicide Application
Herbicides also are classified by application timing with respect to turfgrass and weed seed germination.
Preplant herbicides. These are applied to provide nonselective control of all weeds before turfgrass is planted. Soil fumigants, such as metam-sodium and methyl bromide, and nonselective herbicides such as glyphosate, often are used as preplant herbicides.
Preemergence herbicides. Preemergence herbicides generally are applied to an established turfgrass site prior to weed seed germination. This group of herbicides controls weeds during the early stages of the germination process. Emerged weeds visible at the time of application are not controlled by preemergence herbicides. Although the majority of herbicides may be classified as preemergence or postemergence, atrazine, simazine, dithiopyr (Dimension), ethofumesate, pronamide and others are exceptions. These herbicides have preemergence and postemergence activity on selected annual weeds.
Preemergence herbicides form the base of a chemical weed control program in turfgrasses and are used primarily to control annual grasses and certain annual broadleaf weeds. They are persistent in the soil and control susceptible weeds for an extended period of time. Turfgrass preemergence herbicides generally are effective for two to five months. The length of efficacy depends on the specific chemical being used, soil, physical and chemical properties, soil moisture levels, temperature and application placement. The soil persistence of these herbicides is advantageous in terms of length of weed control; however, it may be a disadvantage if seeding, sprigging or sodding operations are planned for a treated site. Newly-seeded and sprigged turfgrasses have a low tolerance to most preemergence herbicides (siduron and oxadiazon are noted exceptions). Appropriate waiting periods after herbicide application are required before reestablishing the site with turf. The herbicide label should be consulted to determine the length of time required before establishment operations can be conducted safely.
Postemergence herbicides. Postemergence herbicides are applied
directly to emerged weeds. In contrast to preemergence herbicides, this
group of herbicides provides little, if any, residual control of weeds.
A complete chemical weed control program can be accomplished with postemergence
herbicides provided multiple applications are used. Because of the necessity
for repeat applications, possible temporary turfgrass injury may occur.
As a result, most turfgrass managers use postemergence herbicides in conjunction
with a preemergence weed control program. Postemergence herbicides are
useful to control perennial grass and broadleaf weeds not controlled by
preemergence herbicides. Certain postemergence herbicides may be used on
newly established turfgrasses or within three to four weeks after its establishment..
Three types of names normally are associated with an herbicide. The chemical name describes the chemistry of the compound. These usually are a lengthy technical description of the chemical. The common name is a generic name assigned to the chemical and often is a simpler version of the chemical name. Chemical and common names must be approved by an appropriate authority before being accepted by the scientific community. The trade name, used by the chemical company for marketing purposes to promote the sale of a specific product, often is the most recognizable name of an herbicide.
An example of chemical nomenclature would involve the herbicide oryzalin.
Oryzalin is the common name while the chemical name is 4-(dipropylamino)-3,
5-dinitrobenzenesulfonamide. SurflanR is the trade name
for oryzalin. Due to the number and constant change of trade names, most
scientific journals and university publications use the common name of
a herbicide.
Registering a new pesticide has become expensive and time consuming. Only one out of 20,000 compounds tested reaches the market (Hill, 1982). Developing a new herbicide may cost more than $35 million. An additional $40 million to $100 million is required to build a production plant (Hill, 1982). Seven to 10 years of testing normally are required before a compound reaches the market. Chemical companies obtain a product patent prior to the release of the herbicide. After the product is patented, a company retains exclusive or proprietary rights for that herbicide for 17 years. After the patent has expired, other chemical companies are free to market the herbicide under a different trade name. Since a patent protects a compound for 17 years, and it takes an average of seven years for testing, patenting and facility building, only 10 years of exclusive marketing can be expected.
Much of the money for herbicide development is used to generate data from extensive and rigorous testing. These look at crop and weed efficacy, environmental hazards, and a wide array of toxicological tests. Environmental and toxicological data are submitted to the Environmental Protection Agency (EPA) for review and registration. The EPA has the responsibility of insuring the pesticide poses no undue environmental and health hazards when used as instructed. A label providing sufficient information and warnings on the safe and proper handling, environmental precautions, as well as use of the product must accompany each product.
Experimental Use Permits (EUP)
Late in a pesticide's development process, the EPA can be requested
to allow the use or sale of a limited amount of the product under an experimental
use permit (EUP). This usually is sought one-to-two years prior to the
anticipated full registration and usually is done in order to gather performance
information based on real use conditions.
General and Restricted Use Pesticides
Pesticides are classified into general and restricted use categories.
General use pesticides are those not causing adverse environmental
effects and are safe for application by the general public. Restricted
use pesticides represent minor risk to human health or to the environment.
They only are sold to certified applicators trained in pesticide handling.
Two categories of certification exist. A Private Applicator Certificate
is required for those who use restricted-use pesticides on their own or
rented land. A Commercial Applicator Certificate is required for those
who apply either restricted or general use pesticides for compensation.
Herbicides are not sold as pure chemicals, but are formulated or combined
with an appropriate solvent, a diluent, or with adjuvants to form a material
called a formulation. A formulation is a mixture of the active ingredient
and the inert ingredient. Formulations allow uniform application, extend
the stability and storage life of herbicides, enhances its activity, and
allows it to be packaged in convenient containers. Herbicides are available
in a variety of formulations and the same herbicide often is sold in different
formulations. Herbicides are applied to the target site with the use of
a carrier. A carrier is a gas, liquid, or solid substance used to propel,
dilute or suspend an herbicide during its application. Liquid carriers
may be water or fluid fertilizers. Water is the most commonly used carrier.
Granules and pellets consisting of clay, ground corn cobs, sand or fertilizer
commonly serve as carriers for dry herbicide formulations.
Sprayable Formulations
Sprayable formulations are applied with liquid carriers and include:
1. Water Soluble Liquids (S or SL) - These form true solutions, or completely dissolve when mixed with water. A water-soluble herbicide formulation typically consists of the herbicide, water as the solvent for the herbicide and selected surfactants to improve wetting and penetration. It also may contain an anti-freeze. The resulting solution is stable and appears clear.
2. Water Soluble Powders (SP) - These are dry solids that completely dissolve in water to form true solutions.
3. Emulsifiable Concentrates (E or EC) - These are oily (nonpolar) liquids that form emulsions (droplets of oil surrounded by water) in water (polar). The emulsifying agent acts as a binder-coupler between the oil-water surface. It reduces interfacial tension and allows the tiny droplets to remain in suspension. Emulsions are milky-colored and require agitation to keep the herbicide uniformly suspended in the spray tank.
4. Wettable Powders (W or WP) - These are finely ground solids consisting of a dry diluent (usually a hydrophilic clay) plus the herbicide and various adjuvants. The adjuvants in the formulation prevent lumpiness or flocculation of the finely ground materials and improves mixing in the spray tank. Wettable powders form unstable suspensions in water and require vigorous agitation to prevent settling of the suspended particles.
5. Liquid or Aqueous Suspensions (L or AS) - These also are designated as water dispersable liquids (WDL) or flowables (F). These contain finely ground solids suspended in a liquid system. Its particles are smaller than the wettable powders. These formulations form a suspension in water and require agitation. These also settle-out when stored and therefore require vigorous shaking before use.
6. Dispersable Granules or Dry Flowables (DG, WDG, or DF) - These
are finely ground solids formulated in a water dispersable granule and
forms a suspension in water. Agitation is required to prevent settling
of the suspended particles. The granules are made up of finely ground solids
combined with suspending and dispersing agents. Its chief advantage over
wettable powders is its ease of measurement and handling. Note: These formulations
always are applied with a liquid (water or fluid fertilizer) carrier.
Dry Formulations
Dry formulations are not applied with liquid carriers but are applied in its marketed form. The herbicide normally is formulated in relatively low concentrations on the dry carrier in order to aid in uniform distribution.
1. Granules (G) - These are small granular particles (<10 M3) applied in the dry state. It consists of the herbicide plus a dry carrier (clay, sand, corn cobs). Herbicide concentrations typically range from 2 to 10 percent. Granular formulations require slightly more rainfall for activation than sprayable formulations. They tend to be more expensive than sprayable formulations.
2. Pellets (P) - These are similar to granules except its particles usually are larger. See the above comments for granules.
3. Dusts (D) - Dusts are popular homeowner formulations of insecticides
and fungicides. Due to drift hazards, no herbicides are currently formulated
as dusts.
Adjuvants
An adjuvant is a spray additive that enhances the performance, or handling characteristics, of an herbicide. Adjuvants include surfactants, crop oils, crop oil concentrates, anti-foaming agents, drift control agents and compatibility agents. Surfactants, crop oils and crop oil concentrates are added according to label directions because indiscriminate use may result in severe turfgrass injury. Another result of indiscriminate use is decreased herbicide performance. Adjuvants do not improve the efficacy of preemergence herbicides and are used only with certain postemergence herbicides. Some postemergence herbicides have premixed surfactants and no additional surfactant is necessary.
Three major types of surfactants are emulsifiers, wetting agents and stickers. Emulsifiers stabilize the dispersal of oil-soluble herbicides in water so the herbicide will not settle-out. These are usually added by the chemical company during the formulation process. Wetting agents help the spray droplet to spread over the leaf surface by reducing the interfacial tension between the leaf surface and spray droplets. Stickers cause the spray droplet to adhere to the leaf surface and reduce spray runoff during application and reduce rain wash off. Stickers often are combined with wetting agents (spreader-stickers) to increase adhesion and spray droplet coverage. Soap and liquid household detergents are examples of commonly known surfactants but are not recommended since they may cause excessive foaming, or they may form precipitates in the spray tank.
Crop oils and crop oil concentrates are non-phytotoxic light oils that contain varying percentages of surfactants. Crop oils contain one-to-two percent surfactant and are commonly used at concentrations of one gallon per acre. Crop oil concentrates contain 17 to 20 percent surfactant and generally are used at concentrations of one quart per acre. Crop oil concentrates have largely replaced crop oils since reduced amounts of the adjuvant is required. An example of enhanced performance of a herbicide involves sethoxydim which provided better control of goosegrass when applied with a crop oil concentrate than without one (Chernicky et al., 1984).
The use of anti-foaming agents may be necessary if excessive foaming
occurs in the spray tank. Drift control agents reduce spray droplet drift
by reducing the percentage of fine spray particles in the spray mist. Compatibility
agents are added to fluid fertilizer and herbicide mixtures to prevent
these individual components from separating or clumping together.
Selected preemergence and postemergence herbicides may be applied with either liquid or dry fertilizers. Fertilizer-herbicide mixtures enable a "weed-n-feed" treatment in the same application. "Weed-n-feed" treatments may be convenient but certain factors must be considered prior to its application. Depending on the type of turfgrass, the time of year an herbicide should be applied may not coincide with the time of year that a fertilizer should be applied. When using these products it also is important to determine if the manufacturer's recommended rate supplies the correct amount of fertilizer needed by the turfgrass. The amount of herbicide that is required for weed control should also be taken into consideration. Supplemental applications of fertilizer or herbicide may be required if the combined product does not supply either enough fertilizer or herbicide to meet the needs of the turfgrass.
Turfgrass fertilizer-herbicide products should be used with caution near ornamental plants. Products that contain dicamba or atrazine can be absorbed by the roots of ornamental trees and shrubs and may cause severe injury to them.
Limited information is available on the compatibility of herbicides with liquid fertilizers. Compatibility must be determined by the applicator since some herbicides and liquid fertilizers do not mix well. They may form gels or solids that will not flow through spray equipment. The compatibility of herbicide and liquid fertilizer tank mixes should be tested prior to mixing the components for turf application. The herbicide label will provide information on how to conduct a compatibility test. In most tests, the herbicide and liquid fertilizer are mixed in a container in the proportions that would be used in the tank mix. After the herbicide and liquid fertilizer are mixed, the solution is inspected over a period of time for signs of incompatibility (e.g., separation, precipitates, gels, etc.). An agent may be required to improve compatibility between the herbicide and the liquid fertilizer. If the mixture is compatible, it is advisable to determine the turfgrass tolerance to the mixture. Small, representative turfgrass areas should be treated and observed for injury. If fungicides or insecticides are to be tank-mixed with the liquid fertilizer and herbicide mixture, the above compatibility tests also must be conducted prior to field use.
If perennial weeds such as nutsedge and bermudagrass are present, it
is advisable that weed control be implemented before planting or establishing
a turf area. Once the turf is established, control becomes more difficult
and expensive.
Soil Fumigation. Soil fumigants are volatile liquids or gases that control a wide range of soil-borne pests. Soil fumigants such as methyl bromide, are highly toxic and expensive. Their use is limited to small, high cash crops such as tobacco, certain vegetables, fruits, bedding plants and turf. The expense results from a plastic cover necessary to trap the fumigant vapors in the soil. Fumigants control both weeds and many nematodes, fungi and insects. However, weed species that have a hard, water-impermeable seed coat such as sicklepod, white clover, redstem filaree, and morningglory are not effectively controlled with soil fumigants. Important considerations before choosing a particular soil fumigant include its expense, soil moisture level, soil temperature, and the time available before planting.
Several compounds are used as fumigants (Beard, 1973; Ross and Lembi, 1985). The two most commonly used materials in turf are methyl bromide and metham or metam-sodium.
Methyl bromide. Methyl bromide is a colorless, nearly odorless liquid or gas. At 38oF (3.4oC), the liquid turns into a gas and is 3.2 times heavier than air at 68oF (20oC). These properties require a gas impermeable cover be used with it or the material will escape. Methyl bromide is extremely toxic and commonly is combined with a odor detectable warning agent such as chloropicrin (teargas).
When using a fumigant, the soil should be in a condition suitable for planting. Weed control normally is achieved only as deep as the soil is properly prepared. The soil should be moist for adequate fumigant penetration and dispersion. Moisture-saturated or extremely dry soils limit penetration and dispersion which subsequently affects weed seed absorption. Soil temperatures at 4 inches (10 cm) should be a minimum of 60oF (15.6oC). Fumigation is not effective if soil temperatures are below 50oF (10oC). A plastic or polyethylene cover should be placed with ends properly secured prior to application to prevent gas leakage. Once the area is treated, it should remain covered for 24 to 48 hours. The cover then may be removed, and the soil aerated for 24 to 72 hours before planting.
Metham or metam-sodium. Metham (sodium methyl-dithiocarbamate) is a member of the thiocarbamate herbicide family. Metham is water-soluble and upon contact with moist soils, breaks down to form the highly toxic and volatile chemical methyl isothiocyanate. Like methyl bromide, metham should be applied to moist soils with temperatures of at least 60oF (15.6oC). It is most effective when its vapors are confined by a tarp. A water-based soil seal method may also be used. With this method, the soil is cultivated and kept moist for a week before treatment. The material is applied, rototilled, and watered-in immediately to the depth of desired control (approximately 4 to 6 inches [10 to 15 cm]). Approximately seven days after treatment, the area should be cultivated to help release any residual gases. One-to-two weeks later (two to three weeks after initial application), the treated area may be planted. The longer pre-planting waiting period and lowered effectiveness in the absence of using a tarp are the primary disadvantages of metham.
Dazomet. Dazomet recently has been introduced as a soil fumigant. Unlike methyl bromide and metham, dazomet is a granular product and is not a restricted use product. Being a granular, dazomet must be evenly applied and incorporated for maximum effectiveness. Its breakdown characteristics, application preparation, and effectiveness are closely associated to metham, as are its advantages and disadvantages.
Non-selective herbicides. Non-selective herbicides (e.g., glyphosate)
also are used for pre-plant weed control. Normally, multiple applications,
spaced two to four weeds apart, are needed for control of existing weeds.
For example, a minimum of three applications of glyphosate were necessary
to completely control bermudagrass (Johnson, 1988b). Control of subsequent
germinating weeds as well as other soil-borne pests is not achieved with
this method.
Maintaining turf complexes at the desired level of aesthetics requires
knowledge of specific weeds, weed biology and available weed control measures.
Weed identification and biology are discussed in other chapters of this
publication. The following discusses selective weed control options.
Weed control should be a carefully planned and coordinated program instead of being a hit-or-miss operation. Understanding why weeds are present on a site is just as important as the chemical control options. The following is a suggested list to use in developing a logical, long-term effective weed control program in turf.
Maintain healthy turf
As mentioned earlier, the best method for weed control is to encourage a strong, vigorous growing turf-stand which prevents or out-competes the weed. Proper cultural and other pest control techniques must be provided in conjunction with herbicide use. Unless such a competitive turf fully covers the soil surface, continued weed problems will occur regardless of the herbicide program used.
Weed identification, biology, and habitat
Weed identification is the first step in understanding why weeds occur and how to control them. Each plant should be identified by species, if possible. The identification characteristics of common turf weeds are listed in an accompanying chapter. Appendix B also lists several guides which should aid turf managers in weed identification.
Understanding the biology or growth and reproductive characteristics of a weed is the second most important step in developing a weed control strategy. Turf managers should understand environmental and cultural factors influencing weed infestations and these can be manipulated in order to favor turf growth over weed establishment. Weed biology is covered in an accompanying chapter.
Soils
Understanding and knowing the chemical and physical composition of soils
becomes important when trying to disrupt weed habitat when using preemergence
herbicides. Soil texture, colloidal components, condition and pH effect
the effectiveness, availability and length of control of most organic pesticides.
Knowing these about soils will aid turf managers on choosing and using
certain materials and possibly why acceptable control did or did not result.
A. Texture - Soil texture involves its percentages of sand, silt and clay present. This determines properties such as water-holding capacity, tilth, and potential herbicide absorption and movement.
B. Soil condition and colloidal components - Surface moisture, plant residue, surface cracking and evenness are soil conditions impairing the effectiveness of soil-applied herbicides. Organic matter and clay content are colloidal components that influence soil applied herbicides. For example, soils high in organic matter (e.g., muck or peat soils) may bind certain herbicides so tightly they become ineffective.
C. pH - The soil pH determines, in part, the ionization (ability
to attract or release ions) of some herbicides and thus their binding affinities
and availability for weed control.
Proper Herbicide Selection, Application, and Record Keeping
Knowledge on the safety, effectiveness at certain weed growth stages, stage of turf establishment, tolerance or susceptibility of treated turf species, time required for control, and economics are important considerations when trying to choose between herbicides. Enormous amount of information on these is available from local county extension offices, state turfgrass specialists, colleagues, and representatives of the chemical company. However, even the best herbicide is only as good as its application. Many variables influence successful herbicide application. These include: pesticide formulation, proper equipment, environmental factors at the time of application, proper and constant calibration and adequate agitation. Most herbicide failures involve applying a non-labeled chemical or applying the chemical at an improper time, rate, or manner.
Once a particular herbicide is chosen and applied, accurate, detailed
information should be kept. In addition to listing what and where a material
was applied, information on how and at what rate it was applied, who made
the application, and the environmental conditions at the time of application
are needed. Environmental information should include: soil and air temperatures;
soil moisture and pH; relative humidity; wind speed and direction; water
pH; cloudiness; weed growth stage and turfgrass condition at the time of
application; time and amount of irrigation or rainfall following application;
and dew presence. By having this information, turf managers can determine
the effectiveness of a material when applied under specific environmental
conditions and can more accurately pinpoint contributing factors if unsatisfactory
results occur.
Preemergence herbicides are applied to the turfgrass site prior to weed seed germination. The mode of action for most preemergence herbicides (e.g., bensulide, benefin, oryzalin, pendimethalin) is the inhibition of certain phases of cell division (mitosis) or inhibition of cell elongation. As the weed seedling germinates, its root and shoot absorbs the herbicide, stops growth and eventually dies.
Preemergence weed control was first suggested by Leach and Lipp (1927) and Sprague and Evaul (1930). Some of the first chemicals evaluated for preemergence weed control included calcium cyanide (Sturkie, 1933), arsenate (Welton and Carroll, 1938) and naphthylacetic acid (Slade et al., 1945). The first true and consistent preemergence herbicide was available for turf producers by 1959. Dimethyl Tetrachloroterephthalate (DCPA) provided more consistent weed control with less turf damage than was previously available (Engel and Ilnicki, 1969). With subsequent release of dinitroaniline chemistry, the widespread acceptance of preemergence weed control in turfgrass was established.
Turfgrass tolerance
When considering any herbicide, the first consideration is the tolerance
of the desirable turfgrass species to the chemical in question. As a general
rule, preemergence herbicides are not as phytotoxic to established turfgrass
species as postemergence herbicides. Notable exceptions are atrazine, simazine
and pronamide on cool-season grasses. Table 1 lists the most widely used
turfgrass species and their tolerance to herbicides.
Table 1. Established turfgrass tolerance to preemergence herbicides.1
| Preemergence
Herbicides |
Bahiagrass | Bentgrass | Bentgrass
golf green |
Bermudagrass | Bermudagrass
golf green< |
Centipedegrass |
|---|---|---|---|---|---|---|
| atrazine
benefin benefin
benefin
bensulide bensulide
dithiopyr ethofumesate
fenarimol isoxaben metolachlor napropamide oryzalin oxadiazon pendimethalin prodiamine pronamide siduron simazine |
D2
T T
T
T D
T D
T T T T T D T D D D D |
D
I D D
T I-T
T
D T D D D D D I-T D I D |
D
D D
D
T I-T
T D
D D D D D D D D D I D |
I
T T
T
T T
T I
T T T T T T T T T D I |
D
D D
D
T T
T D
D D D T D D T D D D D |
T
T T
T
T D
T D
D T T T T D T T D D T |
Table 1 (cont.).
| Preemergence
Herbicides |
Kentucky
bluegrass | Perennial
ryegrass | St. Augustinegrass | Tall fescue | Zoysiagrass |
|---|---|---|---|---|---|
| atrazine
benefin benefin
benefin
bensulide bensulide
DCPA dithiopyr ethofumesate fenarimol isoxaben metolachlor napropamide oryzalin oxadiazon pendimethalin prodiamine pronamide siduron simazine |
D
T D
T
T T
T T D D T D D D T T T D T D |
D
T D
T
T D
T T T D T D D D T T T D T D |
T
T T
T
T D
T T D D T T T T T T T D D T |
D
T T
T
T T
T T D D T D T T T T T D T D |
I-T
T T
T
T T
T T D D T D D T T T T D T I-T |
2T = Tolerant at labeled rates;
I = Intermediate tolerance;
D=damaging or not registered for use on this turfgrass.
Effectiveness of Preemergence Herbicides
The effectiveness of preemergence herbicides vary. Reasons for this include timing in relation to weed seed germination, soil types and the environmental conditions (e.g., rainfall and temperature) during and immediately following herbicide application. Other factors are the target weed species and biotypes, and the cultural factors (e.g., aerification) following application. Preemergence herbicides generally, are most effective for annual grass control, although some annual broadleaf weeds also are suppressed (Table 2).
Timing. An important consideration in preemergence herbicide use is its application timing. Most preemergence herbicides are ineffective on visible weeds. Applications should occur prior to seed germination. However, if applied too soon, the degradation of the herbicide in the soil may reduce its concentration to an ineffective level.
Crabgrass and goosegrass are two of the most troublesome annual grass weeds in turf. If preemergence herbicides are used for control, application timing is critical. Crabgrass initiates spring germination from February through May when soil temperatures at a 4-inch (10 cm) depth reach 53 to 58oF (11.7 to 14.4oC) (Lewis, 1985a). Alternating dry and wet conditions at the soil surface as well as light intensity also encourage crabgrass germination. Goosegrass, meanwhile, germinates at soil temperatures of 60 to 65oF (15.6 to 18.3oC). Goosegrass also requires high light intensity for seed germination (Fulwider and Engel, 1959). Because of higher temperature requirements for germination, goosegrass normally germinates two-to-eight weeks later in spring than crabgrass (Bingham, 1985; Lewis, 1985a). Therefore, when developing a goosegrass weed control program, preemergence herbicide application should be delayed approximately three to four weeks in spring longer than a crabgrass program.
Annual bluegrass is a winter grass weed that starts germination in late summer and continues germination through the fall and winter when soil temperatures at the 4-inch level drop to the low- to mid-70s F or lower. Preemergence herbicide application should be timed just prior to expected period of peak germination. Annual bluegrass often has a second germination flush in mid- to late-winter. This is important for turf managers to recognize because fall herbicide applications normally do not provide season-long control and repeat applications may be necessary.
Efficacy. Various literature citings discuss several herbicidal control levels for annual grasses commonly found in turf (Barrett and Jagschitz, 1975; Bingham and Shaver, 1980; Johnson, 1976a; and Dernoeden et al., 1984). Most states have turfgrass specialists who periodically report and update such a list. Table 5 lists the expected control of common turf weeds for various preemergence herbicides.
Continued use of a particular preemergence herbicide may control the
intended species but also may increase the cover of other weed species.
For example, bensulide provides good crabgrass control but may increase
the cover of clover and speedwell (Johnson, 1982a). In this same study,
benefin and DCPA increased lawn burweed, wild parsnip and clover, while
oxadiazon increased wild parsnip and sandwort populations.
Table 2. Expected control of selective weeds with preemergence
herbicides1.
| Herbicide | Crabgrass | Goosegrass | Annual bluegrass | Common Chickweed | Henbit | Lawn Burweed | Corn Speedwell |
|---|---|---|---|---|---|---|---|
| atrazine
benefin benefin
benefin
bensulide bensulide
dithiopyr fenarimol isoxaben metolachlor napropamide oryzalin oxadiazon pendimethalin prodiamine pronamide siduron simazine |
P-F2
G-E E
E
G-E E
E P F F-G G-E E G E G-E F F F |
P
F G
G
P-F G-E
G-E P P F G F-G E G-E G-E P P P |
E
G-E E
E
F -
G-E G F F-G G G-E G G-E G G-E P E |
G
G G
G
F --
-- P G -- E G P E -- E P G |
G
G G
G
F -
-- P G -- P G P G -- P P G |
E
F --
--
F --
-- P -- -- E -- P -- -- P P E |
G
G --
--
F --
E P -- -- E -- G E -- G P G |
2E=Excellent, >89% control;
G-Good, 80 to 89% control;
F=Fair, 70 to 79% control;
P=Poor, <70% control.
Sequential applications. For season-long control, repeat applications are necessary, especially in the southern areas of the United States. Most herbicides begin to degrade when exposed to the environment. Degradation generally is enhanced by increasing soil moisture and warm weather, and with soils having low cation exchange and water holding capacities. Most herbicides are effective in preventing subsequent weed seed germination during two-to-five months after application. Afterwards, through losses from degradation and other means, repeat applications become necessary for continued preemergence weed control. Turf managers must recognize and understand that most preemergence herbicides will prevent the germination and establishment of desirable turfgrasses as well as weeds during this two-to-five months after application. Timing, therefore, is important on those areas to be turf-established. Note: On those areas to be turf-established, most preemergence herbicides should not be used two to four months prior to planting. Severe turf root damage and turf germination reduction may result.
Some evidence suggests that after initial normal herbicide application rates over one or two years, subsequent yearly rates may be reduced. When proper mowing height (5.7 cm) and nitrogen fertilizer (98 kg N/ha) were maintained, the application of a preemergence herbicide in a red fescue lawn, in the first year, maintained low crabgrass populations (1 to 3%) during the next four years (Jagschitz and Ebdon, 1985). Similarly, herbicide rates required to control crabgrass or goosegrass could be halved or eliminated in subsequent years when an normal rate was applied the first year (Johnson, 1982a).
Aerification. Core aeration generally has not been recommended or practiced following a preemergence herbicide application. Core aeration was believed to disrupt the herbicide barrier in the soil and stimulate weed emergence. Johnson (1987a) reported that core aeration immediately prior to or one, two, three, or four months after applications of benefin, bensulide, DCPA, and bensulide + oxadiazon to common bermudagrass did not stimulate large crabgrass emergence. Aeration at one or two months after application increased large crabgrass cover 5% for oxadiazon at 2.2 kg ha-1, but not at 4.4 kg ha-1. In a related study, it was shown that core aeration at one, two, or three months after an application of oxadiazon did not decrease goosegrass control on a 'Tifgreen' bermudagrass putting green (Johnson, 1982b). Branham and Rieke (1986) reported core aeration, or vertical mowing, immediately or one month after an application of benefin, bensulide, or DCPA did not affect large crabgrass control in annual bluegrass. Monroe et al. (1990) noted that aeration did not affect the activity of several preemergence herbicides in controlling crabgrass species in either 'Tifgreen' or common bermudagrass. However, in creeping bentgrass, significantly greater amounts of crabgrass occurred in aerified plots with the cores returned than in plots not aerified, or aerified plots with the cores removed.
Annual Bluegrass Control in Golf greens
Annual bluegrass is the most troublesome winter annual weed in golf greens. Its low growth habit and its ability to thrive in moist conditions and compacted areas make it difficult to control. Annual bluegrass has a lighter green color than most grass species used to overseed golf greens. It also produces numerous seedheads that reduce the quality of the putting surface. Because of its low tolerance to heat, annual bluegrass dies quickly in warm weather, leaving many playing areas bare until the bermudagrass has time to fill in (Engel and Ilnicki, 1969). Chemical control of annual bluegrass is difficult due to: (1) the majority of preemergence herbicides cannot selectively prevent annual bluegrass germination while allowing the overseeded grass to establish itself, and (2) most effective postemergence herbicides for annual bluegrass also injure the overseeded grass species.
Preemergence control of annual bluegrass in turf was first noted in
the 1930s when the insecticide lead arsenate was discovered (Sprague and
Burton, 1937). Since then, preemergence annual bluegrass control has been
reported with numerous herbicides.
Bermudagrass golf greens. Preemergence control in bermudagrass golf greens currently is available with several herbicides. Each has its own use precautions. If these are not followed, unsatisfactory results may occur. Bingham, et al. (1969) reported bensulide would provide preemergence annual bluegrass control and that acceptable stands of ryegrasses could be obtained when seeding was delayed one month after its application. This could, however, be influenced by environmental and management practices resulting in a narrow tolerance range. Label directions (Anonymous, 1992a) dictate that bensulide application should be no sooner than 120 days prior to overseeding. This waiting period allows enough bensulide to be in the soil to control the germinating annual bluegrass, but be low enough to not interfere with germination of the grasses used for overseeding.
Benefin, like bensulide, can be applied before annual bluegrass and overseeding if proper timing is followed. At the low application rate, a minimum of six weeks are necessary after treatment before overseeding should be attempted. Twelve to 14 weeks are necessary between application of the high rate and overseeding.
Pronamide also provides preemergence control of annual bluegrass in bermudagrass. Like bensulide, pronamide must be applied in advance of annual bluegrass germination and seeding cool-season grasses. The minimum recommended period between application and overseeding is 90 days (Anonymous, 1992b). It also is recommended that application not be made where drainage flows onto areas planted with cool-season grasses.
Activated charcoal has been used successfully to prevent injury to the desirable overseeded grass when pronamide was applied closer than 90 days to overseeding (Meyers et al., 1973). High rates (e.g., 2.5 to 4 lb/1000 sq.ft. or 122 to 195 kg/ha) of charcoal are necessary. A disadvantage of this method is the inability to re-establish the ryegrass in the event the charcoal treatment fails. Current formulations of activated charcoal also are messy to handle and apply.
Ethofumesate also provides preemergence and early postemergence annual bluegrass control in bermudagrass overseeded to ryegrass (Lee, 1977; Dickens, 1979; Johnson, 1983). To prevent turfgrass injury, application rate, time and frequency are important. In general, an initial 1 kg/ha is applied 30 to 45 days after overseeding. A sequential application at similar rates may be required 30 days after the initial application for season-long control. If ethofumesate is applied before bermudagrass dormancy in the fall, the bermudagrass stops growing (Dickens, 1979). A delay in spring transition from ryegrass to bermudagrass also occurs when ethofumesate is applied in early fall (Johnson, 1983) or in February.
Fenarimol, a systemic fungicide used to control several turfgrass diseases, gradually reduces annual bluegrass populations without adverse effects to overseeded grasses or bermudagrass (Coats, 1986; Anonymous, 1982). Its application should occur prior to overseeding and prior to the germination of annual bluegrass. A treatment scheme has been suggested consisting of one, two or three applications with the single or final application being two weeks prior to overseeding (Coats, 1986). Fenarimol does not appear to deleteriously effect either overseeded perennial ryegrass or bermudagrass, but the necessity of properly timed repeat applications can be a drawback for managers who have limited budgets and labor. Inconsistent annual bluegrass control following fenarimol treatments has been noted (Gaul and Christians, 1988; Johnson, 1988a).
Bentgrass golf greens. Bentgrass is sensitive to most postemergence grass herbicides and many are ineffective against annual bluegrass. As a result, preemergence herbicides are the most common means of controlling this weed. Erratic control of annual bluegrass, however, has been reported (Bingham et al., 1969; Bingham and Shaver, 1979; Callahan and McDonald, 1992; Shearman, 1986). It has been suggested that the presence of perennial biotypes of the species may contribute to this erratic control (Gibeault and Goetze, 1972). Low growing, creeping perennial types become dominant over the annual biotype under frequent close mowing. Moist soil conditions and high soil nutrient levels, which are conditions normally maintained with creeping bentgrass golf greens, also contribute to creeping perennial biotype dominance (Callahan and McDonald, 1992). As a result, preemergence herbicides may control the annual biotype, allowing the perennial biotype to further dominate the green.
Limited research on preemergence control of the perennial biotype suggests poor short-term control (Callahan and McDonald, 1992). Repeat application over multiple years (minimum of four years) are necessary for significant reduction of perennial weeds in bentgrass golf greens. Multiple-year treatments during February and March are considered superior for control other than do August or September treatments.
Ethofumesate has been used safely to control annual bluegrass in bentgrass golf greens (Lewis and DiPaola, 1989). Best control with minimum turf injury was achieved with application rates at 0.56 kg/ha per application for a total of five applications or at 0.84 kg/ha per application for three applications. Applications should be 30 days apart. Control was more consistent when treatments began in October. Applications in late December or in January increased bentgrass injury. Bentgrass, with a shallow root system, grown under stress conditions or in compacted soils, was more prone to injury from the ethofumesate treatments.
With the realization that elimination of annual bluegrass in golf greens is not always achievable with current herbicide technology, research recently has focused on suppressing its growth and seedhead production (DiPaola and Lewis, 1989). The turf growth regulators (TGR's) paclobutrazol and flurprimidol currently are available for annual bluegrass suppression in bentgrass golf greens. Other materials (e.g., mefluidide, maleic hydrazide) also are available, but only for higher mowed turf (DiPaola, 1992). Generally, paclobutrazol or flurprimidol are applied to actively growing bentgrass in mid-fall or in early spring. Differential species susceptibility to the herbicide is attributed to a greater uptake of the TGR by the shallower-rooted annual bluegrass when compared to the deeper rooted bentgrass (Kageyama et al., 1989). Paclobutrazol and flurprimidol are considered Type II growth regulators since they inhibit gibberellin biosynthesis and suppress internode elongation (Kaufman, 1989). These materials are root-absorbed (xylem-mobile) and work by reducing the competitive ability of the annual bluegrass for three to eight weeks after application. This allows the creeping bentgrass to out compete the weed. These materials should only be applied during periods of active bentgrass root growth. Flurprimidol at rates greater than 0.56 kg/ha can reduce germination of annual bluegrass seed as well as bentgrass (Gaussoin and Branham, 1987), thus, should be applied only to established greens. Prevention of annual bluegrass seedhead formation may be inconsistent with these materials but they do prevent seedhead stalk elongation (Kaufman, 1989) which may result in improved turf uniformity and appearance.
Newly Established Turf
Preemergence herbicides such as bensulide, dithiopyr, and members of the dinitroaniline family (e.g., benefin, oryzalin, pendimethalin, prodiamine, trifluralin) should only be used on well established turfgrasses. The previously mentioned herbicide group has been shown to inhibit rooting of immature or newly-established turfgrasses (Bingham, 1974; Lewis et al., 1988). A waiting period of three to five months normally is required between the last herbicide application and establishment of the turfgrass.
Oxadiazon generally is the safest preemergence herbicide when vegetatively establishing grass (Bingham, 1985), while siduron is the safest on seeded grasses. Oxadiazon functions by inhibiting shoot emergence of susceptible weeds and does not prevent cell division of roots as do most other preemergence herbicides. Minimal effects therefore occur on the rooting of vegetatively established turf species, but selective weed control of goosegrass occurs.
Siduron is a member of the substituted urea herbicide family. It appears
to interfere with photosynthesis of the developing weed. Since siduron
does not appreciably inhibit rooting, it safely can be applied to newly
seeded turf. Siduron effectively controls annual grass weeds such as crabgrass
and foxtail but does not effectively control annual bluegrass, goosegrass,
or most of the broadleaf weeds (Ashton and Monaco, 1991). Siduron should
not be used on bermudagrass.
Postemergence herbicides are effective only on germinated and visible
weeds. The timing of the application should be when weeds are young, preferably
during the two-to-four leaf stage. Herbicide uptake and translocation is
favored at this time. Turfgrasses also are then better able to fill in
voids left by the dying weeds.
Generally, postemergence herbicides have a much greater likelihood of injuring a turf species than preemergence herbicides. Turf managers must be careful about the herbicide they select and how they use it. Turf injury usually is more likely when the turf is under stress from temperature, moisture, and other pests such as nematodes. Turfgrass species tolerance to postemergence herbicides are listed in Table 3.
Table 3. Turfgrass tolerance to postemergence herbicides.1
| Herbicides | Bahiagrass | Bentgrass | Bermudagrass | Carpetgrass | Centipedegrass | Kentucky bluegrass |
|---|---|---|---|---|---|---|
| asulam
atrazine bentazon bromoxynil chlorflurenol chlorsulfuron clopyralid clopyralid
2,4-D 2,4-D
2,4-D
2,4-D
2,4-D
2,4-D
dicamba DSMA,
fenoxaprop halosulfuron imazaquin MCPP metribuzin pronamide quinclorac sethoxydim simazine |
D2
I T T T T D D
T T
T
T
T
D
T D
I* T D T D D D D I |
D
D T T D I D I
D-I* D-I*
D-I*
I*
D-I*
D-I*
I I*
I-T T D T D D D D D |
I-T
I T T T T I-T I-T
T T
T
T
T
D-I
T T
D T T T D T T D I |
D
T T - - - - -
I-T I-T
I-T
I-T
I-T
I-T
T D
- T I-T I-T - - - D-I T |
D
T T T T D D T
D-I D-I
I-T
D-I
D-I
D
I-T D
D T T I-T D D D T T |
D
D T T T T T T
T T
T
T
T
T
T I-T
T T D T D D T D D |
Table 3 (cont.).
| Herbicides | St. Augustinegrass | Perennial ryegrass | Tall fescue | Zoysiagrass |
| asulam
atrazine bentazon bromoxynil chlorflurenol chlorsulfuron clopyralid clopyralid
2,4-D 2,4-D
2,4-D
2,4-D
2,4-D
2,4-D
dicamba DSMA,
fenoxaprop halosulfuron imazaquin MCPP metribuzin pronamide quinclorac sethoxydim simazine |
I
T T T T D D D
D-I D-I
D-I
D-I
D-I
D
D-I D
D T T D-I D D D D T |
D
D T T T D T T
T I*
I*
I*
I*
I*
T I
T T D T D D T D D |
D
D T T T D T T
T T
T
T
T
T
T I-T
T T D T D D T D D |
D
I T T T D I-T T
T T
T
T
T
D
T I
I T T T D D T I I |
2T = Tolerant at labeled rates;
I = Intermediate tolerance, use at reduced label
rates;
D = Damaging, do not use this herbicide or is not
registered for use on this turfgrass.
*Tolerance depends on specific product formulation, timing of its application
and its applied rates.
Broadleaf weed control in turf has been traditionally with members of the phenoxy herbicide family (e.g., 2,4-D, dichlorprop [2,4-DP], MCPA and mecoprop) and the benzoic acid herbicide family (e.g., dicamba). All are selective, systemic and foliar-applied herbicides. Broadleaf weeds, especially perennials, often are not adequately controlled with just one of these materials. Usually, two- or three-way combinations of these herbicides, and possible repeat applications 7 to 14 days apart, are necessary for satisfactory control. Combinations of 2,4-D, MCPP, and dicamba appear to be synergistic, therefore, require less of each than if applied alone. Turf safety with this combination depends on the herbicide mixture, its rate, and its formulation as well as environmental conditions at the time of treatment.
Many formulations and mixtures of the phenoxy herbicides are available (Table 4). These include the parent acid, amine salts, esters and inorganic salts. The water-soluble amine formulations are the most common because of their lack of volatility, high water solubility, ease of handling and overall low cost. Although more effective, high-volatile esters normally are not used in turf. The likelihood these will volatilize and drift to nearby desirable plant species is high.
Until recently, these various herbicide combinations were the main chemicals for broadleaf control. Triclopyr and clopyralid have been introduced as alternatives to phenoxy herbicides for broadleaf control.
Table 4. General characteristics of different forms of 2,4-D*.
| Form | Solubility in Water | Solubility in Oil | Color when mixed with water | Precipitates Formed in Hard Water | Volatility Potential |
|---|---|---|---|---|---|
| Acid
|
low | low | milky | yes | low |
| Amine Salts
- water soluble - oil soluble
|
high
low |
low
high |
clear
milky |
yes
yes |
none
none |
| Esters
- low volatile - high volatile
|
low
low |
high
high |
milky
milky |
none
none |
medium
high |
| Inorganic Salts
|
medium | low | clear | yes | none |
Triclopyr and clopyralid belong to the pyridine herbicide family. Compounds in this family have been noted for their high degree of activity. For example, members of this herbicide family are up to ten times more potent than 2,4-D on some broadleaf weed species (Klingman and Ashton, 1982). These herbicides rapidly are absorbed by the roots and foliage of broadleaf plants. They are readily translocated throughout the plants via both xylem and phloem tissues. Problems with this herbicide family include soil mobility and the extreme sensitivity to them by many desirable ornamental. Triclopyr is used alone and in combination with 2,4-D, or clopyralid, for broadleaf weed control. These are used on primarily in cool-season grasses. Table 5 lists the effectiveness of commonly used postemergence herbicides for broadleaf weed control.
Table 5. Susceptibility of broadleaf weeds to turf herbicides1.
| Weed | Life cycle | Atrazine/
Simazine |
2,4-D | Mecoprop
(or MCPP) |
Dicamba | 2,4-D
+MCPP |
2,4-D
+ dichlorprop |
2,4-D +
MCPP + dicamba |
2,4-D
+ triclopyr |
Quinclorac |
|---|---|---|---|---|---|---|---|---|---|---|
| Betony, Florida | P2 | S-I3 | I | I | I-S | I | I | I-S | -- | -- |
| Bittercress, hairy | WA | -- | S | I | S | S | S | S | -- | -- |
| Bindweed, field | P | -- | S-I | I | S | S-I | S | S | -- | -- |
| Black Medic | A | -- | R | I | S | I | S | S | -- | -- |
| Burclover | A | -- | I-R | S | S | S-I | S | S | -- | R |
| Buttercups | WA, B&P | I | S-I | I | I-R | S | S | S | -- | -- |
| Buttonweed, Va. | P | -- | S-I | I | I | I | S-I | S-I | I-R | - |
| Carpetweed | SA | S | S | I | S | S | S | S | -- | - |
| Carrot, wild | A, B | -- | I | I | S | I | S-I | S | - | - |
| Catsear | P | -- | S-I | I | S | S | S | S | -- | -- |
| Chickweed, common | WA | S | R | S-I | S | S | S | S | S | R |
| Chickweed, mouse-ear | WA, P | I | I-R | S-I | S | S | S | S | S-I | R |
| Chicory | P | -- | S | S | S | S | S | S | -- | -- |
| Cinquefoil, common | P | - | S-I | S-I | S-I | S-I | S-I | S-I | -- | -- |
| Clover, crimson | SA | -- | S | S | S | S | S | S | -- | S |
| Clover, hop | WA | S | I | S | S | S | S | S | S | S |
| Clover, white | P | S | I | S | S | S | S | S | S-I | S |
| Daisy, English | P | - | R | I | S | I | I | S | -- | -- |
| Daisy, oxeye | P, B | - | I | I | I | I | I | S-I | --- | -- |
| Dandelion | P | I-R | S | S | S | S | S | S | S-I | I |
| Dichondra | P | S-I | S | I | S-I | S | S | S | -- | -- |
| Dock, broadleaf & curly | P | I | I | I-R | S | I | I | S-I | I | -- |
| Garlic, wild | P | -- | S-I | R | S-I | S-I | S-I | S-I | -- | R |
| Geranium, Carolina | WA | -- | S | S-I | S | S | S | S | -- | -- |
| Hawkweed | P | -- | S-I | R | S-I | S-I | S-I | S-I | -- | -- |
| Healall | P | -- | S | R | S-I | S | S | S | -- | -- |
| Henbit | WA | S | I-R | I | S | I | S-I | S | S | R |
| Ivy, ground | P | -- | I-R | I | S-I | I | S-I | S-I | -- | R |
| Knawel | WA | -- | R | I | S | S-I | S-I | S | -- | -- |
| Knotweed, prostrate | SA | S | R | I | S | S-I | S-I | S | -- | R |
| Lambsquarter, common | SA | -- | S | S | S | S | S | S | -- | I |
| Lespedeza | SA | S | I-R | S | S | S-I | I | S | -- | -- |
| Mallow | P | -- | I-R | I | S-I | S-I | S-I | S-I | -- | -- |
| Mugwort | P | - | I | I-R | S-I | I | I | I | -- | -- |
| Mustard, wild | WA | S | S | I | S | S | S-I | S | -- | -- |
| Onion, wild | P | - | I | R | S-I | I | I | S | -- | R |
| Parsley-piert | WA | S | R | S-I | S-I | S-I | R | S-I | S | -- |
| Pearlwort | WA | -- | S-I | S-I | -- | S-I | S-I | S-I | -- | -- |
| Pennywort, lawn | P | S | S-I | S-I | S-I | S-I | S-I | S-I | -- | - |
| Pepperweed, Va. | A | -- | S | S-I | S | S-I | S | S | -- | -- |
| Pigweed | A,P | -- | S | S | S | S | S | S | -- | I |
| Plantains | P | I-R | S | I-R | R | S | S | S | I-R | I |
| Purslane, common | SA | -- | I | R | S | I | I | S-I | -- | R |
| Red sorrel | P | -- | R | S | S | S-I | I | S | -- | R |
| Shepherd's-purse | WA | -- | S | S-I | S | S-I | S | S | -- | R |
| Speedwell, corn | WA | S | I-R | I-R | I-R | I-R | I-R | I-R | -- | I |
| Spurge, prostrate | SA | S-I | I | I | S | I | S-I | S-I | S-I | I |
| Spurge, spotted | SA | S | I-R | S-I | S-I | S-I | S-I | S | S-I | I |
| Spurweed (lawn burweed) | WA | S-I | I | S-I | S | S-I | I | S | S | -- |
| Strawberry, India mock | P | -- | R | I | S-I | I | R | S-I | -- | -- |
| Thistles | B, P | -- | S-I | I | S | S-I | S-I | S | -- | -- |
| Violet, johnny-jumpup | WA | -- | I-R | I-R | S-I | I-R | I | I-R | -- | -- |
| Violet, wild | P | -- | I-R | I-R | S-I | I-R | I | I-R | I-R | -- |
| Woodsorrel, yellow | P | I | R | R | I | I-R | I-R | I-R | -- | R |
| Yarrow | P | -- | I | I-R | S | I-R | I | S-I | -- | -- |
| Yellow rocket | WA | -- | S-I | I | S-I | S-I | S-I | S | -- | -- |
2A = annual,
B = biennial;
P = perennial;
SA = summer annual;
WA = winter annual.
3S = susceptible;
I = intermediately susceptible, good control sometimes
with high rates, however a repeat treatment 3 - 4 weeks later each at the
standard or reduced rate is usually more effective;
R = resistant in most cases.
Not all weeds have been tested for susceptibility by each herbicide
listed.
Postemergence grass weed control traditionally has been through single
and repeat applications of the organic arsenicals (e.g., MSMA, DSMA, CMA).
Two-to-four applications, spaced seven to 14 days apart are required for
complete weed control (Johnson, 1975; 1976b; 1977). Repeat applications
are needed for control of annual weeds and for most perennial weeds. The
rate and number of applications necessary for control also increases as
weeds mature. Organic arsenicals can be phytotoxic on cool season turfgrasses,
especially when used during high temperatures (>90oF; 32oC).
Control also is reduced if rainfall occurs within 24 hours of treatment
(Bingham, 1985). Recently, new herbicide releases have provided alternatives
to the arsenicals for postemergence grass weed control. Decreased phytotoxicity,
as well as reduced number of applications, often are associated with these
herbicides. The following discusses herbicides available for various turfgrass
species.
Warm-Season Turfgrasses
Bermudagrass and zoysiagrass. Postemergence control of crabgrass species and goosegrass species usually have been with organic arsenicals (Lewis, 1981; Johnson, 1975). As previously mentioned, repeat applications with a short interval between applications are required, especially for mature weed control. Increasing phytotoxicity usually results in bermudagrass and zoysiagrass with repeat applications. Zoysia spp. tend to be more sensitive to organic arsenical applications. Phytotoxicity, or leaf yellowing, normally accompanying these for ten days to two weeks after application. Seven to ten days of phytotoxicity normally accompanies treatments on bermudagrass.
In order to increase herbicidal activity on goosegrass, various combinations of arsenicals with other herbicides have been tested. High rates (e.g., 0.28 to 0.56 lbs. ai/acre) of metribuzin (an asymmetrical triazine) have provided excellent control of goosegrass when used alone but resulted in marginal bermudagrass tolerance. Lower rates (e.g., 0.125 lbs. ai/acre) of metribuzin with arsenical herbicides have shown good to excellent goosegrass control (Bingham, 1985; Lewis, 1981; Johnson, 1976b; Johnson, 1980). This combination is only used safely on well established bermudagrass maintained at mowing heights greater than 0.5 inch. The use of metribuzin increased herbicidal activity on goosegrass but a certain degree of phytotoxicity and a number of escaped weeds still existed. Metribuzin also has inhibited photosynthesis of bermudagrass for a certain period of time (Yang and Bingham, 1984).
Diclofop-methyl, a member of the aryl-oxy-phenoxy herbicide family, is superior in goosegrass control when compared to individual and combinations of the organic arsenicals and metribuzin. Little damage to bermudagrass has resulted and repeat applications usually are not necessary (McCarty et al., 1991; Murdoch and Nishimoto, 1982). This herbicide seems to be more active on goosegrass maintained at lower mowing heights (McCarty, 1991). The addition of MSMA is possibly antagonistic for goosegrass control while the addition of metribuzin to diclofop injures the turf. Goosegrass control is relatively slow and often requires two-to-three weeks to become effective. Weed control spectrums also appear to be limited, with goosegrass being the most susceptible annual grass weed. Treated areas should not be overseeded with perennial ryegrass for at least six weeks after application (McCarty and Murphy, 1993). Diclofop-methyl's registration currently is limited to certain states under section 24c of the EPA's herbicide registration category. Zoysiagrass tolerance to diclofop-methyl has not been reported.
Quinclorac will provide postemergence crabgrass control in bermudagrass and zoysiagrass. Quinclorac also has provided selective torpedograss suppression in bermudagrass, an option previously not available (McCarty, 1992). A series of two or three applications are normally required for these suppression. Selective broadleaf weed control also is available with quinclorac but goosegrass is not controlled by this herbicide.
Fenoxaprop-ethyl, another member of the aryl-oxy-phenoxy herbicide family, has been shown to control annual grass weeds, crabgrass species in particular (Lewis, 1985b; Bhowmik, 1986). Zoysiagrass has shown good tolerance to fenoxaprop (Dernoeden, 1985) while bermudagrass lacks tolerance to it (Higgins et al., 1987).
Centipedegrass. The triazine herbicides were used traditionally for postemergence grass weed control in centipedegrass. However, repeat applications necessary for control often resulted in turfgrass injury (Miller et al., 1985). Triazine herbicides provide variable postemergence grass weed control but lose their effectiveness as weeds mature. The organic arsenicals, commonly used in other turfgrass species, are highly injurious to centipedegrass (Miller et al., 1985).
Sethoxydim, a member of the aryl-oxy-phenoxy herbicide family, provides good to excellent control of many annual grass weeds and also provides suppression of several perennial grass weeds, including bahiagrass (Chernicky et al., 1984). Centipedegrass tolerance to sethoxydim is excellent (McCarty et al., 1986; Willard and Currey, 1985). The discovery of sethoxydim's tolerance to centipedegrass has been a major breakthrough in postemergence annual grass control in centipedegrass.
St. Augustinegrass. One major problem of growing St. Augustinegrass is its relatively poor tolerance to most postemergence herbicides (Turgeon, 1991). Triazine herbicides (e.g., atrazine, simazine) provide fair control of some annual grass weed species, but only in the juvenile growth stage. Repeat applications often are necessary for satisfactory control, usually resulting in some turf damage.
Asulam, a member of the carbamate herbicide family, provides control of some annual grass weeds without serious damage to St. Augustinegrass (Coats, 1986). Repeat applications seven to 14 days apart may be required for control of older weeds. This increases the likelihood of damage to St. Augustinegrass. As with most other postemergence herbicides, asulam should be applied when temperatures are below 85oF (29.4oC) and when the turf is growing.
Bahiagrass. Bahiagrass, like St. Augustinegrass, is somewhat
sensitive to most postemergence herbicides. This sensitivity limits the
number of herbicides available for use on it. Although labeled on bahiagrass,
most postemergence broadleaf (e.g., 2,4-D, dicamba, and/or mecoprop) herbicides
will cause turf yellowing, especially if it is applied when temperatures
are hot, or the growing turf is under stressful conditions. Normally, the
phytotoxicity is not lethal and turf recovery can be expected within one-to-two
weeks. On high maintenance bahiagrass, no selective postemergence herbicide
is available to control grass weeds. Spot spraying or selective placement
such as rope-wicking, with a non-selective herbicide such as glyphosate
are the only means of chemical control of grass weeds.
Cool-Season Turf Grasses
Postemergence grass weed control in cool-season turfgrasses traditionally has been limited to various members of the organic arsenicals (Ashburn, 1988). Specific formulations and specific rates are necessary for its use on most cool-season turfgrasses or injury may result. Proper timing at a young weed-growth stage, during mild environmental conditions and while turfgrass actively is growing should be considered before using any of these herbicides.
Fenoxaprop has been shown to control tillered crabgrass (Chism and Bingham,
1991; Bhowmik, 1986) when adequate soil moisture was present and was tolerated
by some cool-season turfgrasses. Seedling and mature perennial ryegrass
and tall fescue have shown little injury from fenoxaprop (Dernoeden, 1987;
Grande et al., 1984; McCarty et al., 1989). Kentucky bluegrass also is
tolerant (Chism and Bingham, 1991) but sometimes can be injured when applied
in the spring (Bhowmik, 1986; Cisar and Jagschitz, 1984). Mono-seedling
stands of Kentucky bluegrass have been shown to be injured by fenoxaprop.
However, when it is in competition with a severe infestation of crabgrass,
the loss through injury may be outweighed by the improved turf cover the
following fall (Dernoeden, 1989b). Creeping bentgrass also is injured by
fenoxaprop (Bhowmik and Eichenlaub, 1986) but recovers within several weeks
of application (Higgins, et al., 1986). Safeners may reduce fenoxaprop
damage on bentgrass (Bhowmik and Eichenlaub, 1986) as may the addition
of iron with and without methylene urea (Dernoeden, 1989b). A low rate
of fenoxaprop (0.025 lbs. ai/acre) reduces the degree of bentgrass discoloration
(Carroll et al., 1992) but may result in inconsistent crabgrass control.
The predominant nutsedge species in turfgrasses are yellow and purple nutsedge. Other, more local members of the Cyperus genus include annual or water sedge, perennial and annual kyllinga, globe sedge, Texas sedge, flathead sedge and cylindrical sedge. Path or slender rush, a member of the rush (Juncus) genus, also can occur in some turf situations.
Yellow and purple nutsedge are low-growing perennials which resemble grasses. Nutsedges, in general, are yellow-green to dark-green, with triangular stems bearing three-ranked leaves, unlike the two-ranked leaves of the grass family. The root systems are fibrous with deep-rooted tubers or nutlets. Seedhead color is often used to distinguish between the two major nutsedges. Leaf tip shape is another distinguishing characteristic. Leaf tips of purple nutsedge are generally wider and more rounded. Conversely, yellow nutsedge leaf tips are narrow, forming a needle-like tip. Yellow and purple nutsedge are not believed to produce viable seed bu