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Unlike some of their northern counterparts, southern turfgrass managers must control weeds throughout the year. It would seem that after contending with summer weeds such as crabgrass and goosegrass, southern managers would get a break. But, unfortunately, numerous winter annual weeds initiate their growth at the same time that warm-season turfgrasses enter dormancy. Annual bluegrass, henbit, lawn burweed (spurweed), common chickweed and winter perennials such as wild garlic and wild onion are prevalent throughout the South. These species germinate in the late summer and fall months as temperatures cool and moisture becomes available. There is also some germination during the winter or early spring months as temperatures fluctuate. During the cold months of the winter, growth is slow; but, as temperatures warm in February and March winter annuals initiate rapid growth and development.

Besides being unsightly, dense infestations of winter weeds can be extremely damaging to warm-season turfgrasses during spring green-up. Competition for sunlight, soil moisture and plant nutrients can be intense during this time period. As these dense mats of winter weeds die, the warm-season turfgrass cover or stand can be severely stunted or have large openings that can easily be infested by summer annuals such as crabgrass.

Numerous herbicides may used to control winter annual weeds. Options include the use of preemergence herbicides in the late summer or early fall prior to winter annual weed germination or the use of postemergence herbicides on an as needed basis. Traditionally, southern turfgrass managers have relied upon triazine (Aatrex, other trade names) and simazine (Wynstar, other trade names) to control winter annual weeds in non-overseeded warm-season turfgrasses. Both of these herbicides have both preemergence and postemergence activity on a wide array of winter annuals such as annual bluegrass, corn speedwell, common chickweed, henbit, parsley-piert and lawn burweed. Over the past two to three years, several turfgrass managers have reported that atrazine and simazine no longer control annual bluegrass on sites where these herbicides have been used for several years. Atrazine and simazine belong to the triazine herbicide family. Triazine-resistant populations of annual bluegrass have been known since 1977. Continued annual use of triazine herbicides can lead to the development of triazine-resistant weeds by selecting out populations that are naturally present but are inherently resistant. Rotating to a herbicide with a different mode-of-action is the only way to achieve acceptable levels of control. If triazine-resistant annual bluegrass populations are suspected, then rotation to pronamide (Kerb) or diquat (Reward) for postemergence control would be necessary. Alternatively, a preemergence herbicide could be used in the late summer to early fall months.

Because of the application flexibility, low cost and effectiveness of the triazine herbicides, traditional preemergence herbicides such as oryzalin (Surflan), pendimethalin (numerous trade names) and oxadiazon (Ronstar) have not been widely used for winter annual weed control on southern turfgrasses. However, research conducted at several universities have shown that preemergence herbicides do have a place for winter annual weed control. Research conducted in middle Georgia showed that when applied in mid-September to common bermudagrass, oxadiazon, oryzalin, dithiopyr (Dimension), benefin (Balan), prodiamine (Barricade), pendimethalin, benefin + oryzalin (XL) and benefin + trifluralin (Team) provided excellent annual bluegrass control during the subsequent winter and spring months. Previous research has also showed that when applied at a similar time period, oryzalin, dithiopyr, benefin, prodiamine and pendimethalin would control common chickweed, henbit and corn speedwell. However, unlike the triazine herbicides this group of herbicides is not effective for the control of parsley-piert and lawn burweed (spurweed).

Two-way or three-way herbicides that contain 2,4-D, 2,4-DP, MCPP or dicamba are useful for the postemergence control of numerous winter annual and perennial broadleaf weeds and wild garlic. Unlike simazine, atrazine, pronamide and metribuzin (Sencor), two-way and three-way mixtures can be used on warm-season turfgrasses that have been overseeded with perennial ryegrass and on turf-type fescues. In order to prevent injury, applications should be delayed until the cool-season turfgrass has been mowed three to four times. Delaying the application improves the tolerance of newly-established cool-season turfgrasses to phenoxy or dicamba containing herbicides.

There are a large number of two-way and three-way herbicides being marketed. It is extremely difficult to state that one brand consistently provides better results than another brand. Factors such as the species of weed, growth stage, environmental conditions at the time of application and the turfgrass species must be considered in determining which is the best two-way or three-way herbicide. The best results with these herbicides occur when applications are made to small actively-growing weeds at temperatures above 50^o F. Applications on cold, wintery days are almost always less effective. Additionally, two applications of these products at a three week interval will be needed with some weed species such as henbit and parsley-piert.

Other options for winter weed control include imazaquin (Image) and diquat (Reward). Image controls common chickweed, parsley-piert, henbit and wild garlic. However, Image should only be used when the turfgrass is dormant and should not be applied sooner than 6 weeks before spring green-up or during green-up. Applications near or during green-up can retard green-up. Diquat is a fast-acting, contact herbicide. Therefore, adequate spray coverage is essential for good control. Diquat is not effective for the control of perennial weeds such as plantains and wild garlic.

Many two-way and three-way herbicides and atrazine and simazine have been shown to temporarily delay the spring green-up of warm-season turfgrasses if applications are made immediately before or during spring green-up. Usually, this effect is temporary and persists for only 1 to 4 weeks. If spring temperatures and soil moisture levels are conducive to warm-season turfgrass growth, the turfgrass will rapidly outgrow the injury induced by the herbicide. Thick, dense mats of winter annuals can actually be more detrimental to spring transition than the temporary injury associated with the use of postemergence herbicides. Therefore, it is usually advisable to go ahead and apply the herbicide, unless the herbicide label specifically prohibits applications during spring green-up.

Southern turfgrass managers have an excellent arsenal of herbicides to control winter weeds. The use of a preemergence herbicide in early September followed by as needed applications of postemergence herbicides during the winter months is a good strategy on sites where overseeding is not planned, or where triazine herbicides have not provided annual bluegrass control. On sites that will be overseeded, annual bluegrass control is more difficult; however, broadleaf weeds can be controlled with properly timed applications of two-way and three-way herbicides.