Printable Version
PDF

Controlling Grassy Weeds in Warm-Season Turf

B. J. Johnson and T. R. Murphy

Crabgrass (Digitaria spp.) and goosegrass [Eleusine indica (L.) Gaertn.] are problem annual grasses in warm-season turfgrasses grown in home lawns, athletic fields, golf courses, parks, and other recreation areas during the spring and summer. When these weeds are not controlled, turfgrass quality is severely reduced. The amount of reduction in turf quality depends on the infestation level of weeds. However, even when a small amount of weeds (<10% cover) is present, they can gradually increase to an undesirable population level. Therefore, to maintain a high-quality turf in most areas, the use of herbicides must be included into the overall management program.

Various herbicide programs may be used for weed control in warm-season turfgrasses. They include: a) a single application of a preemergence (PRE) herbicide applied at the maximum labeled rate in late winter of early spring, b) PRE herbicides applied as split applications, one-half the maximum labeled rate in late winter or early spring and one-half of the maximum labeled rate 60-days later, c) sequential programs involving the use of PRE herbicides applied in late winter or early spring, and followed by postemergence (POST) herbicides as needed in late spring or early summer for extended weed control, and d) tank-mixtures of PRE and POST herbicides applied as a single application after weeds emerge in early to late spring. Each program has its merits, but may not perform similarly throughout the spring and summer at all locations. Therefore, turfgrass managers should select a program that can be used effectively to control weeds at their location.

The results reported in this paper are from experiments conducted in Georgia from 1986 through 1993. Herbicides used are given in Table 1. The turfgrass in all weed experiments was common bermudagrass [Cynodon dactylon (L.) Pers.]. It should be emphasized that these studies were conducted on a different weedy site each year as it is commonly known that repeated PRE herbicide applications to the same site over years will control weeds more consistently than when treatments are applied once. Therefore, we wanted to determine the performance of these herbicides under very high weed populations.
 

Table 1. Herbicides evaluated for annual grass control in Georgia from 1986 through 1993.
 
HERBICIDEa
COMMON TRADE CHEMICAL ACTIVE 
INGREDIENT		 COMPANY
Benefin + oryzalin XL 1.0 + 1.0 G DowElanco
Bensulide + oxadiazon Goosegrass  
Crabgrass control		 5.25 + 1.13% O. M. Scott
Diclofop Illoxan 3.0 EC Hoechst/Roussel
Dithiopyr Dimension 1.0 EC Monsanto
MSMA Daconate 6.0 L ISK Biotech
Metribuzin Sencor 75 DF Mobay
Oryzalin Surflan 4.0 AS DowElanco
Oxadiazon Ronstar 2.0 G Rhone-Poulenc
Oxadiazon + benefin Regalstar 1.0 + 0.5 Fb Regal
Oxyfluorfen + oryzalin Rout 2.0 + 1.0 G Grace/Sierra
Pendimethalin PRE-M and Southern Weedgrass control 60 WDG Lesco and 
O. M. Scott
Prodiamine Barricade 65 WDG Sandoz
Trifluralin + benefin Team 0.7 + 1.3 G DowElanco
aTrade and company names are included for the benefit of the reader and do not imply any endorsement or preferential treatment. A given herbicide may have more than one trade name, the one used is from the company furnishing the chemicals.
bFormulated on a urea nitrogen fertilizer
 

PRE Herbicides - Single vs. Split Applications. The results of PRE herbicides applied as single or split applications on crabgrass and goosegrass control in common bermudagrass are given in Table 2. The number of experiments was not the same for all herbicides and weed species. In these studies, we selected >80% control as an acceptable level of control. We realize that some turfgrass areas, such as golf greens, require a higher level of control, but that in some turf areas such as roughs, <80% control may be sufficient.

There was no single PRE herbicide treatment over the 8-year period that controlled both crabgrass and goosegrass effectively (>80% in each experiment) (Table 2). Ronstar at 4.0 lb ai/A controlled an average of 91% goosegrass, and provided >80% goosegrass control in all experiments. Crabgrass control with Ronstar at this rate was >80% in 6 of 7 experiments. Likewise, XL applied at 3.0 lb ai/A controlled an average of 80% crabgrass and goosegrass, but goosegrass control was >80% in 2 of 3 experiments. Several other PRE herbicides applied as a single application controlled crabgrass effectively (>80%) throughout the summer. These herbicides were Barricade at 0.5 lb ai/A, Dimension (EC) formulation at 0.75 lb ai/A, and G formulation at 0.38 lb ai/A, pendimethalin at 2.0 lb ai/A, Regalstar at 3.0 lb ai/A, Surflan at 3.0 lb ai/A, and Team at 2.0 lb ai/A. However, goosegrass control was not acceptable from any of the above herbicides applied as a single application, except Ronstar (4.0 lb ai/A) and XL (3.0 lb ai/A). For the other herbicides, goosegrass control ranged from 18% with Team (2.0 lb ai/A), to 78% with Dimension EC (0.75 lb ai/A).
 

Table 2. Summary of PRE herbicides on crabgrass and goosegrass control in common bermudagrass in Georgia 1986 through 1993.
 
 
Percent weed control
Herbicidea                                                                                         Crabgrass                                                                                                  Goosegrass
Rate 
lbs/ai		 Average >80% in all  
experimentsb		 Average >80% in all  
experimentsb
Barricade 0.5 88 2 of 2 62 0 of 2
0.75 88 5 of 7 67 2 of 6
0.75 + 0.5* 94 1 of 1 78 1 of 1
Dimension EC 0.38 50 0 of 2 69 0 of 3
0.5 78 3 of 6 63 2 of 9
0.75 94 3 of 3 78 5 of 8
0.25 + 0.25* 99 2 of 2 77 3 of 4
Dimension G 
(MON 15152)		 0.25 58 1 of 1 58 0 of 3
0.38 90 2 of 2 62 0 of 5
0.5 87 1 of 1 74 4 of 6
0.75 92 2 of 2 91 3 of 4
0.25 + 0.25* 95 1 of 1 65 1 of 3
Goosegrass/ 
Crabgrass 
Control		 7.5 58 0 of 1 61 1 of 3
7.5 + 3.75* 76 0 of 1 66 1 of 3
3.75 + 3.75* 73 0 of 1 67 1 of 3
Pendimethalin 2.0 81 2 of 4 58 0 of 1
3.0 81 2 of 2 79 7 of 11
1.5 + 1.5* 76 1 of 2 79 2 of 3
Regalstar 3.0 85 1 of 1 64 1 of 4
3.0 + 1.5* 90 1 of 1 89 3 of 3
Ronstar 3.0 65 0 of 2 77 2 of 4
4.0 84 6 of 7 91 10 of 10
1.5 + 1.5* 98 1 of 1 95 1 of 1
Rout 1.5 -  - 79  1 of 2
3.0 - - 78 1 of 2
1.5 + 1.5* - - 89 2 of 2
Surflan 3.0 84 1 of 1 67 0 of 2
2.0 + 1.0* 81 1 of 1 61 0 of 1
1.5 + 1.5* 86 2 of 3 78 2 of 2
Team 2.0 80 1 of 1 18 0 of 2
3.0 90 1 of 1 0 0 of 1
1.0 + 2.0* 86 1 of 1 52 0 of 1
XL 2.0 84 1 of 1 51 0 of 3
3.0 80 1 of 1 80 2 of 3
2.0 + 1.0* 90 3 of 3 69 0 of 2
a All herbicides were applied March 1 ± 1 week. An asterisk indicates a second application was made 8 weeks after the initial application. All herbicide rates included in this paper are based on active ingredients per acre.

b The number of experiments was not the same for all herbicides and weed species. For example, Barricade was applied at 0.75 lb ai/A in 7 crabgrass experiments and 6 goosegrass experiments.
 

There was no advantage in crabgrass and goosegrass control when pendimethalin was applied in split applications (1.5 + 1.5 lb ai/A), when compared with a single full rate (3.0 lb ai/A). However, split applications of Dimension EC and Ronstar improved the control of crabgrass and goosegrass, when compared with the control from a single application at a rate equal to the split applications. Crabgrass control was also higher from a split application of Goosegrass/Crabgrass Control and goosegrass control was higher from a split application of Surflan than when compared to the single application.

Although 3.0 lb ai/A is considered to be the normal use rate for Ronstar, the single 4.0 lb ai/A rate controlled both crabgrass and goosegrass consistently higher in Georgia than that obtained from a single 3.0 lb ai/A rate (Table 2). However, the control from split (1.5 + 1.5 lb ai/A) application was equally as good as that obtained from the single 4.0 lb ai/A rate. In our studies, Ronstar has provided more consistent control of crabgrass and goosegrass over the years than any other PRE herbicide.

When a PRE herbicide is included in a weed control program, it should be selected on the basis of consistency in performance over years. Inconsistent weed control from year to year can be a problem for turfgrass managers. For example, Barricade applied as a single application at 0.75 lb ai/A provided poor goosegrass control in 1991 (45%) and 1993 (28%), but excellent goosegrass control in 1992 (92%).  In 1993, goosegrass control with Barricade was improved to 84% when 0.75 lb ai/A was followed by 0.5 lb ai/A 8 weeks after the initial applications. Therefore, split Barricade applications improved weed control consistency during the year when goosegrass control was poor from a single application.
 
 

Sequential PRE and POST herbicides. With increasing environmental and regulatory pressures, reducing the total amount of herbicides for weed control in turfgrass may become necessary. Since single applications of PRE herbicides at labeled rates often have not provided consistent crabgrass and goosegrass control, experiments were conducted from 1991 through 1993 in Georgia to determine if reduced rates of PRE herbicides applied in late winter, and followed by timely use of POST herbicides in late spring or early summer would provide a higher level of crabgrass and goosegrass control with reduced herbicide rates.
 

Crabgrass control. In several instances, crabgrass control was consistently higher from sequential applications of reduced rates of PRE and POST herbicides, when compared with a single PRE herbicide application. Ronstar applied at a one-third rate (1.0 lb ai/A) on February 24, and followed by MSMA at one-half rate (1.0 lb ai/A) on July 13, 1992, controlled 94% crabgrass. The control was higher than when Ronstar was applied alone at 1.0 lb ai/A (60%), or when MSMA was applied alone at 2.0 lb ai/A (20%). Crabgrass control was 95% when pendimethalin was applied at one-third rate (1.0 lb ai/A) in late February, and followed by MSMA at 2.0 lb ai/A in July. This was consistently higher than when pendimethalin was applied alone at the same rate (60%).

Crabgrass control ranged from 83 to 89% in 1993 when Barricade was applied at a single full rate (0.75 lb ai/A) in late February, or from sequential treatments of Barricade applied at one-third rate (0.25 lb ai/A), in late February and MSMA applied at one-half rate (1.0 lb ai/A) in June. Similar results were obtained from sequential Surflan and MSMA treatments, except the rate for MSMA had to be increased to 2.0 lb ai/A when previously treated with Surflan at one-third rate (0.67 lb ai/A). Not all sequential PRE and POST herbicides controlled a higher percentage of crabgrass than the use of PRE herbicides alone. For example, sequential applications of Dimension EC at 0.25 lb ai/A and MSMA at 2.0 lb ai/A in 1992 resulted in 95% control; however, Dimension applied alone at 0.25 lb ai/A provided a similar level of control.
 

Goosegrass control. A sequential application of Dimension EC at 0.38 lb ai/A made in late February followed by MSMA + Sencor at 2.0 + 0.12 lb ai/A in June controlled 90% goosegrass during 1991 and 1992. When either herbicide was applied alone at the same rate goosegrass control was < 51%.  Similar results occurred from sequential applications with pendimethalin followed by MSMA + Sencor in 1991, but not in 1992. In 1992, Ronstar at 1.0 lb ai/A followed by MSMA + Sencor at 2.0 + 0.12 lb ai/A controlled 85% goosegrass, but control decreased to <67% when either herbicide was applied alone. During 1991 and 1992, Illoxan applied at 1.0 lb ai/A in June effectively controlled goosegrass (>95%). There was no advantage from an application of a PRE herbicide if it was followed by POST application of Illoxan.

In 1993, sequential applications of Surflan at a one-third rate (1.0 lb ai/A) on February 22, and followed by Illoxan at a one-fourth rate (0.25 lb ai/A) on June 17, controlled 87% goosegrass at the late August evaluation. The control from this sequential program was higher than when Surflan was applied alone at the full 3.0 lb ai/A rate (56%). There was no improvement in goosegrass control during 1993 from sequential applications of Barricade followed by either Illoxan or MSMA + Sencor, when compared with each herbicide applied alone (Barricade at 0.38 lb ai/A = 82% control; Illoxan at 0.5 lb ai/A = 88% control; and MSMA + Sencor at 2.0 + 0.12 lb ai/A = 100% control).

These results indicate that it may be possible to improve weed control consistency from sequential applications of PRE and POST herbicides. Consistently high levels of control occurred when PRE herbicide rates were reduced by 50 to 75%, provided that the POST herbicides were applied to a small, emerged annual grass during late spring or early summer. In order to achieve consistently high levels of control, POST herbicides must be applied to small annual grasses. It should be noted that the results from programs that involve the use of sequential applications of reduced rates of PRE and POST herbicides were conducted on common bermudagrass, a highly adapted warm-season turfgrass in middle Georgia. Further research will be needed to determine if programs with reduced preemergence herbicide rates are applicable to other turfgrasses.
 

Tank-mixed PRE and POST Herbicides. In this study, Dimension EC and pendimethalin were the PRE herbicides and MSMA, MSMA + Sencor, and Illoxan were the POST herbicides. Tank-mixes of PRE and POST herbicides were applied one time after crabgrass and goosegrass emerged in mid-May during 1991 and 1992. For a tank-mix program to be effective, the POST herbicide must control the emerged weeds and the PRE herbicide must control later germinating weeds. Generally, tank-mixes of PRE and POST herbicides applied as a single application in mid-May did not control crabgrass and goosegrass as effectively as when PRE herbicides were applied at reduced rates in late February, and followed by a POST herbicide application in late spring or early summer. The difference in control between these programs was related to the effectiveness of POST herbicides at time of treatment. In the sequential PRE and POST programs, weeds were smaller at the time of the POST herbicide applications. However, in the tank-mixed experiment, crabgrass and goosegrass were quite large at the time the tank mix was applied. This was due to lack of early season weed control that would normally be provided by a PRE herbicide.
 

Weed Control. Crabgrass control ranged from 80 to 88% over a 2-year period when MSMA at 2.0 lb ai/A was tank-mixed with either Dimension EC at 0.25 lb ai/A, or pendimethalin at 1.5 lb ai/A. Crabgrass control was poor with MSMA alone (48%), but 72% control was obtained in plots treated only with Dimension EC at 0.5 lb ai/A.

None of the tank-mix PRE and POST herbicide combinations controlled goosegrass effectively (>80%) when applied as a single application in each of 2 experiments. However, the control was higher in one experiment from a tank-mix of Dimension EC (0.5 lb ai/A) plus Illoxan (0.5 lb ai/A) (80%) than with Dimension EC at 1.0 lb ai/A (5%) or Illoxan at 1.0 lb ai/A (36%) applied alone.  Goosegrass control from tank-mixes of Dimension EC with MSMA + Sencor, or pendimethalin with MSMA + Sencor, or Illoxan was unacceptable (control ranged from 38 to 60%).
 

Common bermudagrass injury. Most POST herbicides will cause some degree of temporary leaf discoloration within a few days after treatment. It is commonly known that MSMA + Sencor will cause moderate to severe injury for 7 to 14 days after treatment. In 1 of 3 experiments, a tank-mix of MSMA + Sencor at 2.0 + 0.12 lb ai/A with either pendimethalin at 1.5 lb ai/A or Dimension EC at 0.38 lb ai/A injured common bermudagrass more than MSMA + Sencor applied alone. The injury at 2 weeks after treatment was 2% when treated with MSMA + Sencor alone and 56 to 58% when MSMA + Sencor was tank-mixed with either PRE herbicide. Turfgrass in the tank-mix-treated plots required 6 to 7 weeks for recovery. In 2 of 3 experiments, common bermudagrass was not injured any higher from tank-mixes of PRE herbicides with MSMA + Sencor, when compared with MSMA + Sencor alone. The higher injury from tank-mixes of PRE herbicides and MSMA + Sencor occurred in only 1 of 3 experiments. The increased injury that was observed was probably related to high temperature and high humidity that occurred following the application.
 

Summary

Crabgrass and goosegrass can be effectively controlled in common bermudagrass or other warm-season grasses. However, it is important to select a herbicide or combination of herbicides that will provide consistent control throughout the summer of every year. The control results presented in Table 2 will be a good guide in selecting an effective PRE herbicide. However, the performance of PRE herbicides can vary across the warm-season turfgrass geographical range. Therefore, turfgrass managers should obtain and review the results of PRE herbicide experiments that are being conducted in their home state.

Excellent crabgrass and goosegrass control was also obtained from sequential applications of PRE and POST herbicides. In some instances, the PRE herbicide rate can be reduced by 50 to 75% if POST herbicides are applied to small crabgrass and goosegrass plants. However, this approach requires continued monitoring or scouting of the site to ensure that the POST herbicide is used at the correct stage of weed growth. In general, tank-mixes of reduced rates of PRE and POST herbicides have not been highly effective. Additional research will be necessary to refine this type of program.