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Dallisgrass growth, identification and control

By |  June 12, 2015 0 Comments

Dallisgrass (Paspalum dilatatum Poir.) is a warm-season perennial grass native to South America that was introduced to the United States in the late 1800s. Tolerance to close mowing allows it to proliferate in golf course roughs and fairways. It is best adapted to the warm climates and high soil moisture commonly found it the mid-Atlantic and Southeast, but it can survive in more arid climates.

Dallisgrass is identified by a characteristic seedhead that often escapes the mower reel, a bunch-type growth habit, coarse texture and rapid vertical growth compared to bermudagrass (Cynodon spp.) (Figure 1). Unlike most bunch-type grasses, dallisgrass produces short rhizomes that increase the diameter of the plant and store carbohydrates. These reserve carbohydrates make controlling dallisgrass extremely difficult, even with systemic herbicide applications.
 

Previous Research

FIGURE 1: Dallisgrass growth habit in bermudagrass turf and characteristic drooping seedhead (inset).

FIGURE 1: Dallisgrass growth habit in bermudagrass turf and characteristic drooping seedhead (inset).

There are several herbicide options for dallisgrass control. Regardless of the herbicide combination, several sequential applications usually are required for complete control. Several applications of MSMA can be applied on 5- to 7-day intervals at 1-2 lbs./A during the summer. As of this writing, MSMA is still permitted for use on golf courses, but applications are restricted to spot treatments.

The addition of Sencor (metribuzin) to MSMA will improve control, but this combination causes short-term bermudagrass injury. Two or three sequential applications of the sulfonylurea herbicide Revolver (foramsulfuron) alone or tank-mixed with MSMA can improve control compared to MSMA alone, with little to no bermudagrass injury. Alternatively, pre-treating dallisgrass with MSMA two weeks before a Revolver application improves Revolver efficacy. Revolver is registered for use on bermudagrass and zoysiagrass but will cause severe injury or death to cool-season turfgrasses.

Herbicide application timing is important. When evaluating Revolver alone or in combination with MSMA, previous research found that early fall applications often provided better control than summer applications. Research evaluating fluazifop (Fusilade II or Ornamec) determined that spring applications provided better control than early summer applications. Fluazifop will cause severe bermudagrass injury, but it can be applied to zoysiagrass (Zoysia spp.) and tall fescue (Lolium arundinaceum Schreb.)

Growing degree-day (GDD) accumulation is commonly used to quantify the influence of air temperature on plant development throughout a growing season. In turfgrass, superintendents can use GDD accumulation to target applications of annual bluegrass seedhead suppressants as well as herbicides for pre-emergence crabgrass and post-emergence dandelion control. While GDD measures atmospheric heat accumulation above a certain threshold (average daily temperatures above 50 degrees F in experiments reported here), we also used cooling degree-days (CDD) to measure cumulative effects of atmospheric cooling below a certain threshold (average daily temperatures below 72 degrees F in experiments reported here), when dallisgrass growth begins to slow.
 

Objective

TABLE 1
Calendar dates corresponding to herbicide treatment application at GDD- and CDD-based application timings in 2010 and 2011.
Experiment 1 – GDD- and CDDa-based application timings
Application Timing 2010 2011
115 GDDb April 7 March 22
270 GDD April 22 April 14
580 GDD May 18 May 12
1200 GDD June 15 June 13
10 CDD Sept. 8 Sept. 9
Experiment 2 – CDD-based application timings
10 CDD Sept. 8 Sept. 9
40 CDD Sept. 30 Sept. 16
195 CDD Oct. 18 Oct. 13
aGDD = growing degree-day; CDD = cooling degree-day
bAll treatments were applied with non-ionic surfactant at 0.25% v/v

Given the seasonal variability in dallisgrass control reported with Revolver and fluazifop, our first objective was to evaluate the efficacy of single-herbicide applications made at various GDD- and CDD-based application timings. Our second objective was to investigate sequential application programs using newer herbicide chemistries registered for use in bermudagrass and zoysiagrass at application timings identified in the first study.
 

Materials and Methods

Experiments presented here were replicated and arranged in a randomized complete block design and conducted from 2010 to 2013 in Knoxville, Tenn. Treatments were applied using standard small-plot application equipment at 30 GPA.

Experiment 1: GDD- and CDD-based application timings

Single applications of Fusilade II at 6 fl. oz./A were applied at five GDD- or CDD-based application timings: 115, 270, 580, 1200 GDD or 10 CDD. Growing- and cooling-degree-day accumulation was calculated using equation 1 and equation 2, respectively.

GDD = [(Tmax- Tmin)/2] – Tbase
CDD = Tbase – [(Tmax- Tmin)/2]

In these equations, Tmax represents the daily maximum air temperature, Tmin represents the daily minimum air temperature and Tbase is the threshold temperature. GDD accumulation began on Jan. 1 and continued until July 1. CDD accumulation began on July 1 and continued to Dec. 31. Calendar dates for each application are listed in Table 1.

Fusilade II treatments were applied to a stand of Kentucky 31 and Coyote II tall fescue mowed at 3 inches and naturally infested with dallisgrass. Plots were split and subjected to three interseeding treatments: spring, fall or no interseeding at 7 lbs./A of pure live turf-type tall fescue seed.

Visible dallisgrass control was assessed on a zero (no control) to 100 percent (complete control) scale at 2, 4, 8 and 18 weeks after treatment. Grid counts quantitatively assessed dallisgrass control at 52 weeks after treatment. All comparisons were made to a non-treated control. Data were analyzed in SAS, and Fisher’s least significant difference test was used to separate means.

Experiment 2: Fall Application Timings

Experiment 1 evaluated only one fall application timing. To explore efficacy of several fall application timings, Fusilade II applications were made at 10, 40 or 200 CDD in 2011 and 2012. Calendar dates for each application are listed in Table 1. Applications were made and evaluated using the same methods described above, except that interseeding was not evaluated in this experiment.

TABLE 2
Dallisgrass control results with Tribute Total
Application Timing Dallisgrass Control (%)
10 CDD fba 4 wk later fb 210 GDDb 79a
10 CDD fb 4 wk later fb 300 GDD 84a
10 CDD fb 4 wk later fb 470 GDD 88a
10 CDD fb 4 wk later 56b
afb = followed by
bAll treatments were applied with ammonium sulfate (3 lb./A) and methylated seed oil (0.5% v/v). Applications made at 10 CDD were made on Sept. 11, 2012. Treatments applied at 210, 300, or 470 GDD were applied on April 17, May 1 and May 16 2013, respectively.
Dallisgrass control 41 weeks after initial treatment with sequential applications of Tribute Total (thiencarbazone + foramsulfuron + halosulfuron) at 3.2 fl. oz./A. Treatments were initiated at 10 CDD in the fall of 2012. A second application of Tribute Total was applied 4 weeks later. Certain treatments received a third application of Tribute Total in the spring of 2013 at 210, 300, or 470 GDD.

Experiment 3: Tribute Total Programs

Previous experiments evaluated single applications of the acetyl-CoA carboxylase (ACCase) inhibiting herbicide fluazifop. However, fluazifop is not an option for dallisgrass control in bermudagrass and can cause transient injury to zoysiagrass. Tribute Total was released in 2012 and is registered for use in bermudagrass and zoysiagrass. Tribute Total contains three acetolactate synthase (ALS) inhibiting herbicides (thiencarbazone + foramsulfuron + halosulfuron). ALS-inhibiting herbicides usually are less effective against dallisgrass than ACCase inhibitors like fluazifop, so sequential applications usually are required.

The objective of this experiment was to evaluate sequential application programs with Tribute Total at 3.2 fl. oz./A for dallisgrass control. Treatments were initiated at 10 CDD and followed by sequential applications four weeks later. The following spring, Tribute Total was applied at either 210, 300, 470 GDD or not at all. Dallisgrass control was evaluated at 41 weeks after initial treatment.
 

Results

Experiment 1: GDD- and CDD-based application timings

The effect of application timing on Fusilade II efficacy was most apparent at 52 weeks after treatment as applications at 270 GDD and 10 CDD provided greater than 75 percent control in 2010 (Figure 2). Other application timings in 2010 provided less than 50 percent control. In 2011, trends were similar, as applications made at 270 GDD and 10 CDD provided 83 percent and 93 percent control, respectively (data not presented). In both years, when treatments were evaluated four or eight weeks after treatment, differences were not as apparent as they were at 52 weeks after treatment, highlighting the importance of long-term evaluations of herbicidal efficacy for perennial weed control.

Tall fescue interseeding in the fall further increased dallisgrass control provided by applications made at 270 GDD. This emphasizes the value of management practices that increase competition from healthy turfgrass in combination with herbicides.

Experiment 2: Fall Application Timings

When evaluated 52 weeks after treatment, single Fusilade II applications at 10, 40 or 200 CDD provided greater than 80 percent dallisgrass control (data not presented). Control was not affected by application timing, indicating that unlike spring applications, there is greater flexibility for superintendents to time fall Fusilade II applications.

Experiment 3: Tribute Total Programs

Sequential application programs that involved two fall applications followed by one spring application provided the greatest dallisgrass control (79 to 88 percent) at 41 weeks after initial treatment; the timing of the spring application did not affect control. Dallisgrass was controlled less than 50 percent when a spring application was not made.

Superintendents initiating dallisgrass control programs with Tribute Total in the fall should plan for a spring application as well. Programs initiated in the fall offer greater flexibility for spring application timing; applications between 200 and 500 GDD are suggested.
 

Conclusions

Successful dallisgrass control still requires multiple herbicide applications. However, proper seasonal timing can reduce the total number of herbicide applications required for acceptable, long-term control. Using GDD and CDD to target applications may be preferable to calendar date. More recent research indicates that superintendents should begin calculating CDDs on Sept. 1 instead of July 1 to prevent abnormally cool temperatures from initiating an application.

The most successful programs begin in late summer or early fall (see specific application timings below). The objective is to make the application while dallisgrass is still actively growing, but approaching dormancy. This may interrupt processes involved in cold acclimation prior to winter dormancy, making dallisgrass more susceptible to cold injury.

In bermudagrass and zoysiagrass, programs with three applications of products containing the ALS-inhibitors foramsulfuron and/or thiencarbazone-methyl, such as Tribute Total at 3.2 fl. oz./A, have been successful. Making the first application at 10 CDDs and another four to six weeks later (early September to early October followed by mid-October to early November, depending on climate), followed by a spring application at 270 GDDs (late March to mid-April; highly dependent on spring temperatures) is suggested.

For dallisgrass control in tall fescue, a single application of Fusilade II at 6 fl. oz./A or Ornamec at 20 fl. oz./A between 10 and 200 CDDs (typically mid-September through mid-October in the transition zone) followed by another application in early spring at 270 GDDs (typically late March to mid-April; highly dependent on spring temperatures) is suggested. Spring applications are more likely to cause injury than fall applications; apply as a spot treatment to reduce injury risk. Avoid summer applications.

Some product labels have instructions for spot treatments that recommend the herbicide solution be applied until the weeds are wet. Because this “spray-to-wet” volume is higher than typical broadcast application volumes of 40-80 GPA and the herbicide solution concentration is similar for both application techniques, the herbicide rate can be higher for spot treatments. While this may improve control, it also increases the potential for desirable turfgrass injury.

For all products, defer to the label for more information on application rates and adjuvants. Apply Tribute Total to bermudagrass and zoysiagrass only, as it will cause severe injury or death to cool-season species. Don’t apply Fluazifop II to bermudagrass as severe injury will occur.
 

Acknowledgments

This research was conducted while the author was a graduate research assistant at the University of Tennessee. The author would like to thank Greg Breeden, Daniel Farnsworth and Jim Brosnan, Ph.D., for their contributions to the research presented here. Some of the information presented is based on the following article:

Elmore, M.T., J.T. Brosnan, D.A. Kopsell, T.C. Mueller, B.J. Horvath and G.K. Breeden. 2013. Seasonal application timings affect dallisgrass (Paspalum dilatatum) control in tall fescue. Weed Technology 27:557-564.

Mention of herbicide trade names is only for the purpose of providing information and does not imply recommendation or endorsement. The label always takes precedence over the recommendations found in this article.
 

References

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McCarty, B. and F. Yelverton. 2005. Dealing with dallisgrass. Golf Course Management. 73:782-787. www2.gcsaa.org/gcm/2005/july05/pdfs/82-88_mccarty.pdf

Henry, G. M., F.H. Yelverton, and M.G. Burton. 2007. Dallisgrass (Paspalum dilatatum) Control with foramsulfuron in bermudagrass turf. Weed Technol. 21:759-762.

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Photo: Matthew Elmore

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