Putting the brakes on bermudagrass
Tall fescue is a perennial cool-season grass widely used as turf in much of the United States, due in part to its relatively good heat tolerance. Bermudagrass is a warm-season perennial grass whose range substantially overlaps that of tall fescue. Alone, each is a valuable turfgrass. Inevitably, however, bermudagrass invades many tall fescue lawns, and with its spreading habit, it gradually crowds out the fescue creating an unsightly contrast between turf types. This is among the nastiest of turf weed problems.
Growing where the other one isn't
Tall fescue thrives in arid climates when irrigated. Due to its extensive root system, tall fescue also has excellent drought tolerance, as well as good disease and insect resistance. Its adaptation to shade is only fair, but it will survive infertile, saline, alkaline, wet and dry soil conditions. When improperly irrigated or fertilized, however, tall fescue often becomes susceptible to leaf spot and brown patch diseases, as well as white grub damage. Tall fescue stands thin following such damage, allowing opportunistic weeds such as crabgrass and bermudagrass to become established.
Another factor contributing to invasion by bermudagrass is that many housing subdivisions are established on old bermudagrass pastures and fields. This creates an inherent source of bermudagrass contamination in fescue lawns.
Tall fescue's optimum temperature range is 65 to 75°F, while bermudagrass is best adapted to temperatures from 80 to 95°F. Being a cool-season grass, tall fescue's growth during the hot summer months slows considerably. Meanwhile, the warm-season grasses — bermudagrass and crabgrass — grow aggressively during summer. Thus, they are able to compete strongly with tall fescue during this time.
A combination of cultural and chemical strategies will help tip the balance back in favor of tall fescue. However, you should recognize that there is no “silver bullet” for the problem. It takes persistent application of the strategies discussed below.
Several cultural practices encourage tall fescue growth over bermudagrass or crabgrass if you perform them in certain ways or at certain times. The primary ones involve seeding rates, mowing height and proper fertilization and irrigation practices.
- Seeding rates
Sufficient tall fescue seed is necessary when establishing a turf area. However, with excessive seeding rates, seedlings may become spindly, weak and susceptible to Pythium and other diseases. This in turn leads to gaps in the turf canopy, which can lead to invasion by grassy weeds. Most turf specialists recommend a seeding rate of 5 to 8 pounds per 1,000 square feet. It's best to stay within that range. Additionally, a fall seeding date is much preferred to a spring one because the fescue is more likely to successfully become established with a fall seeding.
- Mowing height
The new turf-type tall fescue cultivars grow better when mowed at 2 to 2.5 inches, but may need higher mowing during dry periods in the summer or under heavy shade. The old cultivar Kentucky 31 should be mowed at 3 inches. Use a rotary mower with sharp blades and mow often enough so that you don't remove more than one-third of the leaf height in a single mowing. So, for instance, if you're mowing your tall fescue at 2 inches, mow it before it exceeds 3 inches.
Keeping tall fescue in this height range encourages maximum root growth and provides enough canopy to help shade, and thus discourage, bermudagrass growth. Scalping tall fescue, by contrast, thins the turf, exposes the soil and is almost certain to encourage weed invasion.
Tall fescue will tolerate low fertility, but 2 to 4 pounds of nitrogen per 1,000 square feet per year is the generally recommended range. Typically, turf managers apply a complete fertilizer at the rate of 0.5 to 1 pound of nitrogen two or three times in the fall and winter (September to February) at six- to eight-week intervals. In areas with colder winters (where the ground freezes), avoid fertilization during the coldest months (roughly, December and January). Also, avoid fertilization during the late spring and summer months because this only adds to heat, disease and drought stress.
Irrigation is required to replace water lost via evapotranspiration. For water conservation, irrigate to prevent drought stress on an as-needed basis. Irrigate when 30 to 50 percent of the turf begins to wilt, turns blue-gray in color or recovery from foot or tire tracks is slow. Apply enough water to rewet the soil root zone and then wait until the turf shows signs of drought (e.g., wilting) again before the next irrigation (usually every 3 to 4 days during April to September, depending on soil type and maintenance practices).
For most soils, no more than 0.75 inch of water is necessary for each irrigation to rewet the top 4 to 8 inches of the root zone. This is the equivalent to 465 gallons per 1,000 square feet. Thus, each time you irrigate, you should do so long enough to wet soil to this depth. The length of the irrigation period should stay constant year round; only the frequency between irrigations should change. Therefore, you shouldn't set irrigation programs by automatic timers to operate on a daily schedule. They need only to operate after the turf begins to show signs of drought and then should be programmed to apply 0.75 inch of water. Light, frequent (daily) irrigation is a sure way of eventually weakening tall fescue and encouraging brown patch disease, bermudagrass, crabgrass and other weeds.Herbicidal control
Selective control of grassy weeds in tall fescue is difficult to completely achieve with post-emergence herbicides because of unacceptable or unpredictable tall fescue phytotoxicity. To ensure safety, rates must be reduced, increasing the number of applications required and lowering the degree of bermudagrass control.
Only two currently available herbicides provide suppression of bermudagrass with acceptable phytotoxicity of fescue. One is Aventis' Acclaim Extra 0.57 EC (fenoxaprop-p-ethyl). This material is applied at 1.5 pints per acre every 3 weeks, starting after bermudagrass greenup in spring. During summer stress, you may need to lengthen application intervals or use lighter rates.
Young, actively growing weeds are easiest to control with Acclaim. Apply it in late spring or early summer to actively growing bermudagrass, but do not apply to moisture- or heat-stressed turf or weeds. Repeat in 3 weeks for complete control. Control may be reduced if you apply it within 14 days after a broadleaf herbicide. Tall fescue seedlings should be at least 4 weeks old before treatment. Do not mow for 24 hours after application, and do not tank-mix with phenoxy herbicides.
The other post-emergence herbicide for selective bermudagrass control in tall fescue is fluazifop-p-butyl (PBI/Gordon's Ornamec Over-The-Top and Syngenta's Fusilade T&O II 2EC). We conducted our research with Fusilade, but Ornamec uses the same active ingredient and is similarly labeled for this use, although product concentration, and therefore product application rates, are different.
Add a nonionic surfactant at 0.25 percent by volume when using this chemical. Make the first application in spring after bermudagrass has greened up, and a second application in early fall. Minor, short-term turf phytotoxicity may occur, especially if you apply the chemical during hot, dry weather. However, as Tables 1 and 2 indicate, little, if any, long-term phytotoxicity is normally exhibited with either herbicide.
- Persistence is critical
No selective herbicide will consistently control bermudagrass with just one application or after only one year's use. For example, in research studies, four to six repeat applications of Acclaim at 3- to 4-week intervals were needed over 2 years for satisfactory (about 85 to 95 percent) control. This program is most effective when integrated with an agronomic scheme promoting tall fescue growth over the bermudagrass.
Table 1 (page 26) demonstrates the need for persistence with Fusilade. One application at 6 ounces per acre provided 30 percent bermudagrass control; two applications provided 79 percent control; three applications yielded 83 percent control. I've seen similar trends with single vs. sequential Acclaim applications.
Table 2 (above) demonstrates the need for a multi-year approach to control. Two applications of Fusilade, again at 6 ounces per application, provided only 3 percent bermudagrass control with one yearly application, but provided 90 percent control with two consecutive yearly applications.
- Tank-mixing herbicides
In the never-ending search for the magical combination of products to provide selective bermudagrass control, turf managers have tried various tank-mix combinations.
Two herbicides used in combinations with Acclaim that we've tested are Aventis' Prograss 1.5EC (ethofumesate) and Dow AgroScience's Turflon Ester (triclopyr). Five monthly applications of Acclaim Extra 0.57EC (32 to 42 ounces per acre) plus Prograss 1.5EC at 1 gallon per acre tank-mixed and applied during a single year have provided between 70 and 96 percent common bermudagrass control in several studies. Tank-mix application of Acclaim Extra 0.57EC at 47 to 94 ounces per acre plus Turflon Ester 4L at 18 to 36 ounces per acre have provided 88 to 97 percent control with four repeat applications during one year. Single application provided less than 30 percent control.
Table 1 lists a study in North Carolina comparing Fusilade and Acclaim alone compared to Acclaim with Turflon or Prograss. Three applications of Acclaim Extra 0.57EC at 28 ounces per acre provided 75 percent bermudagrass control by the end of the growing season. When tank-mixed with Prograss 1.5EC at 128 ounces per acre, control increased to 84 percent, and to 90 percent when tank-mixed with Turflon Ester 4EC at 32 ounces per acre. Table 2 again demonstrates that repeat application of any material is necessary and that Acclaim + Turflon tank-mixes provide slightly better control than Acclaim + Prograss mixes. In addition, these herbicides also provide excellent post-emergence control of crabgrass, especially with repeat applications.
Bermudagrass is a common weed problem in tall fescue in much of the United States. A combination of cultural practices and herbicide applications are needed for satisfactory control. It's unlikely you'll ever completely eradicate bermudagrass from a sward, but with persistence, you can achieve acceptable results without have to perform a complete renovation.
Dr. L. Bert McCarty is professor of turgrass science in the Dept. of Horticulture at Clemson University (Clemson, S.C.).
Want to use this article? Click here for options!
© 2015 Penton Media Inc.