Challenges for crabgrass and other annual grassy weed control: the preemergence/herbicide dilemma

By Dr. John R. Street, Pamela J. Sherratt, and Dr. David S. Gardner

The basic concept of weed control in turfgrass ecosystems will really never change. The paramount principle against the establishment of weeds in any turfgrass system is the culture and maintenance of a healthy, dense, competitive stand of turf. A preventative cultural approach is successful only on sports fields if proper fertilization, mowing, irrigation, pest control, core cultivation, overseeding, etc. practices are implemented in an integrated management program. Unfortunately, on sports fields an additional challenge to maintaining a dense stand of turfgrass is foot traffic (in many cases severe) that creates the additional stress of both direct physical wear and tearing/shearing/divoting that weakens the turfgrass and opens up the surface to an increasing opportunity for weed encroachment and for the germination of annual grassy weeds. Basically, annual grassy weed control in any turfgrass system is what I refer to as the “science of voidology” and “ecological niches.” Weed seed present in the soil is lying dormant just waiting for an opportunity under the right environmental and cultural conditions to invade a weakened turf with open voids. Annual grassy weeds like crabgrass prefer these voidology and ecological niche conditions. Weed encroachment on sports fields is much more likely due to “voidology conditions” and more so than any other turfgrass management system.

Sports turf managers therefore require the ultimate expertise in the art and science of turf management as the odds in many cases are against you.

Although there are many potential problematic weeds that can invade athletic fields certainly the most common annual grassy weed across the country is crabgrass as it observes no boundaries. It is a C4 turfgrass in the same physiological class as the warm-season grasses and thus thrives under moist and warm/hot environmental conditions (ecological niche). Give crabgrass an opening/opportunity under the appropriate conditions and it germinates and infests turf quickly with tillering occurring within weeks of germination.

In our opinion the key predictive criteria for crabgrass germination and infestation is soil temperature. Other predictive methods can be used to determine crabgrass germination like growing degree days (GDDs), phenological events like forsythia bloom drop, calendar dates, weather consultant services, historical experience, etc. but soil temperature monitoring is by far the most reliable qualitative method. Crabgrass typically initially germinates in late winter/early spring when nighttime soil temperatures reach 50-55F for several consecutive nights. It continues to germinate throughout the spring and early to mid summer period.

Monitoring soil temperatures at a 2” depth is one of the best ways to predict when crabgrass will initially germinate. The temperatures listed in Table 1 refer to the low nighttime soil temperatures over a period of several consecutive nights. Any method that assists in monitoring soil temperature within your region can be used like a simple soil thermometer or a website weather database; a weather monitoring technology like the Spectrum Technology Watch Dog weather system also can provide a wide variety of other climatological data in addition to soil temperature, local weather consultants, etc. Our Ohio Agricultural Research and Development Center at the Ohio State monitors climatological data at 20 different locations across the state and provides weather data via the OARDC weather system web-site every 5 minutes 24 hours a day 7 days a week. Sports field managers can log onto the site anytime and retrieve past and current nighttime soil temperatures at the 2” and 4” depths every night and day during the late winter/spring. As soil temperatures begin to move into the 50-55F range for several consecutive nights, just like a clock, crabgrass will initiate germination. The OARDC weather system is a great soil temperature monitoring tool. Check with your closest land grant university or state turfgrass specialists to see if such a weather-based system is available to you in your state. The second best attribute of this system is that it is FREE. I think most of us like valuable things that are free.

The relatively new GDD tracker program/model ( developed at Michigan State University is another good way to assist you in monitoring crabgrass germination timing based on growing degree days. It is set up for monitoring in four mid west states at the present time. In Ohio, GDD tracker program use is being financially sponsored by John Deere Landscapes and the Ohio Turfgrass Foundation. You may want to consider working with sponsoring agencies, associations and foundations in your state to bring this program on board.

Most turfgrass managers continue to rely on the use of preemergence herbicides as a standard preventative control for crabgrass and other annual grasses (an “offensive” strategy). Preemergence herbicides provide a chemical barrier or blanket at the soil surface intercepting the young seedling weed and preventing it from emerging/developing. Thus, proper application timing is a key factor in its effectiveness. Many agronomists consider improper (i.e. missing the initial window of crabgrass germination) as the primary reason for preemergence herbicide failure. Emerged crabgrass plants are not controlled by preemergence herbicides, except for Dimension, which exhibits early post and pre activity. The basic “offensive” principle is that the preemergence herbicide be applied before the onset of crabgrass seed germination. It is fairly simple if you engage the “offensive” principle and monitor soil temperatures and/or GDDs. Crabgrass can germinate at significantly different times from year to year. In Columbus (a great example) crabgrass germinated at its typical time of April 20-25 in 2013, but in 2012 crabgrass surprised most turf managers by germinating March 15-20 (4 weeks earlier than normal). Sports turf managers monitoring for soil temperatures and/or GDDs where well ahead on the “offensive” side of the game plan whereas many other managers lost the game in the first quarter (March/April 2012). Really can’t recover when the initial germination window has long passed (like a wide receiver 30 yards down the field and wide open!).

Preemergence herbicides, or the “offensive” strategy for sports turf managers is confounded by the fact that most of these preemergence herbicides will severally damage, kill and/or prevent the emergence of desirable turfgrasses as well as weeds. Sports turf managers have several choices of preemergence herbicides based on species tolerance and efficacy where NO seeding or overseeding programs are planned. Most of the herbicides listed for standard preemergence use cannot be used on turfgrass areas at the time of seeding or within a certain time interval after a preemergence application.

Please note that there are major differences in the tolerance/safety of these herbicides between cool- and warm-season grasses. Pay particular attention to the herbicide label regarding use on more sensitive species like the fine fescues and hybrid bermudagrasses, as well. Never use a preemergence or postemergence herbicide for crabgrass or other annual grassy weed control before fully reading and understanding the use requirements and restrictions on the label. A good example would be Dimension (dithiopyr) that cannot be safely applied at the time of seeding or until the desirable turfgrass has been mowed at least 2-3 times. Also, there is a suggested waiting period or time interval after a Dimension application of 6 to 16 weeks before seeding/overseeding depending on application rate.

Thus the standard “offensive” strategy becomes a problem in attempting to control crabgrass and other weed species during turfgrass establishment in seeding or overseeding operations. One approach is “site specific” management by only applying a preemergence herbicide on sports field areas that DO NOT require seeding or overseeding like outside the hash marks, beyond the 20 or 30 yard lines, end zone areas, and side line areas on football fields.

Where seeding or overseeding is necessary, there are a few options. The list is restricted to only a few but includes siduron (Tupersan), mesotrione (Tenacity) and Pylex (topramazone). Follow the label carefully. When used properly, siduron will reduce crabgrass, goosegrass, foxtail and many summer annual broadleaf weeds by 70-80%. Mesotrione (Tenacity) and Pylex (topramezone) are excellent preemergence tools to use in seedings for reducing spring/summer weed pressure from crabgrass, goosegrass, sedges, and summer annual broadleaf weeds by 90% or greater. These two latter products allow sports turf managers to be more successful with spring and summer seedings by effectively reducing weed competition and actually “widening the window” for successful seeding/overseeding into the summer.

Both Tenacity and Pylex are in the same chemical family and inhibit carotenoid biosynthesis with chlorophyll destruction resulting in all susceptible weeds turning white (bleaching symptom). These two herbicides have both pre- and postemergence activity on crabgrass and many other weeds. Preemergence residual with both these herbicides, however, lasts only about 30 days and, therefore, will not provide season-long preemergence activity. Where longer preemergence activity/residual is required, like in early spring or early summer seedings/overseedings, a sequential or follow-up application can be made at a 30-day interval or at least 4 weeks after seedling emergence. Where perennial ryegrass is a principle component of the sports field turf, it is NOT suggested that the interval on repeat applications be shortened to less than 30 days. Reducing crabgrass and other annual weed competition during seeding operations with these latter two herbicides should greatly enhance your success at spring and summer seedings/overseedings. They are a definite benefit in establishment programs should be included in every sports turf manager’s weed control tool box.

Defensive strategies

Postemergence herbicide options or “defensive” strategies for controlling crabgrass in established turfgrasses include Acclaim Extra (fenoxaprop p-ethyl), a number of quinclorac (Drive DF) products and XLR8, Tenacity (mesotrione), Pylex (topramezone) and a few combination pre/post products including Calvalcade PQ (combo of prodiamine plus quinclorac), Echelon (combo of sulfentrazone plus prodiamine), and Dimension (dithiopyr). Dimension has early postemergence activity on crabgrass so young (3-5 leaf and before tillering) crabgrass is controlled and a preemergence barrier is set in place for the remainder of the season. This is a great herbicide tool in the spring where crabgrass germination has occurred before the application of a preemergence herbicide. A similar “defensive” strategy is the basis for the combination products Calvalcade PQ and Echelon where the quinclorac or sulfentrazone provides post activity on already germinated crabgrass and the prodiamine provides a preemergence barrier for the remainder of the season.

Drive DF products and XLR8 are good “defensive” options where crabgrass has matured beyond the early post crabgrass stage (tillered). It is a foliar absorbed post herbicide that requires a surfactant and needs to be applied at no less than 0.75 lbs. ai/A for best results. XLR8 would be an excellent choice for sports turf managers in late summer where a rescue treatment for quick crabgrass knockdown is required before the beginning of the playing season (a “defensive” save face strategy). XLR8 will discolor and reduce the visibility of crabgrass in the canopy within 3-5 days in conjunction with a good fertility program.

Finally, both Tenacity and Pylex have been evaluated for postemergence crabgrass control in Ohio State research over the past several years. Both again are so called “bleacher” herbicides. Two sequential applications of both herbicides will effectively control mature crabgrass on a consistent basis. The addition of triclopyr with Tenacity (8 oz product/A) increases the efficacy of Tenacity to where a single application of the combo provides good to excellent post crabgrass control. Pylex alone has shown good to excellent postemergence activity on tillered crabgrass in a single application. The inclusion of triclopyr with Pylex also enhances its activity on tillered crabgrass and many other weeds. Pylex is a stellar product for goosegrass control. The inclusion of triclopyr in combos with Tenacity or Pylex also eliminates the bleaching or whitening symptom.

Dr. John R. Street is an extension/research associate professor; Dr. David Gardner is a research/teaching associate professor; and Pamela Sherratt is a senior extension sports turf specialist, all with the Department of Horticulture and Crop Science at The Ohio State University, Columbus.