The basic concept of weed control in turfgrass ecosystems will never really 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 and other 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, which creates the added 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 (Digitaria sp.) prefer these voidology and ecological niche conditions. Weed encroachment on sports fields is much more likely due to voidology conditions more so than any turfgrass management system.
Sports field 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 sports fields, certainly the most common annual grassy weed across the country is crabgrass, as it observes no boundaries. It’s 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 under the appropriate conditions and it germinates and infests turf quickly, with tillering occurring within weeks of germination.
The key predictive criteria for crabgrass germination and infestation is soil temperature. Other predictive methods can be used to determine crabgrass germination, such as 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 to 55 degrees Fahrenheit for several consecutive nights (Table 1). It continues to germinate throughout the spring and early to midsummer period.
Monitoring soil temperatures at a 2-inch 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 soil thermometer, website weather database, or a weather monitoring technology, such as the Spectrum Technology WatchDog weather system.
Table 1. Soil temperatures for annual grass germination.
* Watschke, T.L. 1995. Turfgrass weeds and their management. In “Managing Turfgrass Pests.”
The Ohio Agricultural Research and Development Center (OARDC) at The Ohio State University monitors climatological data at 20 locations across the state of Ohio and provides weather data via the OARDC weather system website every five minutes 24/7. Sports field managers can go to the site (http://www.oardc.ohio-state.edu) and retrieve past and current nighttime soil temperatures at the 2 and 4-inch depths every night and day during the late winter/spring.
The second best attribute of this system is that it’s free. Check with a land-grant university or state turfgrass specialists near you to see if a similar weather-based system is available in your area.
The relatively new GDDTracker (http://www.GDDTracker.net), developed at Michigan State University, is another good way to monitor crabgrass germination timing based on growing degree days. It’s currently set up for monitoring in four Midwest states. In Ohio, GDDTracker use is being 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 preemergence herbicides as a standard control for crabgrass and other annual grasses; this is an “offensive” strategy. Preemergence herbicides provide a chemical barrier or blanket at the soil surface, intercepting the young seedling weed and preventing it from developing. Thus, proper application timing is a key factor in its effectiveness. Many agronomists consider improper application (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 prior to the onset of crabgrass seed germination. It’s 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, Ohio, crabgrass germinated at the typical time (April 20-25) in 2013, but in 2012 crabgrass surprised most turf managers by germinating four weeks earlier than normal (March 15-20). Sports field managers monitoring soil temperatures and or GDDs were well ahead on the offensive side of the game plan, while others lost the game in the first quarter – recovery is difficult when the initial germination window has long passed.
Preemergence herbicides that can be routinely used in the offensive plan of cool and warm-season turfgrass managers are listed in Table 2. The most effective preemergence herbicides are those ranked good (G) to excellent (E) for crabgrass control. The offensive strategy for sports field managers is confounded by the fact that most of these preemergence herbicides will also severly damage, kill and/or prevent the emergence of desirable turfgrasses.
There are 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 frame after a seeding. Please note that there are major differences in the tolerance/safety of these herbicides between cool and warm-season grasses. In addition, pay particular attention to the herbicide label regarding use on more sensitive species, like the fine fescues and hybrid bermudagrasses. 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.
The offensive strategy becomes a problem in attempting to control crabgrass and other weeds 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, such as outside the hash marks, beyond the 20 or 30-yard line, end zone areas and sideline areas on football fields.
Where seeding or overseeding is necessary, there are a few options. Herbicides that are considered to be safe for use at the time of seeding or at four weeks after seedling emergence are listed in Table 2 and include siduron (Tupersan), mesotrione (Tenacity) and Pylex (topramezone). Follow the label carefully.
When used properly, siduron will reduce crabgrass, goosegrass, foxtail and many summer annual broadleaf weeds by 70 to 80 percent. Mesotrione (Tenacity) and topramezone (Pylex) are excellent preemergence tools to use in seedings for reducing spring/summer weed pressure from crabgrass, goose- grass, sedges and summer annual broadleaf weeds by 90 percent or greater. These two products allow sports field 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.
Table 2. Preemergence Herbicides Efficacy and Turf Tolerance
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 pre and postemergence activity on crabgrass and many other weeds. Preemergence residual with these two herbicides, however, only lasts about 30 days. Where longer preemergence activity is required, such as with early spring or early summer seedings/overseedings, a follow-up application can be made at a 30-day interval or at least four weeks after seedling emergence. Where perennial ryegrass is a principle component of the sports field turf, the interval on repeat applications should not be done at less than 30 days.
Postemergence herbicide options, or defensive strategies, for controlling crabgrass in established turfgrasses include fenoxaprop p-ethyl (Acclaim Extra), a number of quinclorac products (Drive 75 DF and Drive XLR8), mesotrione (Tenacity), topramezone (Pylex), and some combination pre/post products including (prodiamine plus quinclorac (Cavalcade PQ), sulfentrazone plus prodiamine (Echelon) and dithiopyr (Dimension).
Dimension has early postemergence activity on crabgrass, so young (three to five leaf and prior to tillering) crabgrass is controlled, and a preemergence barrier is set in place for the remainder of the season. This is a great tool in the spring where crabgrass germination has occurred prior to the application of a preemergence herbicide. A similar defensive strategy is the basis for Cavalcade PQ and Echelon, where the quinclorac or sulfentrazone provides post activity on germinated crabgrass and the prodiamine provides a preemergence barrier for the remainder of the season.
Drive 75 DF and XLR8 are good defensive options where crabgrass has matured beyond the early post-crabgrass stage (tillered). These are foliar-absorbed herbicides that require a surfactant and need to be applied at no less than 0.75 pounds of active ingredient (AI) per acre for best results. XLR8 is an excellent choice in late summer when a rescue treatment for quick crabgrass knockdown is required prior to the beginning of the playing season (a defensive save-face strategy). XLR8 will discolor and reduce the visibility of crabgrass in the canopy within three to five days in conjunction with a good fertility program.
Finally, both Tenacity and Pylex have been evaluated for postemergence crabgrass control in Ohio State University research over the last several years. Both are “bleacher” herbicides. Two sequential applications of either will effectively control mature crabgrass on a consistent basis. The addition of triclopyr with Tenacity (8 ounces of product per acre) 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 combination with Tenacity or Py- lex also eliminates the bleaching or whitening symptom.
Dr. John Street has been a professor in turfgrass science at Ohio State University for the last 30 years. Pam Sherratt is a sports turf specialist at Ohio State University and served on the STMA board of directors from 2010-2011. Dave Gardner is an associate professor in the department of horticulture and crop science at The Ohio State University. He teaches courses in turfgrass management, ornamental plant identification and statistics. His research focuses on turfgrass physiology and weed management.