Crabgrass and goosegrass are both C4 annual grassy weeds that are considered undesirable contaminants in sports turf because of the lack of compatibility with the desirable turfgrass, the inconsistent playing surface they provide, and the fact that they die with the first frost. The crabgrasses (Digitaria sp.) are the most common annual grass weeds in most sports fields. Crabgrass and goosegrass are often put together when talking about annual grassy weed control, but they are actually from different subfamilies, and they respond to control products in different ways (Table 1). In other words, an herbicide that controls crabgrass may not necessarily control goosegrass.
PHOTO COURTESY OF PAM SHERRATT.
Predicting crabgrass and goosegrass emergence
Monitoring soil temperature (most accurately on site) is one of the best ways to predict crabgrass and goosegrass germination. In the spring, soil temperatures within the landscape can vary considerably, with south and west facing exposures heating up most rapidly. Therefore, these areas should be the main target monitoring sites for temperature and degree-day determinations. It is also important to note that critical soil temperatures for annual grassy weed germination vary among species. Crabgrass, for example, germinates earlier in the spring (50 to 55 degrees) than goosegrass (60 to 65 degrees), and so preemergence herbicides will need to be applied earlier in the season for maximum efficacy.
Another useful method for predicting crabgrass emergence is phenology. Typically, phenology refers to using temperature as a basis for predictive methods because plant growth and development depend on temperature. Fidanza and Dernoeden (1996) provided useful information to predict crabgrass emergence as influenced by growing degree-days. By monitoring crabgrass emergence patterns in conjunction with calculating degree-days accumulation, turfgrass managers can develop models as a guide for targeting preemergence herbicide applications in their region. It is important to understand that biological processes don’t suddenly turn on when a specific degree-day total is reached, but they occur within a range of degree-days. For example, in Fidanza’s study at the University of Maryland, crabgrass first germinated within a degree-day range of 42 to 78 degrees. It is also important to recognize that crabgrass will continue to germinate and emerge for several weeks after initial crabgrass emergence is observed. Fidanza reported peak germination occurred at 150 to 225 growing degree-days and then for a considerable time thereafter. Therefore, preemergence herbicide applied after the initial germination period will still provide a significant benefit in preventing additional crabgrass emergence that occurs before the peak. However, crabgrass that has germinated most likely will mature and result in consumer complaints or dissatisfaction. This problem can be addressed by combining a preemergence with a postemergence herbicide if the target date is missed or by using a specialty herbicide like Dimension (Table 2) that has pre and postemergence activity.
Crabgrass and goosegrass control
The most effective control against the establishment of crabgrass and goosegrass in sports fields is the culture and maintenance of a dense, healthy stand of turfgrass. This preventative cultural approach is successful only if proper fertilization, mowing, irrigation, pest control and other practices are implemented in the culture of the turfgrass. Unfortunately, in many turfgrass areas, the desirable turfgrass fails to establish sufficient competition to prevent all the germinating crabgrass or goosegrass from establishing and the need for preemergence and/or postemergence herbicide control results.
Preemergence herbicide control: Most turfgrass managers continue to rely on the use of preemergence herbicides for assistance in the control of crabgrass and goosegrass (Table 2).
Figure 2: Model of dissipation of a preemergence herbicide following application. After the pesticide concentration degrades to below a certain minimum threshold, weeds will begin to germinate through the barrier. The duration of effective control is affected by: product choice, weather (warmer + wetter = faster degradation), amount of thatch (more = faster degradation) and rate of application (more, within label limits = longer control).
The timing of preemergence herbicide applications is the most critical component of an effective chemical control program. As a general rule, apply preemergence herbicides one to two weeks prior to crabgrass seed germination in the early spring. It is a sound agronomic strategy to apply the preemergence herbicide a little earlier in the spring than to delay the application and miss the target date window. Remember, improper timing is considered one of the major reasons for preemergence herbicide failures. Irrigation or rainfall with approximately .5 inch of water within several days after a preemergence herbicide application is critical for maximum effectiveness. Irrigation/rainfall serves to move the herbicide off the turf foliage and/or fertilizer granule and into the zone of influence (1 to 2 inches of the soil surface) where the herbicide becomes active. Remember, the crabgrass seed is germinating in the soil, and it is when the seedlings are emerging (growing toward the soil surface) that contact with the herbicide is made (Figure 1). Preemergence herbicides do not affect ungerminated (dormant) seed. The proper rate and uniform distribution of the preemergence herbicide is also critical to ensure good efficacy. A certain concentration of each preemergence herbicide must be maintained in the upper soil zone at the threshold level during the active germination period for season-long control (Figure 2). Improper distribution, improper rate and improper pattern overlap can allow for weak spots in the surface chemical barrier that may result in concentration of the herbicide dropping below the threshold level, allowing for short and long-term (residual) failures. These factors are also ranked high on the list of reasons for preemergence herbicide failure. Finally, agronomic programs that are consistently exhibiting short-term or long-term preemergence herbicide efficacy failures should carefully check rate and application accuracy, up the preemergence rate more to the upper end of the range, or utilize a follow-up sequential application at half rate. In Ohio State research for example, pendimethalin and benefin/trifluralin provide better and more consistent efficacy at 2 pounds of active ingredient per acre than at 1.5 pounds of active ingredient per acre.
Postemergence herbicide control: After annual grassy weeds have established in turfgrass areas, their removal usually necessitates the use of postemergence control products. The herbicides available for post- emergence annual grassy weed control in 2012 include: Fenoxaprop p-ethyl (Acclaim Extra), Dithiopyr (Dimension), Quinclorac (XLR8 and others) and Tenacity (mesotrione) (see Table 3).
Dimension is unique in that it exhibits both preemergence and postemergence activity on crabgrass. It is most efficacious on young crabgrass that is not beyond the one to two tiller maturity stage. Growth stoppage of crabgrass occurs within three to five days after application, but actual kill takes three to five weeks. It is a good specialty herbicide for use where the critical target date for crabgrass germination has been missed, a postemergence application is required, but seasonal preemergence control is still necessary.
Drive (quinclorac) recently went off patent, so there are several new product formulations containing quinclorac alone and in combination with other herbicides (see Table 3). Quinclorac continues to be a good to excellent postemergence herbicide for crabgrass control. It does appear to provide some variability in efficacy/control in the intermediary crabgrass maturity stage of approximately two to four tiller at times. BASF has released a new formulation of Drive called XLR8 that causes more rapid crabgrass discoloration following application, and in our research provided enhanced efficacy at the intermediate crabgrass maturity stage. The inclusion of sulfentrazone (Dismiss) with Drive (quinclorac) to enhance efficacy has resulted in mixed/variable results to date. In our 2009-2010 research, there were certain adjuvants/surfactants that were more effective than others in enhancing XLR8 efficacy (note: in 2011 postemergence crabgrass research there was little difference in herbicide efficacy among surfactants). Certainly, many surfactants have worked as well as the methylated seed oil (MSO) surfactants. Several products have recently become available that are combinations of quinclorac and preemergence herbicides (e.g. Cavalcade PQ, Sipcam/Advan). Quinclorac can be used for springtime weed control at establishment. Quinclorac is labeled for application any time before or after establishment of tall fescue. Research shows that quinclorac can safely be applied to either Kentucky bluegrass or perennial ryegrass as little as seven days after seedling emergence. The label suggests restricting the use on seedling turf for postemergence applications to 30 days after emergence. Recent research also indicates that not only can a combination of quinclorac and carfentrazone (Quicksilver) be applied to seven-day-old seedlings of either of these species, but that delaying the application to 14 or 21 days after emergence actually results in decreased weed control. This is likely because the herbicides become less effective as the weeds become more mature and capable of competing with the germinating turfgrass. There was some slight phytotoxicity with the seven-day application, but the turf outgrew this by day 14.
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Research undertaken at Ohio State University on a cool-season grass sward. More recently, many product formulations of quinclorac (Drive) have been available on the market, including Solitare, Q4, Onetime and Quincept, that contain sulfentrazone and/or broadleaf weed herbicides like 2,4-D, MCPP and dicamba (see labels for various combinations). Also, product combos containing quinclorac and preemergence herbicides are available.
Tenacity (mesotrione), a Syngenta product, has recently been labeled for postemergence control of a wide variety of annual and perennial grasses and broadleaf weeds (see label for susceptible weeds). Tenacity is a systemic pre and postemergence herbicide for the selective contact and residual control of weeds in turfgrasses. It is in the triketone herbicide family and functions as a pigment inhibitor. This results in a temporary whitening or bleaching appearance to the target weed foliage. In general, symptoms appear five to seven days after application and last for several weeks prior to weed necrosis. Postemergence treatments on established turf usually require a second application at a two to three-week interval, and it is not suggested that the interval be shortened, especially on perennial ryegrass. Apply to young actively growing weeds with an NIS-type surfactant for best results. Tenacity will effectively control crabgrass postemergence. The label states that two applications are required. In 2010-2011 research, Tenacity provided good to excellent postemergence crabgrass control with one application prior to tillering. A combination of Tenacity and a standard preemergence (i.e. Barricade) will allow early postemergence control of nontillered crabgrass and season-long preemergence control with one application. Additional research has been conducted at Ohio State University on Tenacity efficacy on tillered crabgrass at various herbicide rates, maturity stages, and with various herbicide combinations and adjuvants/surfactants. Some of these latter treatments resulted in enhanced postemergence activity of Tenacity on crabgrass and a potential for an effective single application. Two applications of Tenacity at the 5 ounces per acre rate at a two to three-week interval provide excellent postemergence crabgrass control. Tenacity (8 ounces per acre) in combination with triclopyr (at 16 ounces per acre) has shown promise in our 2010-2011 research for making a single application with good post efficacy (this combination will, of course, provide postemergence broadleaf weed control as well).
Dr. John Street has been a professor in turfgrass science at Ohio State University for the last 30 years. Dr. David S. Gardner is an associate professor in turfgrass science at Ohio State University. Pam Sherratt is a sports turf specialist at Ohio State University and served on the STMA board of directors from 2010-2011.