During the past couple of years, parts of the country, notably the Midwest, have experienced greater than normal amounts of weed pressure from crabgrass and other annual weeds.

Compared to postemergence herbicides, preemergence herbicides are the better option in areas with severe crabgrass pressure. The green and weed-free plot near the center of the photograph is where a preemergence herbicide was applied. PHOTOS AND IMAGES BY DAVID GARDNER, PH.D.

In some cases, the weather was unusually warm in the spring, which resulted in crabgrass germinating early before preemergence herbicides had been applied or properly watered in. In other cases, unusually warm and wet conditions during the summertime not only favored the development of crabgrass, but also hastened the breakdown of applied preemergence herbicides. Despite these challenges, preemergence herbicides remain the best choice for the control of annual grassy and broadleaf weeds in turfgrass areas

To deal with the problems listed above, a recent trend in the industry has been the introduction of products that offer effective control of crabgrass both pre and postemergence. Some of these are new chemistries and some of them are combinations of existing products.

Preemergence herbicides

Preemergence herbicides are effective for a finite period of time after application (weeks or a few months). Numerous factors influence the overall performance of a preemergence herbicide and how long it will remain effective following application. Among these are: timing of application; product choice; application rate and whether the application is split; climate and weather post-application; and the amount of thatch and organic matter in the turf/soil profile.

Climate and weather cannot be controlled. Warmer temperatures or higher rainfall will result in faster degradation or leaching that will cause the herbicide to lose effectiveness. The amount of thatch and organic matter can usually only be slowly altered over time. Higher amounts of thatch and organic matter will increase the rate of degradation of the product.

Unlike these two factors, however, you do have a choice of product selection, application timing, and the method in which it applied.

Table 1 lists the preemergence herbicides that are on the market to control annual weeds in turfgrass. Note that not all of these products are registered for use in all species of turfgrass. For example, atrazine, metalochlor, napropamide, oryzalin, pronamide and simazine are registered only for use on warm-season turfgrass. Consult the label to make sure that the product is safe to the turf species that you are managing.

In some cases there are special instructions for using the product on a particular turf species. Preemergence herbicides vary greatly in their duration of effectiveness. Herbicides, such as bensulide, pendimethalin, prodiamine and dithiopyr, can be effective for up to four months. Benefin, and benefin-containing products are older industry standards and provide effective and economic control of crabgrass. The tradeoff is shorter duration of residual activity and slightly less control. This might not be a bad thing, however, if you’re planning to overseed in the near future.

Some new options are available in cool-season turf that combine a preemergence herbicide with a postemergence herbicide. This is a plot at The Ohio State University on July 21, 2011. An application of Cavalcade PQ herbicide was made on May 28th to four-leaf crabgrass. The untreated control is to the right and the edge of the study is to the left.

Many recent product introductions have not been new chemistries, but rather generic versions of older herbicides. In fact, the only preemergence herbicide to be introduced recently is dimethinamid-p (Tower herbicide). Tower is labeled for preemergence control of goosegrass and certain other grassy weeds, and is safe for use on short-cut creeping bentgrass. The other goosegrass product on the list is oxadiazon. There are also products that combine oxadiazon with either benefin, bensulide or prodiamine.

There tends to be little difference in crabgrass control whether the same active ingredient is applied as a liquid or as a granular formulation. Both require light irrigation for activation. One note of caution, however, is that a preemergence herbicide formulated on an excessively large granule can be a problem, because this may result in sporadic distribution of the herbicide around the particles. The granule does not need to be “greens grade,” but it should not be excessively large either.

Proper application of the right preemergence herbicide can result in 95 percent control of crabgrass and other labeled annual weeds for the duration of the season in northern climates.

One concern with preemergence herbicide use is timing of application. In a typical year, the earliest germinating crabgrass may be killed by subsequent frosts. However, in order to be effective, the preemergence herbicide must be applied before the first crabgrass that germinates following the last frost. It must also be activated by either rainwater or irrigation after application. Typically, they are applied two to three weeks earlier, when an indicator plant, such as Forsythia in the northern United States or dogwoods in South, bloom.

Preemergence herbicides registered for use against annual weedy grasses inturfgrass. Products vary in terms of allowed uses, weeds controlled and turfgrasstolerances. Blue shaded products have both pre and postemergence activity. Alwaysconsult the label for specific recommendations prior to use.

If applied too early, these products may dissipate prior to the end of the season, which will result in some late germination of annual grasses

Applying the herbicide as a split-application (half of the product prior to germination followed by the other half about six weeks later) has proven an effective strategy. This depends, however, on what product is being applied and in what part of the country. Always consult the label for specific usage recomendations.

Research conducted at Ohio State shows there is no difference in crabgrass control whether a preemergence herbicide is applied as a single or split application. What’s more important is the amount that’s applied. Herbicide applied at a higher rate takes longer to break down below what is minimally required to prevent germination. Because of this, we recommend that managers apply the maximum label rate in a single application in Ohio and other midwestern states.

In contrast, split applications are almost the norm and are effective in the southern United States where the crabgrass season is longer. Unfortunately, the dividing line for where split applications become more effective isn’t well-understood.

Pre + postemergence herbicides

Perhaps the best option to combat the problem of achieving seasonlong control of crabgrass with a preemergence herbicide application is to use a product that allows for application later in the season. Three herbicides offer both pre and postemergence control of annual weeds. In addition, two products combine a preemergence herbicide with a postemergence herbicide.

Dithiopyr has been on the market for many years and was the first example of a product that offered both pre and postemergence control of crabgrass in cool-season turfgrass. Dithiopyr will provide excellent control of crabgrass preemergence for up to four months. Dithiopyr will also control emerged one to two-leaf crabgrass. This makes dithiopyr an excellent choice for application after it is too late to apply other preemergence herbicides, but also before crabgrass is easily treatable with a postemergence herbicide.

Ethofumesate has been on the market since the mid-1990s for the control of annual bluegrass. The product has both pre and postemergence activity. Research, however, has shown that when ethofumesate is applied to turfgrass, the residual activity is in some cases shortened significantly. Sequential applications according to label directions in the fall are more typical with this product.

Mesotrione (Tenacity herbicide) was introduced in 2007. Research conducted at Ohio State suggests that you may achieve 100 percent control of crabgrass for 160 days when Tenacity is used as a preemergence herbicide in combination with a product such as prodiamine. This combination was effective when applied either in early April (preemergence), early May (two-leaf-stage crabgrass) or early June (tillering crabgrass).

However, researchers in southern states have found Tenacity a bit less effective than this, and you will want to check the label or your state extension specialist for specific recommendations. Tenacity also has good postemergence activity on crabgrass. In trials at Ohio State University, control of crabgrass has been similar to herbicides that contain fenoxaprop or quinclorac.

Researchers have shown for years that combining a preemergence and postemergence herbicide will insure that late-germinating annual grasses will be controlled in addition to those already emerged. But until recently, there were no options to purchase a preemergence herbicide and a postemergence herbicide in one product.

Two such products have been released recently: Cavalcade PQ (which combines quinclorac and prodiamine) and Echelon (which combines prodiamine and sulfentrazone). Each of these is effective on crabgrass up to one to two-tiller stage, in addition to providing a preemergence barrier for the remainder of the season.

After a preemergence herbicide is applied, it will begin to dissipate. This figure shows a model of hypothetical degradation following an application made on April 15. Once the pesticide is degraded to below a certain minimum (in this example 20 percent of applied just prior to July 1), annual grasses will begin to germinate through the barrier. Many factors affect how long an application will be effective.

The timing of application is less critical, because either can be applied anytime from preemergence up until crabgrass is in the one to two-tiller stage, making it less likely that the preemergence barrier in the product will dissipate before season’s end.

It’s also important to note the longevity of control if any fall overseeding projects are planned, as the remaining herbicide residues could potentially interfere with August, or even September, overseeding activities.

In summary, there have been a few new chemistries introduced to the market for the control of annual grassy weeds in the past five years. However, most of our advances in annual weed control of late have come from understanding how to properly use existing products. This has included strategies such as split and/or sequential applications of preemergence herbicides, which are more effective in the southern United States, and the use of products that combine pre and postemergence herbicides into one formulated product.

Connect with Turf Magazine: Like us on Facebook, Follow us on Twitter