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In many parts of the country, control of crabgrass and other annual grassy weeds was more difficult this past year than in typical years. Parts of the Midwest experienced warmer than normal temperatures in the spring which resulted in crabgrass germinating about three weeks earlier than normal.
Thus, anywhere pre-emergent herbicides had not been applied about three weeks earlier than normal saw a lot of crabgrass this year.
Pre-emergent herbicides continue to be our main line of defense against annual grassy weeds.
However, a recent trend in the industry is toward 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.
A review of the various pre and postemergent crabgrass controls, and their advantages and disadvantages follows.
Preemergent herbicides
There are several pre-emergent herbicides labeled for the control of annual grasses, such as crabgrass and goosegrass, in both cool and warm season turfgrass (Table 1). 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. You should consult the label to make sure that the product is safe to the turf species that you are managing.
Benefin and benefin-containing products are older industry standards and provide effective and economic control of crabgrass. DCPA went off the market in the late 1990s but is once again available. Benefin and DCPA have the potential to breakdown more rapidly in thatchy turfgrass. Herbicides, such as bensulide, pendimethalin, prodiamine and dithiopyr, can be effective for up to four months. Many of these products have been around in the industry for a very long time.
A lot of recent product introductions have been not new chemistries, but rather generic versions of older herbicides. In fact, the only recent preemergent herbicide introduction for use in turfgrass 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.
Proper application of the right pre-emergent 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 preemergent herbicide use is timing of application. The product must be applied ideally two weeks prior to germination.
However, if applied too early, these products will dissipate in the soil 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 the product prior to germination followed by the other half about six weeks later) has proven an effective strategy.
Research conducted at The Ohio State University shows that there is no difference in crabgrass control whether a pre-emergent herbicide is applied as a single or split application. The amount applied is more important.
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 LCOs 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 very effective in the southern U.S. (where the crabgrass season is longer).
Unfortunately, the dividing line for where split applications become more effective is not yet well understood.
Postemergent Herbicides
Fewer products are available for post-emergent control of crabgrass and goosegrass.
The organic arsenicals, DSMA and MSMA, do not appear in the Table 1 because their uses in residential turf were phased out after 2009 and all other uses have been severely restricted and are to be phased out by 2012.
Fenoxaprop has been on the market since the mid 1990s and gives good control of young crabgrass. However, control with tillering crabgrass is not as consistent.
Quinclorac can give better control of young crabgrass and also late-tillering crabgrass (>6 tillers).
However, control of actively tillering crabgrass has been inconsistent. We typically see around 90 percent control of 1-4 leaf crabgrass (but then more crabgrass germinates), around 30 percent control of early tillering crabgrass, and then about 70 percent control of crabgrass that has more than six tillers. Also, control of goosegrass with quinclorac is not very good.
Post-emergent options have improved over time and can be effective when timed correctly.
However, as illustrated in Table 1, one should not underestimate the importance of the use of a preemergence herbicide in those areas with severe annual grassy weed pressure.
Combination Products. Dithiopyr has been on the market for many years and was the first example of a product that offered both pre and postemergent control of crabgrass in cool season turfgrass. Most research shows that dithiopyr will provide excellent control of crabgrass preemergence for up to four months. Dithiopyr will also control emerged 1-2 leaf crabgrass. This makes dithiopyr an excellent choice for application after it is too late to apply a preemergence herbicide, but also before crabgrass is easily treatable with a postemergence herbicide.
Mesotrione (Tenacity herbicide) inhibits carotenoid pigment synthesis and results in a bleaching of the affected tissue which causes the target weed to turn white. Research conducted at Ohio State suggests that you may achieve 100 percent control of crabgrass for 160 days when this product is used as a pre-emergent herbicide in combination with a product such as prodiamine.
However, researchers in southern states have found it a bit less effective than this and you will want to check the label for specifics. Researchers have shown for years that combining a pre- and post-emergent herbicide will ensure that late germinating annual grasses will be controlled in addition to those already emerged. Recently, two such products have been released recently, Cavalcade PQ (which combines quinclorac and prodiamine) and Echelon (which combines prodiamine and sulfentrazone).
Each of these products has been effective on crabgrass up to 1-2 tiller stage, in addition to providing a preemergence barrier for the remainder of the season. In addition, the timing of application is less critical, because they can be applied anytime from preemergence up until crabgrass is 1-2 tiller stage. In a year such as 2010, this product was effective whether it was applied on April 15 or on June 1.
The other advantage is that since they can be applied until crabgrass reaches 1-2 tiller stage, it is less likely that the preemergent barrier in the product will dissipate before season’s end, thus reducing the need for split applications or the use of a postemergent herbicide mid-summer to deal with any breakthroughs.
The author is an associate professor of turfgrass science at Ohio State University.
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