crabgrass control Pamela Sherratt

Pamela Sherratt on Crabgrass Control Strategies

Q: Can you recommend a crabgrass control program for athletic fields? We are wary of using herbicides as we overseed regularly.

A: Crabgrass and other weeds can be a serious challenge for the sports field manager. They can interfere with playability and look unsightly. Warm-season annuals such as crabgrass also die at the end of the growing season, leaving behind large patches of bare dirt. Those bare patches are infested with weeds the following spring, and so the cycle continues.

Ultimately, a crabgrass management program should be constructed based on your answer to the following question: Is my stand of turfgrass adequate, or will I require spring overseeding? If your turfgrass stand is adequate and you do not plan to overseed this spring, then a good plan is to apply a preemergence herbicide containing pendimethalin, prodiamine or dithiopyr. However, timing is important. To optimize control of crabgrass, pendimethalin and prodiamine should be applied around April 15 in the Midwest, or when the shrub Forsythia is in bloom. Dithiopyr when sprayed has some post activity so long as the crabgrass is no more than 1-2 leaf so it can be applied in early May. The advantage of applying later is that it increases the chance that you won’t have late-season weed breakthrough. On the other hand, if you are also trying to control knotweed preemergence, then remember that knotweed germinates very early and so a preemergence herbicide would need to go out sometime in mid-March, and you may need some postemergence control for late germinating weeds.

Most of the herbicides labelled for turfgrass use should not be used during seeding or overseeding. This is because the postemergence broadleaf herbicides may injure seedlings until they are mature enough to have been mowed three times and the preemergence herbicides may prevent germination for up to 16 weeks after application. On areas that you plan to seed or overseed, the recommendation is to use mesotrione for weed control. To maximize your chance of success with springtime seeding, you should apply the seed, then make an application of mesotrione according to label directions, then apply a mulch and begin irrigation.

Timing of this seeding operation can have a large impact on success. Even with mesotrione, turfgrass can sometimes have difficulty competing with germinating annual weeds. This will not work for everyone, but if your play schedule allows for it, and if you have irrigation, then your chances of getting good weed-free establishment increase significantly if you wait to seed until after June 1. Of course, you must be able to irrigate many times per day if attempting to establish in the summer months. But the weed competition is far lower compared to when seeding is done in the spring.

Crabgrass can be controlled postemergence with fenoxaprop, quinclorac, mesotrione or topramezone. The issue of when to apply the herbicide for postemergence control is complicated and somewhat dependent on product used. For example, fenoxaprop tends to be very effective on leaf-stage crabgrass but its control of tillering crabgrass is variable. I have seen applications of fenoxaprop to 5-tiller crabgrass that were very effective and others where it barely injured the crabgrass. The conventional wisdom is that fenoxaprop is most effective on leaf-stage or early tillering crabgrass. Quinclorac is effective on leaf-stage crabgrass and late-stage (>5 tiller) crabgrass, but can be frustratingly inconsistent when crabgrass is between 2 and 5 tillers. Topramezone is the most recent grassy herbicide to be marketed for use in cool-season turfgrass. It seems the most flexible when it comes to the stage of crabgrass at application. Our research has found that a mix of ½ label rate of either mesotrione or topramezone combined with ½ label rate of quinclorac is very effective. That said – and this applies to whichever herbicide you use – if you make a postemergence application before crabgrass seed has stopped germinating (typically late June or early July), in most cases you will get a few weeks of suppression followed by a new population of crabgrass from seed. For this reason, lasting control of crabgrass tends to occur with applications made after July 1.

Pamela Sherratt is sports turf extension specialist at The Ohio State University.

This article was co-authored by Dr. David Gardner, professor of Turfgrass Science at The Ohio State University.


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