When to apply crabgrass preventers

Crabgrass has got to be one of the most frustrating weeds in our cool season turfgrasses. Broadleaf weeds are annoying, but there are a number of herbicides and management techniques available to us to get rid of them. Crabgrass control is made difficult by the fact that we are trying to control a grass species in a grass stand, resulting in limited herbicide possibilities. Crabgrass also loves our hot, dry summers. While cool season turfgrasses go dormant at some point during summer, crabgrass is able to take off and gain a foothold.

Preventing that foothold is the idea behind crabgrass preventers. Crabgrass preventers are actually preemergence herbicides that prevent crabgrass seeds from developing into mature plants. They don’t actually kill the seed—or even keep it from germinating. Instead, they work by killing the young plant after it germinates. The timing of that kill is important, and requires application prior to germination since the herbicide will typically have no effect on existing crabgrass plants. Avoid applying too early, as well, since microorganisms and natural processes begin to break the herbicides down soon after application. Most are ineffective after about 60 days. There are exceptions noted below.

Because applications too late or too early are an issue, timing of the application is key. Since most crabgrass will typically start to germinate around May first, April 15th is a good target date for application. This gives the preventer time to disperse in the soil before germination starts. Because every spring is different, a better barometer is to apply when Eastern Redbud trees reach full bloom. A second application is recommended eight weeks later.

Note: Not all products are created equally. Technology advances in the development of crabgrass preventers has resulted in some exceptions with the development of products containing dithiopyr and prodiamine drastically lengthening the application window of crabgrass preventers. Dithiopyr containing products can be applied any time after Marcy first. Prodiamine containing compounds can be applied even earlier—in the fall in some cases. Both will provide sufficient residual strength to last the full growing season. Need to apply later than recommended? Dithiopyr containing products can even kill crabgrass up to the two- to three-leaf stage. They are also the products you should use if the turf was just planted last fall since dithiopyr tends to be gentler on our cool season grass seedlings. When possible, apply preventers prior to fertilizer applications to avoid a flush of early grass growth.

Preemergence herbicides can provide us with some great crabgrass control—when used appropriately. Always read and follow label directions, understanding when and how the products should be applied for best results.