When Should Pre-Emergent Crabgrass Control be Applied?

Crabgrass is one of the most prevalent and challenging weeds to control in New Hampshire. It is a hearty annual plant that will grow in any soil condition including sandy soil. It is highly drought-tolerant and produces hundreds of new seeds per plant each year. Those seeds are viable for a hundred years or more! This creates an environment full of active or potentially active plants.

Because crabgrass is a grass, most broadleaf weed controls will not impact it.

There are several post-emergent controls that will control crabgrass after it has sprouted, but their effectiveness is limited. The most effective way to control crabgrass is with a pre-emergent weed control; however, there are some things that you need to know before applying your control:

Pre-emergent crabgrass controls will kill grass seed.

Are you going to be seeding your lawn in the spring? You may want to hold off on that. Pre-emergent controls are essentially root pruners. Once the seed pops, a root that root is killed off and the seed dies. There are pre-emergent lawn products that claim they will control crabgrass while you are seeding but effectiveness is moderate at best. Standard pre-emergent controls last in the soil for up to 90 days so seeding will have to wait for three months.

Timing of pre-emergent applications is critical.

Crabgrass will start to germinate once soil temperatures reach 55 degrees. It is important to understand that air temperature is not soil temperature and though it may be 60 degrees outside the soil may still be in the mid-forties. Pre-emergent can be applied anytime before the soil gets to 55 degrees but remember the 90-day clock is ticking as to how long it will last once it is applied. A good phenological indicator to watch for is the bloom of the forsythia. The flowers bloom around the time soils are reaching 55 degrees.

Know your pre-emergent weed controls.

Some controls are better at controlling crabgrass at the seed pop stage (early in germination process) and others are better once a tiller or two have popped (later in the germination process). Be sure to choose accordingly.

Here at Groundhog Turf Care, we monitoring soil temperatures is a standard practice with our lawn care services. We apply the first of two crabgrass pre-emergent weed controls at the correct time. The first control is one that is made for the early season control of crabgrass. We follow up with a second application approximately 5 weeks later that is designed to control crabgrass in the later germination stages. The combination of two overlapping applications provides the best pre-emergent crabgrass control available for your lawn.