Crabgrass is an annual grass that often becomes a problem in lawns. Some areas of the country call it by different names. Officially, it’s from the Digitaria genus but may be called finger grass, fonio, and a bunch of other names that I can’t print. Whatever it’s called, it’s is junk. It has a thick, course structure which looks out of place in normal turf. It’s also very aggressive, crabgrass seeds germinate in the late spring and early summer and out-compete the domesticated lawn grasses.
The plants are annual and die out with the first frost, but while they are living, each plant can produce up to 150,000 seeds. The plants leave voids in the lawn when they die, providing a place for the next year’s seeds to germinate.
Fortunately, crabgrass is easy to control. It needs warm bare soil to germinate, so most healthy lawns will block most of its growth. There are several pre-emergent herbicides that prevent the crabgrass seed from germinating. Timing is critical, though, apply it too early, and spring rains can wash it away before it does it work. Apply it too late and, well, you missed the bus for preventing it from coming up. Once crabgrass has sprouted, it’s difficult to kill without harming the rest of your lawn. In most of the northern US, an easy way to tell when to apply a crabgrass pre-emergent is when the forsythias are in full bloom.
Like another big lawn nuisance dandelions, some species of crabgrasses actually serve a useful purpose. The seeds of some varieties of folio are considered millet and are popular in Africa and the Orient where they are ground into flour and used for baking or fermented for beer.
One of the pre-emergents is an organic corn gluten meal. The others are chemical and, like all chemical herbicides should be applied with caution. Crabgrass prefers lawns that are under fertilized and mowed too low, so a healthy lawn is the first line of defense.
Pre-emergents need to be applied before the seed germinates. Throughout the Northeast, the guideline is to apply pre-emergents at about the same time as the Forsythia bloom. Don’t apply it too early as Spring rains may wash it away, or too late as pre-emergent means before (pre) the seeds germinate. Once they start to grow, it’s tough to kill.
Unfortunately, most pre-emergents also prevent regular grass seed from germinating. So if you are overseeding within a few months, use the special pre-emergents that won’t affect regular seed.