By Pamm Cooper, Turf Educator
Whenever areas of a lawn become thin, due to salt, drought, disease, insect, or other sources of damage, weeds are likely to move in. One of the likely invaders will be crabgrass. A warm-season annual grass, it persists by producing seeds the previous year that will germinate the next spring after a few consecutive days of temperatures of at least 60 degrees. Usually in Connecticut, this means sometime after forsythia bloom.
Unless new turfgrass is established, crabgrass will come into where the lawn grass used to be. The trick is to establish the new grass in a small window of time before the crabgrass seeds dropped last summer germinate and crowd out desirable grasses. It can be helpful to allow lawn grasses to grow to a height of 3”, hopefully shading out the crabgrass seedlings so they won’t survive. This will probably not be an effective control strategy if the stand of turf grass thin.
A pre-emergent herbicide can be used if it was not possible to establish turfgrass in the spring, or if the seeded areas did not fill in adequately. This herbicide needs to be watered in before crabgrass seed germination to form a barrier on the soil surface that will prevent the crabgrass from becoming established. In late summer or early fall, the desired grass can then be seeded. Read the label of the pre-emergent herbicide used. Some have a residual effect of three to four months. No grass (including desirable turf) will be able to develop from seed as long as the herbicide remains in place. A pre-emergent herbicide is available for homeowner use that will allow turfgrass seed to develop normally, while preventing crabgrass from coming in. It can be applied at the same time the lawn is seeded. This product is called Tupersan™,
which has the active ingredient, siduron. After six weeks, it can be applied again to the newly seeded areas to ensure crabgrass will not get a foothold in any thin areas.
As with all weeds, crabgrass will be able to get in only where turf has thinned, gone dormant, or even died due to cultural, weather, or other issues. Herbicides are only a tool, and their use can be limited to only areas where crabgrass has been an issue, as along a roadside where salt may have killed some turf over the winter. It is not always necessary or desirable to treat an entire lawn with any pesticide.
Good lawn care practices and proper turf grass species selection go a long way in keeping desirable grasses healthy and better able to withstand many potential problems. A dense, healthy lawn is a better defense against crabgrass invasion than any herbicide. Check with your local UCONN Cooperative Extension System Office, or the UCONN Home and Garden Education Center for herbicide recommendations.
The UCONN Home and Garden Center: