Tips on Crabgrass Control

By Pamm Cooper, Turf Program Coordinator for UConn Cooperative Extension

Unless new turf grass is established, crabgrass will come in 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.

In spring, thinned or dead areas of the lawn can be seeded with desirable grasses as soon as temperatures are around 50 degrees keeping in mind that germination of most lawn grass seed will not take place under 50 degrees. Time is of the essence as crabgrass seeds will germinate as Flowering Dogwoods are starting to bloom.
 
A pre-emergent herbicide can be used in the spring which will allow the grass seed to germinate, but will not allow crabgrass that opportunity. This herbicide is called Tupersan(tm) and contains the active ingredient, siduron. It is applied at the same time as seeding the lawn and needs to be watered in to form a barrier on the soil. It sometimes is sold together with a starter fertilizer. After 6 weeks the product can be re-applied if the seedling turf is still not dense. If no re-seeding will be done until late summer or early fall, another pre-emergent herbicide for crabgrass can be used. Be sure to water it in according to label directions, and do not disturb the soil surface after applying. In late summer or early fall, the desired grass can then be seeded.

Corn gluten meal is sold as an organic pre- emergent crabgrass and weed seed control option. It is applied the same way as chemical control pre-emergent herbicides, while supplying 2 lbs. of nitrogen per 1,000 sq. ft. of lawn. If there are frequent rains in the spring, however, this method of control may not work, as a small period of no rain or irrigation must occur as the crabgrass seed germinates. If rainfall does happen at this time, little control may be forthcoming, and the crabgrass seedlings

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.

The best defense against crabgrass invasion is providing the best conditions possible for your lawn. A dense lawn mowed higher will often prevent sunlight from aiding crabgrass seed germination. Also, do not fertilize cool season lawn grasses such as Kentucky Bluegrass, perennial ryegrass, or fescues in the summer when they are trying to survive heat stress. This can encourage crabgrass growth as it is an annual and must produce its seed heads during the heat of summer at the same time that the desired grasses are slowing their top growth. If crabgrass has produced seed heads, and must be mowed, bag the clippings to pick up the seed, or mow those areas last and wash the mower.

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.