Controlling Crabgrass in the Lawn

Crabgrass is a native summer annual grass species, sprouting from seed each spring and dying in the fall. As such it is well adapted to survival in lawns. The seed are capable of laying dormant in the soil for up to 15 years, waiting for conditions to become right for sprouting and growth.

Annuals depend on their seed for the survival of the species. These seed will lay dormant in the soil until the ground warms in the spring. Crabgrass seeds normally begin germinating at the same time that dogwoods and forsythia bloom. They will germinate all during the summer wherever sunlight hits bare ground.

The best defense is to insure that no bare ground exists in the lawn after the weed(s) begin their spring blooming. This is best accomplished by practicing good lawn management in the fall, namely by removing any live or dormant crabgrass and replacing it with grass seed or plugs. Leaves should be removed in a timely manner, the lawn fertilized and limed according to soil test. Nematodes should be applied between mid-June and mid-October to control grubs of the Japanese Beetle and the Green June Beetle.

The first spring after reseeding the lawn should be mowed no closer than four inches high to increase the shading effect. The lawn must be maintained for optimal growth but not over fertilized. Over fertilization can result in a rank spring growth of the lawn grasses which will make them more prone to foliar diseases. Diseases which result in bare spots or thinned stands will actually encourage the establishment of crabgrass.

Crabgrass preventative herbicides work by inhibiting the germination of the crabgrass seed. Since they will also inhibit the germination of lawn grass seed, they can not be used on lawns seeded in the spring. Even if the lawn is seeded early in the spring the herbicides will be damaging to the newly emerged lawn grasses. For this reason it is recommended that lawns be seeded in the fall and managed to make optimal, but not rank, growth through the next spring. If this approach is successfully followed herbicides will not be required.

Crabgrass can be an annoying problem if proper steps are not taken in the fall to insure a dense stand of healthy grass in the spring. The foresighted gardener can, however, improve the appearance of the lawn while actually reducing the need for lawn chemicals.

Time to wait before seeding grass if using Herbicides
You need to wait three weeks to seed after using a post-emergent herbicide. After seeding, grass needs to be old enough to have been mowed three times before using post-emergent herbicides. Pre-emergent herbicides remain in the soil for months, affecting all seed germination, not just crabgrass.