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Larry Sagers Horticultural Specialist Utah State University Extension Service Thanksgiving Point Office © All Rights Reserved
How do I keep crabgrass out of my lawn?
Crabgrass is a warm season annual grass. It dies out each winter and begins anew from seed each spring. It starts to grow sometime after April 15. As temperatures turn colder in the fall the grass takes on a purple color but it keeps growing until it is killed by the fall frost.
Many different crabgrass herbicides are available from local nurseries. All work well if they are applied correctly at the right time. Since they are pre-emergent herbicides they have to be on your lawn before the weeds start to grow. Many people have the erroneous idea that the herbicides kill weed seeds. They do not.
The pre-emergent herbicides form a barrier over the soil if they are applied correctly. When the seeds germinate, the tender first shoot absorbs the herbicide and it dies. Those seeds that do not germinate remain in the soil until the germination requirements are again right for growth.
Remember, once the herbicide is applied, do not disturb the barrier. Aerate your lawn before applying the herbicides, otherwise you may disturb the barrier and the crabgrass seed will emerge in the disturbed areas.
Timing is critical as you have to put the herbicides down before the seeds germinate. The common calendar date for the application to be on the grass is April 15. A better biofix is to look at the blooming plants in your neighborhood. The herbicide needs to be down on the lawn before the ‘Radiant’ crabapples bloom or before the lilacs bloom.
The first line of defense against this pest is your lawnmower. No, you are not going to kill this grass by mowing it. The secret is in the mowing height. Crabgrass seed must have light to germinate. For those who are willing to raise the mower height to at least three and a half inches, the threat almost vanishes.
Many crabgrass problems arise from improper edging or string trimming. Keep the lawn along the edges at least as long as the rest of the lawn. Most crabgrass invasions come in areas where the lawn is cut to short or the grass dies out for another reason.
Crabgrass is a very opportunistic. The seeds lie dormant in the soil for years waiting for the right germination conditions. When they get light, they grow. To prevent problems, keep the lawn long and lush. Fertilize as needed and avoid problems that thin the lawn or interfere with its growth.