Fixit: Is it quack or crab grass?

  • Article by: KAREN YOUSO , Star Tribune
  • Updated: April 25, 2003 - 11:00 PM

QHow can a person tell the difference between quack grass and crab grass? Apparently, crab grass can be killed using a selective herbicide, but quack grass has to be killed with a non-selective herbicide.

AYou're right. Effective weed control is based on correct identification. For example, it's useless to treat quack grass with crab grass control products. They're effective only against crab grass and other annual grass-type weeds.

Many books and charts are available at nurseries and online to help identify common lawn weeds, or contact your county extension service.

Crab grass is an annual weed that sprouts from seed in spring, usually from May to June. During the summer it develops into a ground-hugging weed that spreads over surrounding grass. In late summer it produces hundreds of seeds that will sprout the following year. Crab grass seeds can remain in the soil for many years and sprout when the soil is disturbed.

If the weed grass is present in early spring, it is probably a perennial weed grass such as quack grass, not crab grass.

Quack grass is extremely vigorous, and is taller, faster-growing and lighter green than desirable lawn grasses. Quack grass has a broad grass blade and a tough, wiry network of underground stems.

The most effective method of controlling lawn weeds is to maintain a dense and vigorously growing turf. Weeds are often an indication of problems in the grass environment, and killing the weeds without correcting the underlying problem won't get you the results you want.

Often simply altering practices to favor the grass rather than the weeds will eliminate weeds. That may include raising or lowering the mowing height, changing the frequency of mowing, lengthening or shortening the period between watering, increasing or decreasing fertilizer applications or aerifying the soil.

Sometimes changing how you care for your lawn isn't enough -- you'll need to use chemicals to control weeds. Whenever herbicides are used, always read and follow label directions carefully.

• Preemergence herbicides affect germinating seeds. To be effective, the herbicide should be applied two to three weeks before the seeds germinate. They are most effective against annual weeds. For control of summer annuals such as crab grass, application of preemergence herbicides between May 5 and May 20 in the Twin Cities area is most effective in a normal (temperature, precipitation) year. When in doubt, contact your county extension office.

• Postemergence herbicides are used to kill weeds after they begin to grow. To be effective, most postemergence herbicides must be absorbed through the leaves; consequently, liquid sprays generally work better than dry, granular materials. However, granular formulations may be the most practical way for homeowners to apply these materials. Postemergence herbicides are most effective when weeds are young and growing vigorously.

• Nonselective postemergence herbicides kill all plants, both desirable and undesirable. These herbicides can be used to spot treat perennial grassy weeds that are not affected by selective herbicides. To spot treat an area, thoroughly wet the weed foliage with herbicide solution.

Includes information from Brad Pedersen and Bob Mugaas at the University Extension Service and Rittenhouse Fine Tools Website

Send your questions to Fixit in care of the Star Tribune, 425 Portland Av. S., Minneapolis, MN 55488, or call 612-673-9033, or e-mail columns are available at, Fixit cannot supply individual replies. Fixit appears every day except Friday.

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