Fixit: Get rid of nuisance grasses

  • Updated: May 4, 2004 - 11:00 PM

Crab grass

Q How do I kill crab grass and quack grass in my yard?

A Effective weed control is based on correct identification. 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.

You can prevent crab grass by applying a pre-emergent herbicide at the appropriate time. That time depends on where you live and which parts of your yard are involved, according to University of Minnesota Extension Service horticulturist Deb Brown.

If you live in the Twin Cities area, apply and water-in a pre-emergent herbicide in early May. For each 100 miles north or south of the metropolitan area, Brown said, you should add or subtract a week. The applications should be done later in the north and earlier in southern Minnesota.

She noted that "hot spots," such as along a driveway or on a south-facing slope, might need to be treated a week earlier than the general time recommended for your location. Brown said the herbicide must be introduced into the soil with a light watering for it to become active. Spreading the product over the lawn without watering is useless. Don't use a pre-emergent herbicide if you've planted grass seed this spring, unless it is a product that is designed for newly seeded lawns.

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.

If you have only a few clumps or patches of these perennial grasses, you might be able to successfully dig them out and replace them with either new seed or sod.

Another approach to eliminating these weeds is to spot treat the area, using an herbicide known as glyphosate (Ortho's Kleenup or Monsanto's Roundup). This is a non-selective herbicide. That is, it will kill or damage any green plant, including desirable lawn grasses or garden plants.

Glyphosate must be used when the plants are actively growing. The chemical is most effective when temperatures are 60 to 85 degrees Fahrenheit. Preferably, it should be applied at least 24 hours before rainfall. The plants in the treated areas will turn yellow, and, ultimately, brown. These areas can be reseeded or resodded after seven to 10 days. Remember, always read and follow herbicide label directions. For more information, contact your local extension office.

Often, weeds can be eliminated simply by altering practices to favor the growing of grass rather than 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 aerating the soil.

Karen Youso, Fixit Editor

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

Send your questions to FixitHG in care of the Star Tribune, 425 Portland Av. S., Minneapolis, MN 55488, or call 612-673-9033, or e-mail fixit@startribune.com. Sorry, Fixit cannot supply individual replies. Fixit also appears in the Variety section every day except Wednesday and Friday.

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