Great Plains Gardening

By Edith Koeller

May 21, 2004

Last time I mentioned ground covers. Your lawn is a considered ground cover and the most common form. This week I would like to share with you what CSU fact sheet #3,101 has to say about control of grassy weeds in the lawn.

"Crabgrass, foxtail, barnyard grass, and goose grass can be important lawn weed problems at the lower elevations in Colorado. These weeds belong to a group called the summer annual grasses, and it is important to understand their growth cycle if you want to control them."

"The seed of summer annual grasses deposits on the ground from the previous fall and germinates the following year, from midspring through midsummer. Germination is based on soil temperature, generally beginning when surface soil temperatures reach 55 to 60 degrees. Once germinated these grasses grow quickly during the summer months. Their growth is favored by warm temperatures and a good supply of water. Each annual grass plant produces thousands of seeds from midsummer through the early fall, when the plants are killed by the first frost. There are several approaches to consider in the management of annual grasses in home lawn, involving both cultural techniques and the possible use of herbicides."

For non-herbicidal control "mow as high as practical during the summer months for the particular grass species present in your lawn. Maintain bluegrass, tall fescue, fine fescue, and ryegrass at 2 ½ to 3 inches during the summer. The seeds of some weeds require high light intensity to germinate, and the shaded environment near the soil surface in a high mown lawn helps deter weed seed germination. In addition, the higher mowing height produces a healthier grass plant. Crabgrass and other annual grassy weeds are much more common and more aggressive in lawns that are mowed at heights less than 2 inches. Also mow at intervals frequent enough so that no more than one third of the grass blade is removed in a single mowing. Letting a turf grow tall and then cutting it back to a low height will cause a reduction in turf density…a lawn may require mowing every three to five days during the spring and early summer."

"Irrigate properly to help reduce annual weed infestation. Light, frequent irrigation will encourage weed seed germination – even if a preemergent herbicide has been applied. Fertilize according to the needs of your lawn species and core cultivate the lawn at least one per year to reduce compaction and to control thatch."

There are two types of herbicide control—preemergent and post emergent. "The preemergent herbicides must be applied before the crabgrass seed germinates to be effective. In southern and western Colorado, crabgrass seed can germinate from late March to early April. Along the northern Front Range, crabgrass can germinate from mid April to mid May. Apply preemergent herbicides two to four weeks before the above dates. Remember the actual germination of crabgrass varies from year to year, depending on the weather. Warm, moist springs cause earlier germination and cool, dry springs delay it. It is essential to remember that a preemergent herbicide application will not control annual weedy grass after the seed germinates and the weed begins to form leaves. The preemergent herbicide must be applied uniformly across the lawn to establish a chemical barrier on the soil surface. It is essential to avoid skips and streaks, which may allow weeds to appear in the lawn later in the year. These preemergent herbicides break down during the summer months, most quickly when summers are warm and precipitation/irrigation is plentiful."

"With normal weather patterns, most preemergent herbicides give good to excellent control of crabgrass, foxtail and barnyard grass. Control of goose grass and field sandbur will often be less satisfactory, depending upon herbicide used. In all cases, read the specific pesticide label for more detailed information before using the product."

"Post emergence herbicides can be used on annual grasses after they germinate and begin to mature. These are called organic arsenicals. The most commonly sold type, called MSMA (monosodium methanearsonate), is often sold under trade names like "Crabgrass Killer." These herbicides are most effective against young, seedling annual weeds and can only be applied as a spray. Once the weeds become larger and more mature, MSMA is largely ineffective. The organic arsenicals do not act as preemergent herbicides. Some points to consider when using MSMA and related organic arsenicals include:

"They may cause a temporary discoloration of the turf. If the soil is dry, water the turf area so that the moisture penetrates to a 3-inch depth the day before the treatment. During hot, dry weather, apply another one half inch of water two days after the lawn has been sprayed. With a late summer or early fall treatment it is best to wait two weeks before over seeding the treated area. Do not apply post emergent crabgrass herbicides to a new seeding until it is mowed a least three times. A second application of organic arsenicals may be needed four to seven days after the initial application. This is generally needed for more mature weeds. Irrigate with 1 inch of water two days after the last treatment to minimize damage to the turf."

"Apply organic arsenicals when air temperatures range from 70 to 85 degrees. Shade or cloudy weather accompanied by cooler temperatures, decreases the effectiveness of organic arsenicals."

This fact sheet also contains a table of comparisons of the different herbicides and their efficiency on the different weeds mentioned in this article. If you are having problems with a weedy lawn, I urge you to pick up the fact sheet at the local extension office.

Until next time…