Dr. Grow: Know types of weeds to control them in lawns

The seeds of a Dallisgrass seed head are 1 /16-inch in diameter.

Nutsedge is found in moist soils, thus the common name, watergrass.

At this time of year, lawns, especially those composed of cool season grasses (bluegrass, fescue and rye), are under stress due to the hot temperatures we have been experiencing. They are also under stress due to diseases that grow rapidly when temperatures are high and humidity is high. Stress will also occur during a drought.

When lawns are under stress, their growth rate slows down. This slower growth rate reduces the ability of grasses to compete with weeds. Consequently at this time of year many weeds begin to appear in lawns.

In today's article I want to concentrate on grassy-type weeds. Because the chemicals used to control these weeds differ, you must first learn to identify the weeds. The easiest way for you to do this is to examine the structure of the seed head.

Crabgrass is an annual grass with a seed head that consists of three or more spikes sitting on the top of a main stem. Each spike is lined with two rows of seeds. Crabgrass blades are usually wider and lighter green than lawn grasses. Crabgrass plants usually grow in clumps.

Foxtail has a busy seed head that resembles the tail of a fox. It, like crabgrass, is an annual broad-leaved grassy weed and grows in clumps. Its color is usually lighter green than surrounding cultivated grasses. The leaves of foxtail are quite often slightly twisted.

Dallisgrass is a perennial grass, often found on poor soils near streets, sidewalks and driveways. Its seed head consists of one to three spikes on a main stem. The seeds are in two rows along each spike. Each seed is 1/16-inch in diameter but is flattened on one side.

Johnsongrass is a perennial weed that normally grows to 4 or 5 feet tall before seed heads develop. In lawns, where mowing is done weekly, it seldom develops enough to maturity to produce a seed head. Its 1-inch wide or wider grass blade has a pronounced center white vein. Run your finger along the blade edge carefully. Its sawtooth edge can easily cut your finger. If you think a clump of grass is Johnsongrass, dig it up. You will find roots that are often 1/4-inch or greater in diameter.

Nutsedge is not a grass at all, but a sedge. You will find small nutlets attached to the roots. The angular, light yellow foliage, grows extremely fast. One day after mowing it may be 1 to 2 inches taller than the surrounding cultivated grass. You generally find nutsedge in moist soils, thus the common name, watergrass.

At this time of year crabgrass and foxtail are controlled by using MSMA or DSMA. You must make two or three applications to kill these weeds. Although MSMA and DSMA are selective herbicides, they may burn your cultivated grasses. To reduce burn, water your lawn one day, spray the next, and then water again the following day.

According to most labels, MSMA or DSMA should control Dallisgrass. I find they seldom do. Therefore, control Dallisgrass and Johnsongrass with glyphosate. Make sure the weed grasses are growing rapidly and not under stress. Apply glyphosate as a fine mist or with a paint brush. Spray soon enough in the day so the weeds are exposed to four to six hours of sunlight. Unfortunately glyphosate is a nonselective herbicide. The surrounding grasses will also be killed if you get the spray on them. Plan on reseeding in the fall.

Control nutsedge with two or three applications of basagran. This selective herbicide will not kill surrounding cultivated grasses, although some burn may occur. Reduce burn by following the water spray, water cycle mentioned above.

The different textures and colors of grassy-type weeds can ruin the uniform appearance of your lawn. Learn first to identify them. You can then choose the right chemical to rid your lawn of these nuisances.

Paul Schnare of Cape Girardeau has been in the lawn and garden business for more than 20 years. He's an adjunct professor in horticulture at Southeast Missouri State University.