Clamping down on crabgrass should be top of mind this summer

There are a few spots in my lawn where a grassy-looking weed seems to be taking over. The leaf blades are wider and lighter green in color than the rest of the grass in my lawn. What is this and what can I do to get rid of it?

— Sarah Horton, Wheaton

The description of the weed in your lawn sounds like you have crabgrass moving in. Crabgrass is an annual, weedy warm-season grass that prefers the hot and dry summer weather over the cool spring and fall temperatures that bluegrass prefers. It can blend into your lawn during the summer if you do not pay close attention. When you look more closely, crabgrass can be identified by its wide leaf blade and light green color. It is very easy to recognize when it grows in severely stressed areas like along a driveway where the soil has been compacted by driving on it. Open areas in the lawn provide an opportunity for crabgrass and other weeds to move in. During the summer, this low-growing weed can also be identified by its seed spike that resembles a turkey foot. The seeds turn a maroon color when cool temperatures begin to cool down in fall. Freezing temperatures kill this weed, resulting in patches of brown throughout your green lawn in fall.

The best way to prevent crabgrass is to maintain a healthy, thick lawn that outcompetes and excludes crabgrass. Mow your lawn at 3 to 3 ½ inches to promote a deep root system in your grass that will enable it to better withstand stress and remain dense to help shade out weeds like crabgrass. Thin areas of the lawn like those in yours will allow light to penetrate the soil, giving any crabgrass seed, as well as other weed seed, an opportunity to germinate. Light, frequent watering also favors the development of crabgrass. There are two common species referred to as crabgrass — smooth or small crabgrass (Digitaria ischaemum) and hairy or large crabgrass (Digitaria sanguinalis). Smooth crabgrass is the smaller of the two species, typically less than 15 inches in height, with dull green leaves that are slightly hairy at the base. Each leaf will be only 1 to 4 inches long. Large crabgrass can be up to 3 feet in height and can form thick mats of growth, with pale blue-green leaves that are 2 to 6 inches long. The leaves will be hairy on both sides over their entire length. Large crabgrass can spread rapidly by tillering or from sprouts at the base of the plant, which rapidly increases the width of the crabgrass clump. Regular mowing prevents the plants from attaining their mature height, and they will still set seed even mowed at a low height.

Both types of crabgrass are considered summer annuals and will die when the weather gets cold. They produce lots of seed that can remain viable in the soil for years so this needs to be considered in your strategy to eradicate this weed. You could manually remove the crabgrass currently growing in your lawn, if that is feasible, and seed the bare areas to improve the health and thickness of your lawn. There are products to control crabgrass after it has germinated but they work best when applied to plants when they are small much earlier in the season. The best way to control a big outbreak of crabgrass in your lawn is to use a pre-emergent herbicide in spring. This herbicide must be applied before crabgrass begins to germinate after soil temperatures reach 50 to 60 degrees Fahrenheit. In general, apply the herbicide in early to mid-April before forsythias begin flowering, though you may need to adjust the timing if it is an exceptionally warm or cold spring. You can apply a second application of pre-emergence crabgrass weed killer six to eight weeks after the first application at half the recommended rate if you need to be more aggressive for a major crabgrass problem. The product used to prevent crabgrass from germinating will also prevent new grass seeds from germinating.

For more plant advice, contact the Plant Information Service at the Chicago Botanic Garden at plantinfo@chicagobotanic.org. Tim Johnson is senior director of horticulture at the Chicago Botanic Garden.