I seeded some bare areas in my lawn in late spring and now it seems I only see grassy-looking weeds. The leaf blades are wider and lighter green than the bluegrass in the rest of my lawn. What is this, and what can I do to get rid of it?
— Jonathon Jacobsen, Wilmette
It sounds from your description like you have a crabgrass infestation. This summer, I have seen a lot of crabgrass in home lawns. The crabgrass in one friend’s lawn choked out most of the bluegrass that was seeded in late spring, resulting in a very frustrated gardener. There may have been a crabgrass seedbank in the existing soil. Or, when my friend bought soil to use for grading, crabgrass seeds may have been mixed in with the soil. I even have some crabgrass moving into my lawn at home for the first time. This seems to be a big year for crabgrass, and I have seen stands of newly sprouted crabgrass and older plants that are going to seed in many lawns. I have also seen another grassy weed called nutsedge in a lot of garden beds and lawns.
Crabgrass is an annual, weedy, warm-season grass that prefers the hot, dry summer weather over the cool spring and fall temperatures that bluegrass prefers. It can blend into your lawn during early 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 come in. During the summer, this low-growing weed can also be identified by its seed spike that resembles a turkey’s foot. The seeds turn a maroon color when cool temperatures begin to arrive in fall. Freezing temperatures kill this weed, resulting in patches of brown throughout the lawn.
The best way to prevent crabgrass is to maintain a healthy lawn that is thick and outcompetes and excludes crabgrass. Mow your lawn at 3 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 allow light to penetrate the soil, giving crabgrass seed as well as other weed seeds an opportunity to germinate. Light, frequent watering also favors the development of crabgrass. Water deeply and less often to help prevent these weeds.
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 tall with dull green leaves that are slightly hairy at the base. Each leaf is only 1 to 4 inches long. Large crabgrass can be up to 3 feet tall and can form thick growth mats, with pale blue-green leaves that are 2 to 6 inches long. The leaves are hairy on both sides over their entire length. Large crabgrass can spread rapidly by tillering or from sprouts at the base of the plant. This rapidly increases the width of the crabgrass clump.
Both types of crabgrass are considered summer annuals and die when the weather gets cold. They produce lots of seeds that can remain viable in the soil for years, so consider this as you strategize 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 much earlier in the season when they are small. I decided to treat some crabgrass patches with an herbicide called Q4 Plus two weeks
ago and got great results even with older plants. The infestations were too big to remove manually, and I wanted to reseed the lawn this fall.
Another effective 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. Generally, apply 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 dose of pre-emergent 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. Keep in
mind that the product used to prevent crabgrass from germinating will also prevent new grass seed 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
