Jennifer Schultz Nelson
Extension Educator, Horticulture
Battle lines are being drawn in lawns across much of the U.S. this time of year. It's time to face the big bad weeds threatening the American dream of a great looking lawn full of beautiful green grass.
One of the weeds that is an ongoing threat in most lawns is crabgrass. Though we haven't really shaken the last of the wintry dismal weather, this is the time to wage war against this grassy weed.
There are actually two common species we call crabgrass–smooth or small crabgrass (Digitaria ischaemum) and hairy or large crabgrass (Digitaria sanguinalis). Both are members of the grass family.
Smooth crabgrass is the smaller of the two species, typically less than 15 inches in height, dull green leaves slightly hairy at the base, each leaf only one to four inches long. Large crabgrass can be up to three feet in height and can form thick mats of growth, with pale blue green leaves two to six inches in length, and hairy on both sides over the entire length.
Large crabgrass can spread rapidly via tillering, or sprouts at the base of the plant which rapidly increases the width of the crabgrass clump. But for both types of crabgrass, their true power lies in seed production. A single crabgrass plant, if allowed to grow and mature, will produce anywhere from 150,000 to 180,000 seeds in one season. This translates to somewhere around 10,000 to 20,000 seeds per square foot!
Worsening the seed problem is that although the seeds need a cold period before they will germinate, they will remain viable in the soil for years. They are just waiting for their golden opportunity where the temperature and moisture is just right for germination so they can attempt a hostile takeover of the lawn.
Both types of crabgrass are warm season grasses that are considered summer annuals. The secret to eradicating them is to use their prolific seed production and unique cultural requirements against them.
The first line of defense against crabgrass is a healthy lawn. A thick, properly fertilized and watered lawn will outcompete and exclude crabgrass. A lawn which is thin allows light to penetrate to the soil and any crabgrass seed there has an opportunity to germinate. Shallow watering, in the first three inches of soil also promotes germination of crabgrass seed while not promoting development of healthy deep-rooted turf that can withstand drought and outcompete crabgrass and other weeds.
Grasses typically grown for turf in this part of the country are cool-season grasses–they prefer the cool weather of spring and fall, and will go dormant during the hot, relatively dry summer. Fertilizing in the summer is typically only recommended when the turf is being watered, which keeps it from going dormant.
Since crabgrass is a warm-season grass that actually prefers the hot dry summer over cool spring and fall temperatures, it thrives in our summer heat. Many sources recommend omitting summer fertilization of lawns as a method to help control crabgrass. A summer application of fertilizer will actually give the crabgrass more energy and vigor, which will definitely make it harder to control.
To chemically control crabgrass, the recommended method is pre-emergence herbicides. These are typically included in the "spring" lawn fertilizers. You may also find them as separate stand-alone products. They are typically applied in granular form with a spreader. These pre-emergence herbicides work on the germinating seeds in the soil, disrupting key enzymes in the germination process, and the seeds never germinate.
The timing of pre-emergent herbicide application for crabgrass is crucial. Crabgrass seed will only germinate after soil temperatures reach 50 to 60 degrees Fahrenheit for several consecutive days. There is some disagreement in the specific temperature and time period depending on the information source used. Nonetheless, if you apply a pre-emergent herbicide after the crabgrass seed in your lawn has germinated, you have just wasted your time and money. Pre-emergent herbicides will not affect seedlings. It must be in the soil before the crabgrass germinates.
So when should you put down your pre-emergent herbicide? The bag in our garage says February through April- not a very specific timeframe. One source I found says that the average date of crabgrass seed germination for this part of Illinois is between April 20 and 23rd during a year of "normal" weather, and pre-emergent herbicides should be applied at least two weeks prior. While more specific, I challenge you to tell me what "normal" weather is around here!
The best advice I can give is to keep an eye on the weather, and if you are really ambitious, track the soil temperature too. When the weather warms consistently, put your pre-emergent herbicide out. One source I found said to apply pre-emergent herbicides when the local Forsythia blooms are wilting. The good news is that pre-emergence herbicides do stick around in the soil for awhile, so if you're a little off in your timing, it will still work.
If you are trying to seed a new lawn or over seed an existing lawn this spring, do not put down pre-emergent herbicides, or none of your desirable grass seed will germinate. The best strategy is to start a new lawn or over seed an existing lawn in the late summer or fall, get it growing well, and then you can apply pre-emergent herbicide in the spring without an issue.
There are some post-emergent herbicides for crabgrass, but they are most effective on very small crabgrass plants, and are more difficult to use than pre-emergence herbicides. You should always follow the label directions of any herbicide, but the directions for post-emergent crabgrass herbicides need to be followed as strictly as possible to increase the chances of it working properly and controlling the crabgrass.
Mowing practices are another defense against crabgrass if done properly. Mow at 2 ½ to 3 inches if possible, to shade the soil which will help reduce the number of crabgrass seeds that can germinate.
While you may find great satisfaction in pulling crabgrass out by hand in mid-summer, you are probably only making the problem worse. Typically, when a crabgrass plant is removed, it leaves a large gap in the lawn. Even if the removed plant has not produced its own seed, it has successfully created a gap in the lawn and allowed light and moisture a way to get to the thousands of seeds ready and waiting to grow in the soil.
The good news is that crabgrass is an annual, so the plants are all killed with the first frost in the fall. We are all given a clean slate each spring.