Why is this plant a pest? A single plant can produce up to 150,000 seeds. These seeds fall to the ground and stay dormant until mid-spring. When soil temperatures reach 60-70 º, about 80% of the germination has taken place. This ugly plant matures in summer, blooms, and starts producing seed again. It grows rapidly in hot, dry weather and produces seeds for next year’s lawn invasion. Crabgrass loves to grow in those bare, thin spots of your lawn.
It is very important to apply pre-emergent herbicide before the crabgrass seeds sprout. Late application or fall application will do nothing to control crabgrass. The optimum temperature for application is when soil temperatures approach 55º. Other than testing soil temperature, a rough indication for pre-emergent herbicide application is when the forsythias are blooming. Crabgrass seeds will start sprouting when forsythia blossoms start fading. This year, the forsythias have bloomed, but our unusually cold spring has not warmed the soil past the mid-fifties yet. There is still time to prevent crabgrass from invading your lawn and from producing thousands of seeds.
The most common preventative treatment is to apply a pre-emergent weed killer that will prevent the growth of crabgrass seedlings. Do not apply the pre-emergent to a newly seeded lawn, because the herbicide will kill the germinating lawn seedlings also. Pre-emergent herbicides include dithiopyr, pendimethalin, prodiamine, prodiamine + quinclorac, sulfentrazone + prodiamine, and others. To prevent a major lawn disaster or an ineffective application, be sure to read and follow the package recommendations carefully; as with any chemical substance, wear gloves and appropriate body protection. Be sure to water the herbicide into the soil with a hose, sprinkler, or natural rainfall, applying at least ¼ to ½ inch of water. Control generally lasts at least 2-3 months, and sometimes 4 months or longer.
A natural herbicide product is corn gluten meal. It contains 60% protein and is used in livestock feeds. Corn gluten meal is 10% nitrogen by volume, and besides its weed control properties, it is an excellent, natural, slow-release lawn fertilizer. Part of the protein in corn gluten meal contains a chemical that prevents root formation in sprouting seeds. It has absolutely no effect on weeds that have already sprouted. Again, do not use it on newly seeded lawns. The sprouting grass seed will be killed, as well as the sprouting weed seeds.
Corn gluten meal comes in two forms, powdered and an easier-to-apply granulated form. Spread it over the lawn at package-recommended rates. Give the application a light watering to move it into the soil. Control may not be optimum in the first year but expect better control results if the product is used year after year. It is a very safe product but may cost more than chemical products. If it is overwatered, or a large rainfall occurs, the effective portion of corn gluten meal may be diluted or washed away. Use the commercially prepared corn gluten meal product. Less expensive substitutes, like feed store products, often are processed differently and do not contain as much active weed-suppressing protein material.
Dr. Sam Bauer, Turf Specialist at the University of Minnesota, recommends spring fertilization of lawns when the lawn is actively growing (mid-May to early June). Using a weed and feed product in early spring up may just end up wasting the fertilizer portion. The still dormant lawn will not take up the fertilizer and you risk losing nutrients by leaching or runoff into the nearest waterway. This spring, the weather has been too cold for much active lawn growth yet. If you do use an herbicide/fertilizer combination, look for one with slow- release rather than fast-releasing nitrogen.
There are post-emergent herbicides (such as dithiopyr, quinclorac or fenoxaprop) for crabgrass control that work best when the crabgrass is still young and has not developed more than one or two sprouts (tillers) at its center. If you have just a small amount of crabgrass, spot treat it with herbicide or remove by hand weeding before the plants start producing seed. Consider over-seeding those bare spots in late August to mid-September with good quality lawn grass. Late in the season is the optimal time to establish our cool season grasses, since new grass seedlings do not have to face the blazing summer sun nor the competition from spring-sprouting annual weeds.
One of the best ways to combat crabgrass is to establish a thick, lush lawn and maintain it well. One of the biggest errors in lawn care is mowing too low. Mowing low forces the lawn to replace leaves, rather than directing its energy into deep root development. Most of us are not trying to turn our beautiful lawns into putting greens. If you mow higher (at 3”), the lawn will establish a good root system resistant to droughts and will shade out crabgrass and other weeds. Water your lawn less frequently, but when you do, water it deeply so that water penetrates to at least 6 inches deep. Monitor rainfall amounts. Frequent, light watering will encourage a shallow root system and will stress your lawn during drought.
You may have to mow more often or maybe even less often; look at your lawn growth and then decide when to mow. Don’t stick to a rigid schedule; monitor the lawn growth to see if it is long enough to mow. Make sure to keep your mower blade sharp, so that it cuts rather than tears the lawn. Remove no more than one third of the lawn height at any one time. Use a mulching mower to return clippings to the soil and to provide some nutrients and organic material for the lawn. Make sure that the grass is dry when you mow. Mowing in the evening is best; the grass will recover better overnight than in hot sun. Mow in different directions each mowing time to develop a nice even lawn.
If you have been giving your lawn plenty of intelligent, tender loving care, you may not need to apply an herbicide to your lawn at all.
Be sure to check the University of Minnesota web site for more information, www.extension.umn.edu. Type “crabgrass” into their search box and you will become a crabgrass expert. Also check out “lawn care” for the very best information on lawns in Minnesota. Here’s hoping that you win the never-ending war with crabgrass and end up with the nicest lawn in your neighborhood.