Thick, scraggly patches of crabgrass are an unpleasant sight, but a frustratingly common one in lawns around the country. While getting rid of crabgrass takes some effort, it usually doesn’t require extreme measures. Once you eliminate existing plants through natural methods or herbicides, prevention is just a matter of good lawn care and vigilance.
How Crabgrass Thrives
Knowing how crabgrass grows helps you plan your attack. One of the reasons this grass is so successful is its low, crab-like growth pattern consisting of a central clump with stems radiating out from the center like crab legs. This growth pattern allows it to quickly take over any thin or bare spots in your lawn.
A native of warm climates, it thrives in hot, dry conditions where most lawn grass species struggle. Worse yet, through a process known as allelopathy, crabgrass produces its own herbicides that weaken nearby plants. It’s an annual, so each individual plant dies in fall, but before it fades away, a plant can produce as many as 150,000 seeds for new plants the following spring. Those that don’t sprout the next spring can lay dormant for years waiting to exploit a moment of weakness in your lawn.
Large crabgrass (Digitaria sanguinalis) and smooth crabgrass (Digitaria ischaemum) are the most common crabgrass species found in the U.S., but most species in the Digitaria genus respond to the same types of treatment.
Get Rid of Crabgrass This Year
In spring, when the crabgrass plants are still small, pulling them out is the most effective way to get a minor crabgrass problem under control. Water the lawn first to soften the soil. Dispose of the plants far from your lawn. Leaving them on the lawn, dumping them in the compost, or turning them into mulch gives lingering seeds a chance to re-infiltrate your lawn. If you want to compost the plants, leave them in a closed bag in a hot, sunny place for at least four weeks to kill any seeds.
Older plants might be too well established to pull, but you can smother them by covering them with cinder blocks or other objects that completely block sunlight. After four to six weeks, the crabgrass will be dead and easy to remove. Pouring boiling hot water is a faster natural way to get rid of individual crabgrass plants, but you’ll need to cover a 3-foot radius around the plant to kill the roots, and this will kill any grass or other plants around the crabgrass.
For larger crabgrass infestations, herbicides are a more practical option. If you want to keep things natural, try vinegar with an acidity of 5 percent or higher, or an organic herbicidal soap. Spray each crabgrass clump until it’s soaked and repeat one to three times a day until the plant dies, which could take several weeks. Just remember these treatments can kill any other plants they touch, including turf grass.
If you’d rather go with a commercial herbicide, choose a selective, post-emergent herbicidal product specially formulated to kill crabgrass. Before you buy, read the label carefully to find out exactly what types of grass the product kills and what types it won’t harm.
Quinclorac is one of the best herbicides for getting rid of crabgrass without harming most turf grass species, but it isn’t safe for St. Augustine grass. Another effective option, Mesotrione, is generally safe for St. Augustine, but not for Zoysia grass. After treatment, let the crabgrass die, remove it, then reseed or re-sod the bare spot.
If your crabgrass problem is so extensive it engulfs 40 percent of your lawn or more, it might be time for soil solarization. This method kills both the crabgrass and your lawn, but it lets you start over with weed-free soil.
During the hottest part of the summer, mow the lawn as short as possible. Water thoroughly, then cover the lawn with sheets of clear, UV-resistant plastic of 1.5 to 2 mm thick. Seal the edges by weighing them down with soil. The plastic will heat the ground enough to kill all the seeds under it, usually within four to six weeks or up to eight weeks in milder weather. You can then reseed immediately afterward.
Keep it from Coming Back
Preventives known as pre-emergent herbicides are available to stop crabgrass seeds from germinating in spring. While most are harmless to established turf grass, take care to choose a product that’s labeled as safe for your grass species.
While corn gluten is moderately effective at suppressing spring crabgrass germination, you’ll need to apply it at a rate of 20 pounds per 1,000 sq. ft. to see much effect. Because of the product’s high nitrogen content, applying this much is prohibited by law in some areas. Check your local fertilizer and lawn care regulations before you use it. Overall, commercial pre-emergent herbicides are more effective and safer.
Crabgrass starts growing when the soil reaches 55 degrees for three to 10 days in a row, depending on exactly how warm the soil gets. Apply your preventive too early, and the effect won’t last to the end of the season. Then you’ll have crabgrass popping up in the middle of summer. Act too late, though, and the weeds will be so well established the chemicals won’t bother them.
In the north, the time to apply pre-emergent herbicides is approximately the same time the forsythia blooms start falling, but this is an imprecise guide at best. Use a soil thermometer to time the application correctly.
Follow the product directions carefully. Some products require an application in early spring and in late spring. Because pre-emergent herbicides also inhibit turf grass from germinating, wait at least 60 days and two mowings before you overseed your lawn again.
Once the crabgrass is gone, keeping your lawn healthy and dense is the best way to prevent the weeds from coming back. Lack of care weakens your lawn and gives the competitive edge to weeds that have adapted to growing in adverse conditions. In a healthy lawn, crabgrass has few opportunities to take hold.
Start by setting your mower blade to the highest setting in the range recommended for your grass species. Cutting higher lets the lawn fill out, leaving fewer spots for crabgrass to grow. In spring, taller grass shades the soil, protecting your lawn’s roots, but depriving crabgrass seeds of the sun they need to germinate. If you don’t know your grass species, choose one of the two highest settings, and never cut shorter than 2 1/2 inches.
Start watering in spring before the soil temperature reaches 55 degrees. This gives your lawn a chance to start actively growing before the crabgrass germinates, so it can more easily compete with the weed. Water infrequently, but to a depth of 6 to 8 inches. This encourages your grass to grow deep roots that can carry it through hot, dry periods.
Crabgrass roots are relatively shallow by nature, and although the plant tolerates heat and drought well, it struggles to compete with turf grass that’s grown deep roots.
Fertilize well in spring, but first use a soil test kit to determine at least the soil pH and, ideally, the buffer pH and mineral levels. This information helps you decide how much of which fertilizers to apply, so you don’t do more harm than good. To encourage dense growth, overseed your lawn in the fall if you’re in the north or in spring if you’re in the south. Proper mulching, dethatching, and aeration also help.
Weeding and the right herbicide are often enough to get rid of the crabgrass in your lawn for one year, but keeping it gone for good requires on-going attentive lawn care. To be sure you’re using the best crabgrass control methods for your region and lawn type, talk with a landscaper or expert at a garden supply store.