The best weapon you have against this annual weed is crabgrass preemergence herbicide (also called crabgrass preventer). You apply this product in the spring before the crabgrass seed sprouts. This granular herbicide works by creating a chemical barrier at the surface of the soil. As the seeds begin germination, they take in the herbicide and die.
The most cost-effective way to apply a preemergent herbicide is to use a fertilizer with crabgrass preventer added to it. These combination products are readily available in the spring and cost about $20 for a 5,000-sq.-ft. bag at garden centers. Apply it when you would normally apply your first application of fertilizer, and do it just before it rains to work both the fertilizer and the herbicide into the soil. The fertilizer will help thicken the turf. Thicker turf helps to squeeze out crabgrass plants missed by the herbicide. Common brands include Ferti- Lome’s Weed-Out, Sta-Green’s Crab-Ex Plus and Scotts Turf Builder.
This sounds easy enough, but where you’re likely to get it wrong is in the when. Apply too early and microorganisms and natural processes in the soil break down the herbicide. By the time it’s needed, much of the product has lost its potency. Apply too late and you’ve missed the early stage of germination when the herbicide works. There is nothing in preemergence herbicide that kills crabgrass seedlings once they’ve sprouted. You’re just pushing around that spreader for the exercise, not to mention wasting expensive products. As far as prevention goes, you’re out of luck until next spring.
So how do you select the exact instant for application? Don’t depend on the calendar. Pay attention to your grass instead. Fill up the spreader with preemergence granules between the second and third mowings of the year. The window is short, only about a week and a half, when the soil hits the ideal temperature—about 52 degrees F. You can also buy an inexpensive soil thermometer (sold at garden centers) to monitor soil temperature. If you’re going to err on timing your crabgrass preemergence application, err on the early side. While you’ll lose some effectiveness, you’ll still probably kill lots of crabgrass. If you go late, you’re likely to miss early sprouts.
If you’ve had a particularly bad crabgrass problem, you’re not done for the season. Chances are that the crabgrass will germinate and spring up later in the summer. Preemergence herbicides have a life of about 50 days (check the label; product life spans vary). Once that chemical barrier breaks down, dormant crabgrass seeds, which can remain viable for years, may germinate into seedlings. Or if your yard butts up against property that has a thriving crabgrass crop, you can bet that thousands of seeds will blow into your lawn, just when your herbicide is calling it quits. You don’t need to reapply the preemergence herbicide to your whole yard, but hit areas again where crabgrass thrives, like right next to driveways and walking paths. Because they absorb heat, the soil around them gets warmer and encourages the growth of crabgrass.
Reseed or Kill – Not Both
Herbicides that kill crabgrass will also kill desirable grasses such as bluegrass, ryegrass and fescue. If you treat your lawn with a preemergent, you cannot seed. And if you seed, you cannot use a preemergent herbicide. The solution is to control crabgrass in the spring and do your seeding in late summer or early fall, making sure to keep these two chores at least eight weeks apart. There are a few preemergent herbicides, such as Tupersan, that are compatible with newly established seed, but they’re expensive and can be hard to find.
Check the Key Ingredient
There are many different trade names for “weed and feed” products on the market. Chemical names can be confusing. Look carefully at the ingredients panel for dithiopyr, prodiamine or pendimethalin. These active ingredients, which are sold under various brand names such as Dimension, Barricade and Scotts Halts, will kill crabgrass in most areas of the country and in many different kinds of turf. However, it’s always wise to ask your local extension service which chemicals are best for your area and turf species.
OK, your lawn has been growing for a couple of months and you notice light green blades thickening up your Kentucky Blue. Before you think your lawn is having an exceptional season, think again: It's likely to be young crabgrass (see Photo 1).
Pulling, at this early stage, is a surprisingly effective way to get rid of crabgrass. But if the weed has pushed up three or four rows of leaves, inspect it carefully before you snatch it. If you spot a slender, green seed head that is still closed and folded up against the leaves of the plant, go ahead and pull it, too (Photo 3). However, after the seed head tines have spread out like a fork, leave it alone (Photo 4). Otherwise you'll scatter scads of seeds right over that nice big hole you've just created by removing the mature weed. You might as well be trying to cultivate new crabgrass!
Come fall, seed bare and patchy areas. With good lawn care practices, you'll soon crowd out those fallen crabgrass seeds.
Spray postemergence herbicide directly on crabgrass after it has sprouted. Pulling is equally effective, but if the roots are deeply embedded in your lawn, it may be tough to pull them out without pulling grass chunks too. It's not worth spraying a postemergence product on crabgrass that has gone to seed. It takes about two weeks for the herbicide to work, which is about how long it takes the plant to finish its seeding process. If it has gone to seed, you're better off waiting for next spring and applying a preemergence product then.
Post-emergence herbicides are most effective when the soil is moist and the plants are dry. Read the label for specific instructions. Typically you apply it with a hand pump sprayer. It's best to apply it on a hot day when there's low wind. If temperatures are too low, the product may be ineffective. Unless the crabgrass is young, you'll probably have to reapply the product a few days later (according to the label) to kill the plant.
After postemergence application(s), keep an eye on the treated area. In extremely dry conditions, water two days after the application to aid absorption. If your grass near the treated area is turning brown, you probably were a little heavy handed. Soak the damaged area with water to dilute the chemical and avoid further damage. Also be on the lookout for new crabgrass sprouts. These will require another herbicide treatment, or if there aren't too many, simply pull them. Be sure to seed these areas in the fall.
Don't waste your money on a postemergence herbicide in the fall, when the temperatures are falling. The herbicide won't be effective and the plant will soon die anyway.
Fight Crabgrass With a Healthy Lawn
The best way to stop crabgrass is to shade it out with a thick, healthy lawn. A thick lawn provides a dark canopy of grass blades over the seeds, so they won't sprout. Follow these good grass-care practices.
Watering: A thorough watering once a week will encourage the grass's root system to go deeper, making the whole lawn more hardy and heat tolerant. Avoid short, frequent waterings. These “sips” will promote a shallow, weaker root system in your lawn.
Mow: As a rule, grass should be mowed to a height of 2 to 3 in. Mowing it shorter than 2 in. will reduce the grass's vitality and give weeds a chance to move in. Be sure to keep your lawn mower blades sharp so they won't tear the grass. Leave grass clippings on the lawn as a natural fertilizer.
Reduce compaction: Weeds thrive in areas where compacted soil deprives the grass roots of the air and water circulation they need. If your yard is prone to compaction, rent and run an aerator over it every other year, especially if your soil contains a lot of clay.
Fertilize right: Avoid lawn fertilizers that say “quick green-up” on the label. These have excessive nitrogen ingredients that will actually weaken your lawn over time, making it more susceptible to weeds. Instead, select a fertilizer product with half of its nitrogen in a slow-release form. For a 1,000-sq.-ft. lawn, use less than 3 lbs. of nitrogen annually.
Reseed: Weed-damaged or thin areas should be seeded (sometimes called “overseeded”) in the fall, when the days are warm, the nights are cool and you have dew in the mornings.
Lawn near driveways, sidewalks and curbs or on south-facing banks absorbs a lot of heat during the summer months, which makes it more susceptible to crabgrass. Limit crabgrass growth in these areas by doing a targeted double treatment. After you’ve treated your entire lawn, go back and make another pass, about 6 to 8 ft. wide, along these areas (and make sure to sweep it off hard surfaces afterward). This will help keep crabgrass from taking hold along these heat absorbers.
While we all admire those who relentlessly defend their turf against crabgrass, there comes a time when the best strategy is to give up. That time is when your lawn only has 30 to 40 percent desirable grass left in a given area and the rest is lost to crabgrass and other weeds.
Begin by killing all the vegetation. On a low-wind day, apply a nonselective herbicide that is approved for lawn use, like Round-Up or Kleen-Up. Follow the label directions exactly. Depending on the product, weeds and grass will die and dry up in five to 14 days following application. Then renovation can proceed.
Thoroughly soak the area to give your new grass its best chance for a good start. Check your watering depth by pushing a spade into the ground and pulling it back to get a deep view of the soil. If the soil is moist to a depth of 6 to 8 in., you're ready.
For patchy bare areas and turf-free areas up to about 8 ft. square, use the spade technique for seeding (Photo 2). It's very effective, although it would be slow and tedious on areas that are much larger. Scuff up the dead vegetation with a rake and, using a spade, make 1/4-in.-deep furrows about 2 in. apart. Broadcast your grass seed, then flip a rake upside down and knock the seeds into the furrows. These furrows ensure that the seeds will make good contact with soil; they provide some moisture-retaining shelter as well. Then be sure to keep the seeds and soil moist. Continue to baby your new grass until after its first mowing. Do not apply crabgrass preventer to freshly planted areas.
Consider Chemical-Free Control Methods
Preemergent herbicides are the most effective and economical way to control crabgrass. But if you’d rather not use herbicides, you can try hand-weeding individual crabgrass plants in late spring before they get too big. They pull easily in soft ground after a rain.
Corn gluten meal (CGM), a corn byproduct, is another method used to control both crabgrass and broadleaf weeds such as dandelions and clover. It releases a protein that slows the development of weed seedling roots. CGM requires a heavy application rate (20 lbs. per 1,000 sq. ft.), which makes it cumbersome to use and expensive. It costs about $30 for 25 lbs. at garden centers.