Crabgrass lives for only one summer, yet its roots and creeping stems let a single plant claim a surprising amount of lawn. It comes back every year because of seed, not because roots survive the winter.1Cornell University College of Agriculture and Life Sciences. Large crabgrass That makes lasting control mostly a matter of timing: stopping each new crop before it sprouts, or dealing with it while the plants are still small.
How Deep Crabgrass Roots Go and Why They Grip So Hard
Crabgrass has fibrous roots, a tangle of many fine roots rather than one main taproot. Those roots can reach up to 6.5 feet (about 2 m) deep.2Cornell University College of Agriculture and Life Sciences. Large crabgrass Most of the dense mass that actually holds the plant sits in the top few inches of soil, though. So when you pull, the deep roots usually tear off and stay behind, while the dense upper clump comes out with the crown.
A mature clump doesn’t give up easily, because hundreds of fine roots grip the soil at once. In dry, hard soil the roots snap and leave pieces behind. In moist soil the whole mass slides out more completely, and a big clump often lifts a patch of soil with it, leaving a bare spot.
That broken-root problem sounds worse than it is. A dandelion regrows from a piece of its single thick root, but a crabgrass root fragment left in the soil does not grow back into a plant. What matters is removing the crown, not every last root.
Deep roots don’t make crabgrass a perennial. The first hard frost kills it, and whatever roots remain over winter are dead, not dormant.
You can confirm you have crabgrass by looking at the base. Mature stems are flattened and turn red at the base, and the leaf sheaths wrapping the stem are flattened and hairy.3Cornell University College of Agriculture and Life Sciences. Large crabgrass A reddish, flattened base at soil level is a useful clue, though it’s best checked along with the hairy, flattened sheaths.
Crabgrass also does well where lawn grass struggles. It is well suited to hot summers and uses water and sunlight efficiently in heat.4Weed Research. Biology, Ecology, Distribution and Management of Large Crabgrass (Digitaria sanguinalis) It tends to thrive in hard, compacted soil.5Emerald Lawn Care. Compacted Soil: The Silent Saboteur of Healthy Lawns and Welcome Mat for Weeds
Rooting at the Stem Joints Lets One Plant Spread Wide
Crabgrass spreads mainly by seed, but it is one of the few annual weeds that can also spread from stems.6Cornell University College of Agriculture and Life Sciences. Large crabgrass Its outer stems lie flat on the ground and grow roots at the joints along the stem, especially the first joint.7Cornell University College of Agriculture and Life Sciences. Large crabgrass Single plants up to 10 feet (3 m) across have been seen.8Cornell University College of Agriculture and Life Sciences. Large crabgrass If a rooted joint gets cut off from the main plant, it can keep growing as a separate plant.9Rutgers New Jersey Agricultural Experiment Station. The Crabgrass Root System and How to Control It
By midsummer a spreading plant looks like a star-shaped clump, with stems radiating outward along the ground and smothering the lawn grass beneath them. Once seed heads form, each clump also starts producing next year’s seeds.
Mowing doesn’t slow this down. The low-growing stems sit under the mower blade, so they keep creeping.
Raking or string-trimming can actually split one plant into several. Tearing stems loose can leave rooted joints behind in the soil, and each rooted piece can keep growing on its own. When you pull a plant, lift the outer stems along with the center.
Small plants are far easier to deal with. A young plant has one center and hasn’t rooted at its joints yet. After it branches, there are several anchor points to remove. Acting in early summer means far less work than in August.
How to Pull Crabgrass So It Doesn’t Regrow
The key to pulling crabgrass is removing the entire crown, the plant’s center growing point.10Almanac.com. How to Get Rid of Crabgrass—and Stop It From Coming Back If the crown is left behind, the plant can grow back.11Almanac.com. How to Get Rid of Crabgrass—and Stop It From Coming Back
Water the area the day before so the roots release more easily. Grip the plant at the base and lift slowly, or pry it up with a weeding tool. Then fill the hole with soil and lawn seed so new crabgrass seeds don’t sprout there.
Plants that have gone to seed need more care. Pull them before the finger-like seed heads open, since shaking or dragging a seeding plant scatters seeds across the lawn. Bag them instead of composting, because seeds can survive a home compost pile.
Hand pulling stops making sense when the infestation is big. A few scattered plants are quick to pull. Dense patches leave large bare spots that fill with weeds again, so for widespread crabgrass a herbicide or late-season seeding works better.
Late in the season you can pull or wait. Frost will kill the plants anyway, but pulling before seeds drop cuts down next year’s crop. Dead clumps leave bare spots that should be reseeded in fall.
Killing Established Crabgrass With Herbicides
A postemergence herbicide kills weeds that are already growing. Fenoxaprop (sold as Acclaim Extra) moves slowly through the plant and can kill crabgrass that has already branched, often in a single application.12Penn State Extension. Postemergence Control of Crabgrass and Other Summer Annual Grasses in Lawns Quinclorac is another widely sold crabgrass killer for lawns, but some smooth crabgrass has become resistant to it.13Pesticide Biochemistry and Physiology. Mechanism of resistance to quinclorac in smooth crabgrass (Digitaria ischaemum) If quinclorac fails on a patch, switch to a product with a different active ingredient.
Read the label before you buy. Check that it lists crabgrass and your type of lawn grass, since some products can harm certain lawn grasses. Then follow its directions on how much to apply and when to water.
Timing matters. Young, actively growing plants die more easily than older ones, and drought-stressed plants take in less of the herbicide. Late-summer plants are about to die from frost anyway, so spraying them adds little.
It pays to look again a few weeks later. Spraying crabgrass that has already formed seed heads kills the plant but does nothing about the seed it has already shed on the soil. Because seeds keep sprouting through summer, new seedlings can show up after a spray, so a clean-looking lawn in July isn’t proof the job is finished.
Stopping Next Year’s Crabgrass Before It Sprouts
Soil temperature sets the timing. Crabgrass rarely sprouts while soil temperature is below about 59°F (15°C).14Weed Science. A Model for Predicting Large Crabgrass (Digitaria sanguinalis) Emergence as Influenced by Temperature and Water Potential The first seedlings in lawns appeared after a week of soil averaging around 57–63°F (14–17°C) at 1 inch deep.15Crop Science. Degree‐Days for Predicting Smooth Crabgrass Emergence in Cool‐Season Turfgrasses Large numbers of seedlings came up once soil averaged above about 73°F (23°C).16Crop Science. Degree‐Days for Predicting Smooth Crabgrass Emergence in Cool‐Season Turfgrasses Sprouting is fastest in warm soil around 77–95°F (25–35°C).17Planta Daninha. Abiotic Factors Affecting Seed Germination and Early Seedling Emergence of Large Crabgrass (Digitaria sanguinalis) So plan to apply a preventer before the soil warms to about 55°F (12–13°C).
A single warm afternoon doesn’t mean the soil is warm; what counts is the soil averaging near that temperature for several days, so check readings over a week. Waiting until the lawn looks fully green often means you are already late.
You can time it with signs in your yard. Forsythia blooming is a common local cue. An inexpensive soil thermometer pushed about 1 inch deep gives a direct reading. Bare, sunny spots next to driveways warm up first.
A preemergence herbicide is a weed preventer that stops seeds from sprouting, and it does nothing to plants already growing. Some crabgrass preventers also stop roots from forming at the stem joints of creeping grasses such as Bermuda grass for up to 12 weeks, but they did not harm roots that were already established.18Cambridge University Press. Influence of Herbicides on Root Development of Bermudagrass After applying one, wait to seed or sod the lawn according to the label.
Water it in, then leave it alone. The product needs watering in to form a barrier in the top layer of soil, and raking or aerating afterward breaks that barrier up. A split application, with a second dose about two months later, extends protection.
Mowing High and Fixing Soil to Crowd Crabgrass Out
Mowing bluegrass and fescue lawns at about 1.5–3 inches (4–8 cm) reduces crabgrass.19Crop Science. Cultural Management of Weeds in Turfgrass Taller grass shades the soil, keeping it cooler and blocking the light crabgrass seeds need to sprout. Scalping a lawn does the opposite: it exposes bare soil to sunlight and heat, exactly what the seeds want. Cut no more than about a third of the blade height at one time, since removing much more stresses the grass and thins the turf. A sharp mower blade makes a clean cut, so the grass loses less moisture and recovers faster.
Soil matters too. In one experiment, crabgrass seedlings grew less as soil pH rose, so liming very acidic soil may make crabgrass less able to compete with lawn grass.20Weed Technology. Effects of Soil Calcium and pH on Seed Germination and Subsequent Growth of Large Crabgrass (Digitaria sanguinalis) But another experiment found no change in crabgrass growth between pH 4.8 and 6.3.21Forage & Grazinglands. Soil pH Effects on the Shoot and Root Yield of Crabgrass Test your soil and lime only if the test shows it’s needed. Core aerating compacted soil also favors lawn grass over crabgrass.
Thickening the lawn in fall helps most. Overseeding fills the thin areas where crabgrass gets started, and a dense lawn leaves seeds no bare soil to land on.
Water deeply and less often. Light, frequent watering favors shallow-rooted weeds, while deep watering helps lawn grass roots grow deeper.
Is Crabgrass Ever Useful
Outside the lawn, crabgrass has a record as a crop. It can produce summer livestock feed on acidic soils.22Forage & Grazinglands. Soil pH Effects on the Shoot and Root Yield of Crabgrass Researchers are also testing a fungus that attacks crabgrass leaves as a natural weed killer, which reduced crabgrass growth by 60–76% in field trials.23Biocontrol Science and Technology. Isolation, pathogenicity and safety of Curvularia eragrostidis isolate QZ-2000 as a bioherbicide agent for large crabgrass (Digitaria sanguinalis) No such product is sold for home lawns.
The same traits that help on a farm, such as fast growth, heat tolerance, and spreading stems, make it hard to keep out of a lawn. Whether a plant is a weed depends on where it is growing and what you want from that spot.