Why is my lawn full of crabgrass?

Crabgrass is a symptom of thin or stressed grass. Its seeds need open, sunlit soil to sprout, so it fills in wherever the lawn is mowed too short, worn down, underfed in fall or patchy after a hard summer. The fix is to close those gaps: mow taller, seed thin spots in fall, and put down a crabgrass preventer at the right moment next spring.

How crabgrass works

Crabgrass is a summer annual. Crabgrass starts germinating once soil temperatures hold at about 55°F and there is moisture.^[1] In Virginia that is usually late March to late April in the Piedmont, mid April to early May in the mountains, and early March to mid April on the coastal plain.^[1] It competes with the lawn all summer, sets seed in fall and dies with the first frost.^[1]

That cycle explains the pattern most homeowners see: a lawn that looked fine in April is lumpy and light green by July, then has brown, bare patches by November where the crabgrass died. Each plant leaves seed behind, so the problem grows year over year unless the grass fills the space first.

Make sure it is crabgrass

Smooth crabgrass and large crabgrass make up most of the crabgrass in Virginia lawns.^[2] Large crabgrass is hairy on both sides of the leaf, and its stems trail along the ground and root where they touch.^[1] Smooth crabgrass usually has no hairs, turns red at the base as it ages, and tolerates very close mowing.^[2] Both carry seedheads of three to five slender spikes on a thin stalk,^[1][2] which look like spread fingers.

The common mix-up is wiregrass, the name Virginians use for common bermudagrass. Bermudagrass in a cool-season lawn shows up as distinct brown patches in the colder months, when it is fully dormant.^[6] If the same patches go tan every winter and green up again in late spring, you are looking at a perennial, and a crabgrass preventer will not touch it.

Why it is winning in your lawn

The grass is mowed too short. Virginia Cooperative Extension's lawn guide traces most weed problems in Virginia to a single mistake: mowing the lawn too short.^[3] Short grass lets sunlight reach the soil, which is exactly what crabgrass seed is waiting for. Raising cool-season grass to 3 to 4 inches in mid to late spring, before summer stress, deepens roots and gives the grass more leaf to compete with.^[3]

The lawn is thin. Bare spots from summer drought, disease, dog traffic or shade are open ground. Edges along driveways and sidewalks are classic starting points, because they warm early and get scalped.

Feeding is backwards. Excessive spring nitrogen reduces a cool-season lawn's summer stress tolerance and can increase weed and disease pressure.^[3] Tall fescue does better fed mostly in fall, which thickens it before crabgrass season.

Watering at the wrong time. Extension advises against extra irrigation during the weeks weeds are expected to germinate.^[3]

Preventing next year's crop

Crabgrass preventers work by forming a barrier of herbicide on the soil surface that sprouting seedlings must grow through; plants that have already emerged are generally not affected.^[3] Timing is everything. Virginia Cooperative Extension advises applying during full forsythia and daffodil bloom, since crabgrass emerges when about half the forsythia blooms have fallen.^[3] For the Richmond area, Henrico County's Extension office puts the first application after forsythias bloom but before dogwoods, about March 15.^[4] Mountain lawns run later.

A preventer must be watered in by irrigation or rain, and light can break it down, so water it within a few days.^[3] Spread it evenly, since gaps in the barrier are where crabgrass breaks through.

How long it lasts depends on the rate. At full professional rates most preventers stop emergence for about two months, and many consumer products carry one-half to three-quarters of that rate.^[3] In a lawn with persistent crabgrass, two to three applications may be needed to carry control through the season,^[3] within the label's limits. A fall application is an acceptable second choice, but control will not last as long into summer as a spring application.^[3]

Plan around seeding. Seeding may be restricted for 60 to 120 days after a crabgrass preventer, so check the label before you plan to seed.^[3] Siduron and mesotrione prevent crabgrass without harming germinating cool-season grass,^[3] which makes them the choice if you must seed in spring. Better still, seed in late summer to mid-fall, the best time to establish cool-season grass, when the main weed worries are annual bluegrass and winter annual broadleaf weeds rather than crabgrass.^[5]

Dealing with crabgrass you already have

Early in summer, scattered young plants pull out easily after a rain. For larger infestations there are selective crabgrass killers. Quinclorac is the most common active ingredient for killing emerged crabgrass, but it does not control goosegrass or bermudagrass.^[3] Younger crabgrass is easier to kill than older plants, and treatments are typically repeated at two to four week intervals.^[3] Check that the product is labeled for your lawn grass.

By late August, it is usually better to let frost finish the job, rake out the dead mats, and put your effort into fall seeding and feeding. Our guide to getting rid of lawn weeds without killing grass covers application safety, and when to fertilize in Virginia sets out the fall feeding schedule.

Stop and call a pro if

• More than a third of the lawn is crabgrass or bare after frost. A full fall renovation with seeding usually beats patching.
• The "crabgrass" turns out to be wiregrass, nutsedge or another perennial that comes back from roots.
• A preventer or weed killer has damaged the lawn, or new seed failed to come up after a treatment.
• You would rather have a professional time and apply the products. Check the company's state credentials first; see what a lawn care company needs.

Browse more lawn and landscaping answers, compare fertilization and weed control services, or find lawn businesses around Blacksburg. To get a crabgrass and fertilization program quoted, describe your lawn once and local businesses respond.