Crabgrass (Digitaria): how to identify it in your lawn

Crabgrass vs goosegrass, quackgrass and tall fescue

The table draws on NC State's pages for goosegrass and tall fescue and the RHS page on couch grass.

Goosegrass is the closest match in a summer lawn. NC State says it has a flat habit and is often white in the centre, like a wagon wheel. Unlike crabgrass, it does not root at the joints, and it sprouts when the soil reaches 60 to 65°F, usually at least two weeks after crabgrass. It favours compacted soil and heavily used turf, such as paths and sports fields.

Quackgrass, called couch grass in the UK, is a perennial. The RHS describes a grass up to 1 m tall with flattened leaves up to 40 cm long and a dense network of pale rhizomes that run in straight lines from the parent plant. If a clump comes up with long white underground stems, it is not crabgrass.

Tall fescue is a lawn grass in its own right, the most widely grown cool-season grass in North Carolina according to NC State, but it becomes a weed as coarse clumps in a finer lawn. As a perennial it is still there after frost has killed the crabgrass, and its blades are deeply ridged on top, glossy underneath and rough at the edges.

UC IPM adds two more: bermudagrass, whose finger-like seed-head spikes all start from one point, and dallisgrass, whose spikes are spaced widely along the stem. Both are perennials that spread underground.

For broadleaf weeds as well as grasses, see the lawn weed identification guide.

Where crabgrass grows, and why lawns get it

UC IPM says smooth crabgrass is most often found in lawns, landscapes and vegetable gardens, while large crabgrass tolerates close mowing less well and is more common in orchards, vineyards and other farmland. Both grow in vacant lots and on roadsides.

Timing matters. Seeds start to sprout when the soil reaches about 55°F for several days in a row, according to UC IPM, often earliest near warm pavement and buildings. In North Carolina, NC State gives March to early May, at soil temperatures of 53 to 58°F at a 4-inch depth, and notes that alternating dry and wet soil in spring encourages germination. The plants die with the first frost, which is why crabgrass leaves bare patches in fall.

NC State lists the conditions that favour it: thin, open turf, mowing at the wrong height for your grass, summer fertilizing and light, frequent watering. A single large crabgrass plant can produce more than 100,000 seeds, and UC IPM says they can stay viable in the soil for at least 3 years.

Controlling crabgrass in a lawn

Extension advice centres on keeping the lawn dense. UC IPM recommends choosing a grass suited to your area, mowing at the right height for it and removing no more than about a third of the leaf at a time, feeding at the right time of year, and watering deeply and less often rather than a little every day. Overseeding thin areas helps keep turf thick.

Stop it before it seeds. UC IPM says to pull established crabgrass plants before they set seed; in garden beds, hand pulling and hoeing while plants are young, followed by mulch, controls it well. Because the seed lasts years in the soil, a patch you let go to seed will be back for several summers.

Herbicides exist for crabgrass, but extension services treat them as a supplement to good lawn care. If you use one, follow the label and local rules, and ask your local extension office about timing for your grass type and region.

To track where patches appear from year to year, the Plant Map in Plant Identifier keeps each located identification. For the general method on any unknown plant, see how to identify plants, and for creeping broadleaf weeds in the same lawn, see creeping Charlie.