Crabgrass (Digitaria species) is a low-growing, sparse grass that has invaded gardens and yards in every part of North America. It’s an annual that self-seeds readily and can grow seed even if you cut it down as low as a quarter inch above the ground.
The most widespread species originated in Eurasia and traveled to North America after the U.S. Patent Office brought it over in 1849 to see if it could be used as a forage crop for animals. It escaped cultivation and has become an unwelcome weed in lawns and gardens.
Not all crabgrass are alien. There are over a dozen species indigenous to North America. All species prefer dry, warm environments, and all are considered weeds when they grow in cultivated areas.
Range
Different species appear in different areas throughout the world. In North America, smooth crabgrass (D. ischaemum) and large or hairy crabgrass (D. sanguinalis) are the most common species, and they can be found in every state, including Hawai’i.
The plant grows in turfgrass, lawns, ornamental gardens, disturbed areas, vineyards, field crops, and abandoned lots. It prefers dry, bare spots.
Identifying
Crabgrass is a low-growing plant, spreading out wider than it does high, though some species grow up to two feet tall. The stems are multi-branched with long leaves extending from the stems.
The young seedlings are bright green, which makes them easy to identify against the deeper green of turfgrass. These are half as wide as they are long. They look similar to a small, young corn stalk.
The mature foliage is darker green and more long and narrow, making it difficult to differentiate from lawn.
The leaf blades are long, pointed, and most species are smooth though they have fine, silky hairs. They grow up to six inches long and up to an inch wide.
Crabgrass leaves lack distinct auricles. This is the area where the leaf clasps the stem and is prominent on many, but not all, true grasses. The ligule, which is a little outgrowth where the sheath and the stem join, is tinted purple and covered in stiff hairs.
Flower stalks, known as inflorescences, emerge from the main stem and form every half inch or so, depending on the species.
Crabgrass can change its appearance to adapt to its environment, making identification more challenging. When competing with taller plants, crabgrass will take on a more upright habit. It takes on a more spider-like, sparse, sprawling habit with leaves spread far apart.
Plants in some shade tend to be darker than those in full sun. Some plants have a purple streak in cooler regions, and some take on a bluish hue in moist areas.
Lookalikes
Goosegrass (Eleusine spp.) grows in the same dry, open areas that crabgrass prefers. Goosegrass has white stems radiating out of the center and has no hairs on the leaf sheath. Goosegrass requires warmer temperatures to germinate. The seeds remain dormant until the temperatures are above 65°F. The seeds are larger and the corresponding racemes are larger, as well.
Dallisgrass (Paspalum dilatatum) has a large underground rhizome system, which crabgrass lacks. Dallisgrass is a perennial and will return each year from the same root system.
Bermudagrass (Cynodon dactylon), as with dallisgrass, has a rhizomatous root system. Bermudagrass is grown as a lawn species in parts of North America, meaning its a welcome plant rather than a weed in those areas.
Bermudagrass is more upright and grows thicker than crabgrass, though the leaves look similar.
Control
Crabgrass spreads both by seed and by rooting where the leaf joints, known as culm nodes, touch the ground. Seed heads start forming in midsummer and continue until the frost.
One plant can produce tens of thousands of seeds, so controlling relies on preventing the plant from going to seed. Once the seeds drop from the ground, they sit dormant on the surface of the soil until temperatures are consistently at 55°F or above for five days. Then they germinate and begin growing.
Because of the abundance of the seeds, control will take several years. The first year will kill the existing plants, and the second year will kill plants that germinated from the previous year’s seeds. A third year might be necessary if any plants are able to go to seed.
In small gardens, hand cultivation before the weed goes to seed is all that’s necessary to eliminate the problem, but it requires consistency.
Otherwise, herbicides are necessary, particularly in large, established patches of crabgrass.
If crabgrass is growing in your lawn, the best strategy is to maintain your lawn so that it outcompetes with crabgrass. Most lawn species are dormant in hot weather, while crabgrass thrives in it, making it difficult to maintain the health of the lawn over the weed.
Keep your lawn at the recommended height for the species and feed it as directed. A majority of species do best right around three to four inches. You should also ensure you’re following best watering practices and that the lawn has appropriate sun exposure.
Remember that deep, infrequent watering is better than frequent, shallow watering. Don’t fertilizer during the summer because most lawn is at least partially dormant during the summer, but crabgrass isn’t and it will use the nutrients to grow rapidly.
It might be necessary to use crabgrass-specific herbicides if a lawn is infested with many large patches of the weed. There are both preemergent and post-emergent herbicides available for crabgrass. Both might be necessary depending on how extensive the infestation is.
Preemergents contain ingredients such as benefin, oxadiazon, trifluralin, pendimethalin, dithiopyr, prodiamine, or corn gluten. Post-emergents that contain quinclorac are safest for lawns.
Reseed your lawn each spring while you are battling crabgrass to allow your lawn to fill in the areas that have been left bare and to outcompete the crabgrass.
