Crabgrass is a common weed that almost everyone knows. There are two species of crabgrass common in USA: smooth crabgrass, Digitaria ischaemum, and large or hairy crabgrass, D. sanguinalis. Both species were introduced from Eurasia and are widespread throughout the United States. Crabgrass is found in turfgrasses (mostly smooth crabgrass) and in ornamental landscapes (primarily large crabgrass). Large crabgrass is also found in orchards, vineyards, and other agricultural areas. Crabgrass also has many other names including crowfoot grass and summer grass

IDENTIFICATION AND LIFE CYCLE

Smooth crabgrass is a low-growing, summer annual plant that spreads by seed and from rootings of the culm nodes (joints) that lie on the soil. When unmowed it will grow upright to about 6 inches, but it will tolerate mowing in turf at 1/4-inch and will still produce seed at this height. Seedling leaves are light green and smooth. True leaves are dark green and smooth, and the leaf blade is from 1/4- to 1/3-inch across, up to 5-inches long, and pointed. Crabgrass often forms patches in lawns, and plants can grow together to form large clumps. The ligule (collar) is small and inconspicuous without prominent appendages or auricles. The leaf sheath and upper leaf surface are smooth, but a few hairs may be found on the lower leaf surface. There may be a reddish tint at the base of the leaf. The inflorescence (flower stalk) has branches that originate from the main stem at 1/8- to 1/4-inch intervals. The branches are 1/2- to 2-1/2-inches long at the end of the stalk.

When found in turf, large crabgrass is a low-growing summer annual that spreads by seed and from rootings of nodes that lie on the soil. When unmowed it may grow upright to a height of 2 feet. It will not tolerate close mowing as well as smooth crabgrass. Seedling leaves are light green and hairy. True leaves are generally 3 inches long and hairy on the upper surface of the leaf and leaf sheath. The collar region and flower stalk are similar to that of smooth crabgrass, but the branches are longer, about 2 to 5 inches, at the end of the stalk.

The flowering stems of crabgrass are similar to those of bermudagrass (Cynodon dactylon), but the spikelike branches on the flowering stems of bermudagrass originate at the same point whereas those on crabgrass originate about 1/8- to 1/4-inch apart at the end of the stem.

In southern California, the major germination period for both crabgrass species is from January 15 to early April, depending on the temperature, and seeds continue to germinate throughout spring and summer. While germination is early in warm winter areas, growth is slow during spring months until mid-May. In June and July the plants produce tillers and shoots, and flower in late July and August. In the absence of a frost, crabgrass may overwinter in warm areas or during warm winters and produce new growth and a second crop of seed in spring or early summer.