Large crabgrass (Digitaria sanguinalis)
While crabgrass can resemble your grass at first glance, it does look very different upon further inspection:
Newly sprouted crabgrass can appear a light green (lighter than your turf) but may eventually turn a dark, dull green as it continues to grow.
The leaves of crabgrass are broader than grass blades.
Crabgrass grows in "clumps" low to the ground.
The stems of the grass grow laterally, or outward instead of upwards (like grass), and resemble the legs of a crab extending outward from the middle of the plant, hence the name of this weed.
Large crabgrass is a widely distributed summer annual grass that germinates throughout the summer. However, new seedlings may begin to appear as early as February. In California large crabgrass is found in the North Coast, southern Cascade Range foothills, Sierra Nevada foothills, Central Valley, central-western region, South Coast, and probably North Coast Ranges up to 3900 feet (1200 m) in elevation. It inhabits agricultural land and other disturbed sites. Once established, it is difficult to remove because it roots from portions of lower stems.
Crop fields, orchards, vineyards, gardens, landscaped areas, turf, nurseries, pastures, roadsides, ditches, and other disturbed places.
Seedlings sprout quickly. They are pale green and covered with coarse hairs. They form clumps with extensive but shallow roots where the soil is moist. Young leaves are rolled in the bud and unroll as they grow out of the center. Large crabgrass seedlings resemble those of witchgrass but are covered with shorter hairs.
Plants grow in clumps to about 2-1/2 feet (80 cm) tall, mostly upright, but sometimes prostrate. Stems are flat in cross-section, often purplish, and usually branched at the base. Leaves are flat, rolled in the bud and have a prominent midvein. It can be distinguished from smooth crabgrass, D. ischaemum, by its longer, narrower leaf, and presence of long stiff hairs on the sheath and on the upper and lower leaf surfaces. Leaves are about 1-1/4 to 8 inches (3–20 cm) long. In California smooth crabgrass is most often found in turfgrass whereas large crabgrass is more often found in gardens and landscaped areas.
There is a membranous ligule but no auricles.
Flowering takes place from June through October. Flowers cluster along 3 to 7 slender, fingerlike branches that are located toward the end of the flowering stem. Although similar in appearance to the flower head of bermudagrass, Cynodon dactylon, the spikelike flower branches of large crabgrass arise separately from the stem; those of bermudagrass radiate from the same point. Spikelets are lance to egg shaped, and mostly less than 1/10 of an inch (about 3 mm) long.
Reproduces by seed.