Grass family (Poaceae)
Description: This adventive grass is a summer annual about ½2' tall that is tufted at the base; its culms branch occasionally. The lower culms are decumbent, while the upper culms are ascending to spreading. Each slender culm is terete, glabrous, and pale green to pale purple. The blades of the alternate leaves are up to 4" long and 1/3" across; they are light to medium green or blue-green, rather flat, and mostly hairless; sometimes there are scattered white hairs toward the base of the blades. The inflated sheaths of the leaves cover most of the culms; these sheaths are finely veined, hairless, and the same color as the blades. The upper culms terminate in digitate clusters of 2-6 racemes. These racemes are very narrow, spike-like, and up to 5" long. Each raceme has 2 rows of spikelets and a flattened central stalk (or rachis) about 1 mm. across. These spikelets occur in pairs or individually along the racemes on very short pedicels. Each spikelet is about 2 mm. long, dark green or purplish green, ellipsoid or ovoid in shape, and flattened; it consists of a pair of glumes, a pair of lemmas, and a single floret. The outer sides of the spikelet consist of the 2nd glume and the outer lemma; the 1st glume is small and insignificant, while the inner lemma is dark-colored and hidden from view. The blooming period occurs from mid-summer into the fall. Pollination is by wind. The small seeds are ellipsoid or ovoid and flattened. The root system is shallow and fibrous. This grass reproduces primarily by reseeding itself, although it can spread vegetatively by forming rootlets at the lower nodes of its culms.
Cultivation: This weedy grass is typically found in full to partial sun, moist to mesic conditions, and a somewhat heavy soil containing loam or clay-loam. It can spread aggressively in lawns. Most vegetative growth occurs during the summer as the seeds are slow to germinate.
Range & Habitat: Smooth Crabgrass is occasional to locally common throughout Illinois (see Distribution Map). It is adventive from Eurasia. Habitats include weedy meadows, trampled ground along paths, fields and pastures, vacant lots, lawns, edges and cracks of sidewalks, roadsides, and waste areas. Areas with a history of disturbance are strongly preferred.
Faunal Associations: Various insects feed on Digitaria spp. (Crabgrasses), including the caterpillars of Hylephila phyleus (Fiery Skipper) and the moth Mocis texana (Texas Mocis). The seeds of Crabgrasses are an important source of food to many species of upland gamebirds and granivorous songbirds (see Bird Table). The foliage is eaten occasionally by mammalian herbivores (e.g., rabbits and livestock).
Photographic Location: The trampled ground of a path along a powerline clearance in Busey Woods, Urbana, Illinois.
Comments: This weedy Digitaria sp. (Crabgrass) is less common and slightly less aggressive than Digitaria sanguinalis (Common Crabgrass). The latter species is a little larger in size and has hairy sheaths; it more common in dry sunny areas than Smooth Crabgrass. Both of these species are native to Eurasia, rather than North America, and they are common pests in lawns. In high quality natural areas, they are rarely observed. There are also native Digitaria spp. in Illinois, but they are less common and largely confined to sandy areas (including sand prairies). They are more erect than the Eurasian Crabgrasses and the stalks of their racemes are more narrow and wiry (less than 1 mm. across).