Flower:

A whorl of 2 to 6 spike-like clusters 1½ to 4 inches long in a finger-like array at the tips of branching stems. Spikelets (flower clusters) are elliptic to egg-shaped, 1.7 to 2.2 mm long, are mostly paired along the stem, one short-stalked and the other longer stalked, with 1 fertile flower per spikelet. The glumes (2 bracts at the base of a spikelet) are unequal in length, the first glume—on the front of the spikelet—is inconspicuous or absent and the second—on the back—is short-hairy and about as long and wide as the spikelet. The lemma (2 bracts surrounding a flower) are also about as long as the spikelet, the sterile lemma short-hairy, the fertile lemma covered in tiny pits. Flower styles and stamens poke out from the tip of the spikelet. The spike stem (rachis) is green or purple, about 1mm wide and winged, the wings usually broader than the rachi, with the spikelets all arranged on one side.

Leaves and stems:

Leaves are all alternate, up to 4 inches long and to ¼ inch wide, hairless except at the sheath, and mostly flat.

Sheaths are open at the front, often with a few long hairs at the tip or along the edge near the top but are otherwise hairless, loosely surround the culm (stem), and usually keeled (ridged) on the back. The ligule (membrane where the leaf blade meets the sheath) is white and less than 1mm long. Nodes are hairless and green to purplish. Culms are hairless, multiple from the base, branching, mostly covered by the sheaths in the lower plant, and light green but often tinged purple towards the base. Culms are typically sprawling, radiating out from the base, and may root at the nodes.

Fruit:

The spikelets turn dark purplish as they mature, the absence of the first glume and hairs on the second glume and lemma more evident. Seed is oval-elliptic, dark brown to nearly black.

Notes:

The two weedy crabgrasses in Minnesota are easily recognizable from their finger-like arrays at the tips of stems and are both common weeds of lawns, gardens, trail edges, empty lots and pavement cracks. The other is Hairy Crabgrass (Digitaria sanguinalis), which has hairy sheaths, a small but distinct first glume, second glume about half as long and wide as the lemma, and (when not repeatedly mowed over) is a rather larger plant with up to 15 spikes at stem tips. Some references list 2 varieties of Digitaria ischaemum, but they are not currently recognized in Minnesota.