Flower:

2 to 15 spike-like clusters 2 to 7 inches long in 1 to 3 whorls, forming finger-like arrays at the tips of branching stems. Spikelets (flower clusters) are lance-elliptic, 3 to 3.5 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 triangular and less than 1 mm long but distinct, and the second—on the back—is short-hairy and about half as long and wide as the spikelet. The lemma (2 bracts surrounding a flower) are about as long as the spikelet, the sterile lemma short-hairy, the fertile lemma greenish gray to light brown. Flower styles and stamens poke out from the tip of the spikelet. The spike stems (rachis) are green or purple, about 1mm wide and winged, the wings usually broader than the stem, with the spikelets all arranged on one side.

Leaves and stems:

Leaves are all alternate, 2 to 10 inches long and up to 2/3 inch wide, mostly flat, and densely to sparsely hairy, occasionally hairless or becoming hairless with age.

Sheaths are open at the front, loosely surround the culm (stem), usually keeled (ridged) on the back, and variously covered in long, spreading hairs. The ligule (membrane where the leaf blade meets the sheath) is white to brownish and 1 to 3mm long. Nodes are green to purplish and usually hairless. 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 may be erect to ascending or sprawling, radiating out from the base, and root at the nodes.

Fruit:

The spikelets turn dark purplish as they mature, the first glume and hairs on the second glume and lemma more evident as spikelets dry. Seed is lance-elliptic, gray to light brown.

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 Smooth Crabgrass (Digitaria ischaemum), which has mostly hairless sheaths, no distinct first glume, second glume about as long and wide as the lemma, and is a rather smaller plant with up to 6 spikes at stem tips, though repeated mowing can result in both species having a similar, small size. We suspect this is a highly under-reported species and is likely much more widespread in the state than current records indicate.