Flower:

Branching cluster at the top of the stem, 2 to 10 inches long, the 5 to 25 branches spreading to ascending, up to 4 inches long, the longer branches usually further branched with short branchlets. Spikelets (flower clusters) are crowded on one side of a branch, 2.5 to 4 mm long (excluding awns), green to purplish, flattened on 1 side, with 1 sterile and 1 fertile floret but appearing single-flowered. Long hairs are commonly at the base of main branches and scattered on the spikelet stalks; these hairs may be longer than the spikelets.

At the base of a spikelet is a pair of bracts (glumes), the lower glume nearly half as long as the spikelet, pointed at the tip, the base completely wrapped around the spikelet, the upper glume as long as the spikelet, 5-veined, sparsely hairy at least along the veins, pointed at the tip or tapering to a straight awn less than 2 mm long; hairs may or may not have an inflated, pimple-like base. Surrounding a floret is a pair of bracts (lemma and palea), the lower lemma sterile and like the upper glume but awns varying from less than 1 mm to 50 mm (to 2 inches) long, usually variable within a branch; the lower palea is thin and nearly as long as its lemma. The upper lemma is fertile, as long as the upper glume or nearly so, shiny and hardened, lacks an awn, the tip membranous and withering with a line of minute hairs separating it from the hardened body; the palea is similar but hairless, the hardened portion rounded at the tip, the membranous portion commonly folded down like a flap.

Leaves and stems:

Leaves are alternate, to 24+ inches long, ¼ to 1+ inches (5 to 30mm) wide, flat and hairless. Sheaths are usually hairless, occasionally with a few hairs along the edges, and lower sheaths are commonly purplish. The ligule (membrane where the leaf joins the sheath) is lacking, the juncture smooth and pale to purplish. Nodes are hairless, sometimes the lower nodes minutely hairy. 

Stems are hairless, usually branched near the base, multiple from the base forming clumps, sometimes erect but more often prostrate from the base and rising at a lower node (geniculate) or near the tip (decumbent).

Fruit:

The whole spikelet drops away when mature, leaving a naked stem behind. Grains (seeds) are 1.3 to 2.2 mm long, brown, and enclosed within the persistent lemma and palea.

Notes:

Barnyard Grass, introduced from Eurasia, is one of the most common weeds world-wide and is noted as a serious agriculture pest, especially in rice fields, said to significantly reduce crop yields by removing as much as 80% of soil nitrogen. In Minnesota, it is a robust annual typically found along roadsides, shores, river and creek banks, trail edges, meadows, agricultural fields and other areas with disturbed soil.

Echinochloa is recognized by its absent ligule, (usually) awned spikelets that have 1 sterile and 1 fertile floret, and the lower glume rather shorter than the spikelet. Of the 3 Echinochloa species in Minnesota,  E. crus-galli is distinguished by the minute details of its fertile florets, specifically the membranous tips on lemmas and paleas with the line of minute hairs separating the lemma tip from its shiny, hardened body. Hairs on the glumes and sterile lemmas may or may not have an enlarged, pimple-like base (papillose). Lemma awn lengths are highly variable, frequently variable within a branch; it is not uncommon to find most spikelets nearly awnless with just a few long-awned. Some references list multiple vars of E. crus-galli but they are not recognized in Minnesota.

Without strong magnification (25x), it is very difficult to distinguish from the native Rough Barnyard Grass (Echinochloa muricata), which lacks the membranous tip on lemmas and paleas and hairs are more consistently papillose. While other references make much ado about the differences in the lemma tips, from our observations the differences between the fertile paleas may actually be more easily seen—E. muricata is solid all the way to the sharply pointed tip where E. crus-galli has a pretty obvious separation and is decidedly rounded. The third Echinochloa species in Minnesota, Walter's Barnyard Grass (E. walteri), is distinguished by its upper glumes having awns more than 2 mm long, its generally larger and often nodding panicle, and sheaths are usually hairy.