Flower:

Branching cluster at the top of the stem, 2½ to 14 inches long, the 8 to 25 branches spreading to ascending or sometimes erect, up to 3+ 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 5 mm long (excluding awns), green to dark purplish, flattened on 1 side, with 1 sterile and 1 fertile floret but appearing single-flowered. Long hairs are commonly scattered on the spikelet stalks and sometimes at the base of a branch; these hairs usually have a pimple-like base (papillose) and are usually shorter than the spikelet, rarely longer.

At the base of a spikelet is a pair of bracts (glumes), the lower glume more or less 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 are usually papillose. 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 20 mm (to ~¾ inch) long and may be 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, tapering to a sharply pointed tip, the edges rolled around the edges of the similar palea.

Leaves and stems:

Leaves are alternate, to 10+ inches long, to 1+ inches (1 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, erect or prostrate from the base and rising at a lower node (geniculate) or near the tip (decumbent), sometimes rooting at a lower node.

Fruit:

The whole spikelet drops away when mature, leaving a naked stem behind. Grains (seeds) are 1.2 to 2.5 mm long, yellowish, and enclosed within the persistent lemma and palea.

Notes:

Rough Barnyard Grass is a common warm season grass of wetland edges, shores, river and creek banks, floodplains, abandoned fields and the occasional roadside, usually in moist to wet sandy or gravelly 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. muricata is distinguished by the minute details of its fertile florets, specifically the sharply pointed tips on lemmas and paleas that have no clear separation from the shiny, hardened body. Hairs on the glumes and sterile lemmas usually have enlarged, pimple-like bases (papillose). There are 2 recognized vars, both of which are found in Minnesota: var. muricata spikelets are 3.5 to 5 mm long, the sterile lemma usually with an awn 6 to 20 mm long; var. microstachya, the more common, has spikelets 2.5 to 3.8 mm long with sterile lemmas awnless or awns up to 10 mm long.

Without strong magnification (25x), it is very difficult to distinguish from the introduced Barnyard Grass (Echinochloa crus-galli), which has a membranous tip on lemmas and paleas, the lemma tip with a line of minute hairs separating it from the hardened body, and only some hairs may be 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 membranous tip often folded down like a flap. 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.