Flower:

Erect, very slender, branching cluster 1½ to 6 inches long at the top of the stem. Panicle branches are appressed with a few to several short-stalked spikelets (flower clusters) per branch. Spikelets are lance-elliptic in outline, about 2 mm (less than 1/8 inch) long, overlapping along a branch but not tightly crowded, and usually have a single floret, occasionally 2.

At the base of a spikelet is a pair of bracts (glumes), both thin, whitish to purplish, veinless, hairless, rounded or sometimes notched at the tip, the lower glume less than .2 mm long or absent altogether, the upper glume up to .3 mm long and much shorter than the spikelet. Florets are surrounded by a pair of bracts (lemma and palea), both dark green to purplish, hairless, pointed at the tip, the lemma 3-veined, about 2 mm long with a straight awn 1.5 to 5 mm long at the tip; the palea is 2-veined and slightly shorter than the lemma. The thickened base of the floret (callus) is sparsely to moderately covered in long, white hairs.

Leaves and stems:

Leaves are alternate, 1 to 3 inches long, 1 to 4 mm wide, mostly flat, smooth to slightly rough-textured, hairless except sometimes for a few long hairs near the base.

Sheaths are hairless except for a few long hairs at the tip. The ligule (membrane where the leaf joins the sheath) is up to .5 mm long and may have a fringe of hairs. Nodes are smooth. Stems are slender, usually branched, hairless or sometimes minutely hairy below the nodes, prostrate from the base and arising at a lower node (geniculate), rooting at the nodes and forming loose clumps and mats of mixed flowering and vegetative shoots.

Fruit:

Mature florets drop off individually leaving the glumes behind on the stalk. Grains (seeds) are 1 to 1.4 mm long, amber-colored to brown.

Notes:

Muhlenbergia is a rather variable genus: clusters spike-like or an open panicle; glumes or lemmas awned, or neither; the callus (base of the floret) covered in long hairs or not; some with hairy stems or sheaths, others hairless; some branched, some not; clump forming or not; annual or perennial. What they have in common are spikelets usually single-flowered (occasionally with 2 or 3 florets), membranous ligules (occasionally also fringed with hairs), narrow leaves, glumes usually 1-veined, lemmas usually 3-veined, paleas 2-veined, and mature florets usually dropping off above the glumes (occasionally at the spikelet branch). There are about 70 species native to North America and more than 150 species worldwide.

Nimblewill has only been recorded a handful of times in Minnesota, where it reaches the northern edge of its range. It's been found on a sunny hillside, a wooded slope, a creek bottom, a wooded roadside, in a residential sidewalk crack, and in high-quality, semi-wooded transition areas between the upper portion of a bluff prairie and the surrounding oak woodlands. Of the 10 Muhlenbergia species known to be in Minnesota, it is the only one with underdeveloped glumes (the lower glume often absent altogether) and an awned lemma with a hairy callus. It has a mat-forming habit, rooting at the lower nodes, a few long hairs around the tip of the sheath but is otherwise essentially hairless.

Mat Muhly (Muhlenbergia richardsonis) is also mat-forming, but has awnless spikelets, well-developed glumes up to half as long as the spikeket, a hairless callus, and is usually found in wetter, high quality habitat such as calcareous fens, bogs and wet prairies. The spike-like panicle also resembles some Sporobolus species, which typically have some or all the panicle branches hidden inside the sheaths.