Yellow Foxtail is an annual warm season non-native grass with erect or low-lying stems, growing from 1/2 to 4 feet high; with stems branching from the base. Stem nodes are swollen and usually reddish-purple at maturity.

Leaf blades are up to 3/8 inch wide (10 mm) and up to 10 inches (25 cm) long. Blades can be flat or V-shaped, twisted, drooping, distinctly veined, slightly rough to smooth on upper and lower surfaces. Blades of ssp. pumila are yellowish-green. Ligules are 1 mm long with a fringe of fine hairs fused at the base. The blade sheath is split, sharply keeled and distinctly veined and with long twisted hairs near the base of the upper surface.

Inflorescence: An erect panicle with the spikelets in a tightly packed cylindrical shape 2 to 4+ inches long, densely bristly.

Spikelets and florets: The spikelets are closely crowded, with one fertile floret and a sterile lemma (usually staminate only), each subtended by a cluster of 4 to 12 upwardly barbed bristles, 3 - 8 mm long, which in ssp. pumila are yellow. Spikelets are 3 to 3.4 mm long, much shorter than the bristles, and strongly swollen on one side (widest at the middle) and flattened on the opposite side. The lower glumes are about 1/3 the length of the spikelets and 3-veined. The upper (2nd) glumes are about 1/2 the length of the spikelets and 5-veined. The lower lemmas and upper lemmas are about equal to each other, oval in shape. The lower is veined and sterile (or staminate only). The upper is fertile and transversely ridged right up to the tip, with the widest point at the middle. There are 3 anthers and 2 styles.

Seed: Seeds are darkly yellowish-brown, ovoid, about 3 mm long, with the palea and wrinkled lemma usually adherent.

Subspecies: The other major subspecies, ssp. pallidefusca, has reddish bristles and the spikelets are 2 to 2.5 mm long, but that subspecies is known in only two states and not found in this area although one population was once reported in Anoka County.

Habitat: Yellow Foxtail grows from fibrous roots in open places and in cultivated areas. It thus does well in sunny area with mesic to dry mesic moisture conditions. It is generally considered weedy and unwanted and can be an impurity in commercial clover and grain seeds.

Names: Yellow Foxtail was formerly classified as Setaria glauca. The genus Setaria is derived from the Latin seta, meaning a 'bristle', and aria, meaning 'possessing', referring to the bristly inflorescence. The species name, pumila, means 'dwarf', referring the size of the plant compared to others in the genus. The author names for the plant classification are complex. The first author, ‘Poir.’ refers to Jean Louis Marie Poiret (1755-1834) French clergyman, botanist and explorer and Professor of Natural History at the Ecoles Centrale of Aisne. His work was updated by two others: ‘Roem.’ is for Johann Jacob Roemer (1763-1819) Swiss botanist who with Joseph Schultes published the 16th edition of Linnaeus’ Systema Vegetabilum and ‘Schult.’ is Roemer's co-author Josef August Schultes (1773-1831), Austrian Botanist.

Comparison: There is also a Green Foxtail, not found in the Garden, S. viridis, whose bristles number only 1 to 3 and are 5 - 10 mm long, with smaller spikelets, only 2 - 2.5 mm long.