Setaria pumila subsp. pumila is an annual C4 grass (Downton 1971), tufted, 30-100 cm, erect to prostrate from fibrous roots, branching from base, stems somewhat flattened; sheath hairless (smooth), flattened, keeled; ligule a dense band of short hairs to 0.3 cm.
Leaves alternate, to 30 cm long 0.4-1.0 cm wide, blade twisted in a loose spiral, rough above, usually loosely hairy at base on inside, hairs with swollen bases (Hitchcock 1950).
Flower spikelets to 0.3 cm, second glume only ½ as long as the sterile lemma, which looks like a third glume on the flat side of the finely wrinkled, green to purplish fertile lemma; inflorescence spike-like, stiffly, cylindrical, 5-10 cm, erect, axis bristly-hairy, bristles 4-12 below each spikelet, yellow, 0.3-1 cm long yellow to tan at maturity; blooms and fruits July-Oct. Seeds eaten by numerous birds and small mammals (Martin 1951). Seeds germinate May-June.
Wetland status: FAC.
Frequency in NYC: Infrequent in natural areas.
Habitat: Common along roadsides, late summer-autumn, open, disturbed areas. Often forming large stands on open soil (not adequately represented by samples collected). Very common first year successional annual plant, requires high nitrogen soils, very intolerant of shade (Gregg 1972), intolerant of salt, fire, drought; soil pH 5-7 (USDA, NRCS 2010).
Notes: Plants eaten by rabbits (Martin 1951). An alternate host to the northern corn rootworm, Diabrotica longicornis (Chrysomelidae, Coleoptera). It is infected by the fungi: Claviceps purpurea (ergot, Ascomycotina); downy mildew, Sclerospora graminicola; brown root rot, Pythium graminicola, (water molds, Mastigomycotina); the smut Ustilago neglecta (Basidiomycotina) (Agrios 1988; Steel et al. 1983). Host of the rusty plum aphid (Hysteroneura setaria) first discovered on yellow foxtail (Hilty 2006).