21. Panicum philadelphicum Bernh. ex Trin. (Philadelphia witchgrass)

Pl. 170 e–g; Map 682

P. philadelphicum var. tuckermanii (Fernald) Steyerm. & Schmoll

P. tuckermanii Fernald

P. gattingeri Nash

Plants annual, forming tufts. Flowering stems 10–60 cm long, relatively slender, more or less circular in cross‑section, sparsely hairy, especially near the nodes. Leaves scattered along the stems. Leaf sheaths rounded on the back, hairy, the hairs with pustular bases, the ligule 0.5–1.3 mm long, a line or band of hairs. Leaf blades 2–15 cm long, 3–8 mm wide, relatively soft, ascending to spreading, glabrous or more commonly hairy, often yellowish green, flat. Inflorescences 5–25 cm long, up to 1/3 as long as the entire flowering stem, less than 2 times as long as wide, remaining attached to the flowering stem at maturity, not becoming a “tumbleweed,” the primary branches ascending to mostly spreading, not spikelike, with tufts of hairs at the bases, rebranched several times, the spikelets not appearing 1‑sided, single or in small clusters (often paired) at the branch tips, mostly short‑stalked, not curved or angled with respect to their stalks. Spikelets 1.6–2.4 mm long, elliptic to ovate in outline, narrowed or tapered to a short, sharp point at the tip, glabrous. Lower glume 0.5–1.2 mm long, 1/3–1/2 as long as the rest of the spikelet, ovate, bluntly to sharply pointed at the tip, 3‑ or 5‑nerved. Upper glume 1.4–2.4 mm long, elliptic, abruptly narrowed or tapered to a short, sharp point at the tip, usually 7‑nerved. Lowermost floret sterile and without a palea, the lemma 1.4–2.4 mm long, elliptic, 7‑ or 9‑nerved. Fertile floret 1.3–2.0 mm long, elliptic, bluntly pointed at the tip, tending to darken or turn blackish at maturity. Anthers 0.7–1.0 mm long. Fruits 1.2–1.5 mm long, broadly elliptic in outline. 2n=18. July–October.

Scattered nearly throughout Missouri, but most common south of the Missouri River (eastern U.S. and adjacent Canada west to Minnesota and Texas). Glades (on both calcareous and acidic substrates), openings of mesic to dry upland forests, gravel bars and banks of streams, and margins of sloughs; also moist, open, disturbed areas.

Minor variations in inflorescence pubescence and spikelet size are too variable to permit segregation of varieties or species from P. philadelphicum (Zuloaga and Morrone, 1996).