10. Panicum dichotomum L.

Pl. 164 i, j; Map 671

Dichanthelium dichotomum (L.) Gould

Plants perennial, without rhizomes, forming tufts. Flowering stems 15–80(–100) cm long, erect to spreading, initially unbranched, but becoming highly branched during the summer and autumn (often becoming top‑heavy and leaning or spreading), the nodes glabrous or bearded with long, spreading to downwardly pointed hairs, the internodes glabrous or the lowermost ones rarely sparsely hairy. Foliage in a dense rosette of shorter, somewhat broader leaves at the base and longer, narrower leaves well distributed along the main stems, those of the later branches usually appearing as axillary fascicles of even shorter, narrower leaves. Leaf sheaths glabrous or hairy along the margins, sometimes with pale, glandular spots, the ligule 0.3–1.0 mm long, an inconspicuous line or band of stiff hairs, rarely absent. Leaf blades of the main stems 3–12(–15) cm long, 3–13 mm wide, more or less spreading, abruptly rounded to truncate at the base and not clasping, glabrous or less commonly sparsely hairy on the surfaces and along the margins near the base. Inflorescences 2–10 cm long, the earlier ones larger and more open with mostly spreading to loosely ascending branches (later ones sometimes reduced to small clusters of long‑stalked spikelets), the branches mostly rebranched 1 or more times, not spikelike or 1‑sided, the spikelets appearing mostly long‑stalked at the tips of the branches. Spikelets 1.5–2.5 mm long, elliptic to slightly obovate in outline. Lower glume 0.5–0.8 mm long, about 1/3 as long as the spikelet, broadly ovate, bluntly to sharply pointed at the tip, glabrous or sparsely hairy, nerveless or faintly 1–5‑nerved. Upper glume 1.4–2.5 mm long, elliptic, rounded, or rarely pointed at the tip, 5–9‑nerved, glabrous or hairy. Lowermost floret usually sterile and with a reduced, inconspicuous palea, the lemma 1.4–2.5 mm long, elliptic, rounded or rarely pointed at the tip, 5–9‑nerved, glabrous or hairy. Fertile floret 1.2–1.9 mm long, elliptic, rounded to bluntly pointed at the tip. Anthers 0.2–0.8 mm long. 2n=18. May–September (vernal), July–November (autumnal).

Common in the Ozark, Ozark Border, and Mississippi Lowlands Divisions, uncommon elsewhere in the state (eastern U.S. and adjacent Canada west to Minnesota and Texas; Mexico, Central America, Caribbean Islands). Upland prairies, glades, mesic to dry upland forests, bottomland forests, acid seeps, fens, ledges of bluffs, margins of ponds and lakes, and banks of streams, spring branches, and rivers; also old fields, roadsides, railroads, and open, disturbed areas.

Taxonomic treatments of the P. dichotomum complex have varied greatly in terms of the number of taxa and levels in the classification recognized by various authors (reviewed by Lelong, 1984). The present treatment follows that of Lelong (1984), who recognized seven varieties of P. dichotomum, five of which occur in Missouri. Although there is considerable overlap between some of these, for the most part the Missouri specimens are more easily assignable to varieties than are specimens of the closely related and even more difficult P. acuminatum. Perhaps this is because only two of the varieties are relatively common in the state, the others being known from only one or a few collections. The following key is adapted from those of Lelong (1984, 1986).