24. Cyperus rotundus L. (purple nutgrass, coco grass)

Pl. 73 j, k; Map 268

Plants perennial with long, wiry rhizomes with ovoid, tuberlike thickenings. Aerial stems 10–50 cm long, sharply trigonous, smooth. Leaf blades 5–30 cm long, 2–6 mm wide (sometimes folded longitudinally and thus appearing narrower), shorter than the stems. Inflorescences irregular umbels with usually 1 sessile spike and 3–7 rays, each ray smooth, rarely branched again near the tip, each ray or branch ending in a spike. Inflorescence bracts 2–5, mostly shorter than the rays, spreading to somewhat ascending. Spikes 10–25 mm long, with 3–10 spikelets, broadly elliptic-ovate in outline, relatively open, appearing somewhat flattened, the spikelets spreading to ascending, attached alternately on the axis, the spikelet bases readily visible. Spikelets 4–40 mm long, linear to narrowly elliptic, pointed at the tip, strongly flattened in cross-section, with 12–40 florets, the fruits and scales shed successively from the base to the tip, leaving the persistent axis. Spikelet axis winged. Spikelet scales 2.5–3.5 mm long, strongly overlapping, ovate, sharply angled along the back, rounded and sometimes shallowly notched at the tip, straight to slightly incurved, with 7–9 nerves, reddish brown to purplish brown with narrow, whitish margins, the midrib green. Stamens 3, the anthers 1.0–2.5 mm long. Stigmas 3. Fruits 1.4–1.8 mm long, narrowly elliptic-obovate in outline, sharply 3-angled in cross-section, the sides slightly concave to more or less flat, the surface finely pebbled, dark brown to black or less commonly gray, shiny. 2n=16, 26, 32, 84, 96, 100, 108, 152. August–October.

Introduced, known thus far from a single collection from Dunklin County (native range unclear, but widely introduced in tropical and warm-temperate regions of the world; in the U.S. occurring in the southeastern states west to Missouri and Texas, and in California). Weed in crop fields.

The single Missouri collection consists only of vegetative material and has not been relocated in recent years. Southeastern Missouri is at the climatic extreme for this mainly tropical species, which would not survive the colder winters further north. The species can spread rapidly vegetatively from rhizome fragments. Purple nutgrass is considered one of the world’s worst agricultural weeds, and infestations should be documented with a voucher specimen, then destroyed. Care must be taken in excavating plants, because the sharp, hard rhizome tips can cause puncture wounds. The tubers are edible and said to be relished by turkeys and other wildlife.