2. Thelesperma megapotamicum (Spreng.) Kuntze (Colorado greenthread, Navajo tea, Hopi tea)

Pl. 292 c, d; Map 1236

Plants perennial herbs, with short, somewhat woody rhizomes. Stems 30–80 cm long. Leaves sometimes more or less crowded below the stem midpoint, in other plants more evenly distributed along most of the stem. Leaf blades 3–12 cm long, all but occasionally those of the uppermost leaves 1 or less commonly 2 times pinnately dissected, the ultimate segments 5–50 mm long. Heads discoid (radiate elsewhere). Involucre with the outer series of 3–5(6) bracts 1–3 mm long, lanceolate to oblong or ovate; the inner series of bracts 7–10 mm long, fused in the basal 1/3–2/3, the free portion narrowly ovate to ovate-triangular. Ray florets absent (rarely present elsewhere but with inconspicuous, yellow corollas 3–6 mm long). Disc florets with the corollas 7–9 mm long, yellow to orangish yellow, usually with reddish brown veins. Pappus absent or of 2 short, stout awns 1–2(–3) mm long. Fruits 5–8 mm long. 2n=22, 44. May–July.

Introduced, known thus far from Jackson County and the city of St. Louis (southwestern U.S. east to South Dakota and Texas; introduced farther east). Railroads.

Steyermark (1963) reported a record of T. ambiguum A. Gray (Pl. 290 a, b) in the appendix to his manual, based on a collection made by Viktor Mühlenbach in the St. Louis railyards. However, Mühlenbach (1979) later reported that the material had been redetermined as T. filifolium (as T. trifidum). Thelesperma ambiguum, which probably is better treated as T. megapotamicum var. ambiguum (A. Gray) Shinners (Greer, 1997), is native to Texas and Mexico, differing from T. megapotamicum mainly in its shorter stems, usually radiate heads, and reddish brown disc corollas.