5. Ambrosia tomentosa Nutt. (perennial bursage)

Franseria discolor Nutt.

Pl. 272 d, e; Map 1146

Plants perennial, colonial from deep, often widely creeping rhizomes. Stems 10–35 cm long, sparsely to densely pubescent with short, appressed and sometimes also longer, woolly hairs. Leaves alternate except sometimes toward the stem base, mostly short- to long-petiolate (the uppermost ones sometimes nearly sessile). Leaf blades 2–10 cm long, ovate to lanceolate in outline, irregularly 1–3 times pinnately lobed (at least the larger leaves with 7–15 primary lobes), the lobes irregularly elliptic to ovate, the margins mostly toothed and often slightly thickened or curled under, the upper surface moderately roughened-pubescent with minute, stiff, more or less appressed, white hairs, the undersurface more densely hairy, pale or whitened, the hairs sometimes appearing more or less tangled. Staminate heads in spikelike racemes, these usually not in paniculate clusters, the staminate involucre 4–7 mm wide, with 7–12 shallow to moderately deep lobes, densely hairy. Pistillate heads in small axillary clusters (or sometimes solitary), the involucre enclosing 2 florets and with 2 stout beaks, 4–6 mm long at fruiting, ovoid to ellipsoid, with 1 or few series of relatively short, sometimes slightly flattened spines scattered across the surface (these rarely absent or nearly so), otherwise minutely hairy. July–September.

Introduced, uncommon, known thus far only from the city of St. Louis (Wyoming to Arizona east to South Dakota and Nebraska; introduced sporadically elsewhere in the U.S.). Railroads.

Mühlenbach (1979) observed the colony that he discovered in the St. Louis railyards for fourteen years and remarked that it persisted in spite of repeated sprayings with herbicides.