2. Eupatorium capillifolium (Lam.) Small (dog fennel)
Pl. 266 e, f;
Map 1110
Stems 50–250 cm
long, not hollow, moderately to densely short-hairy above the often nearly
glabrous basal portion, green to yellowish green, sometimes purplish-tinged or
brownish-mottled, occasionally slightly glaucous, the nodes often with small
fascicles of axillary leaves less than 1/2 as long as the main stem leaves.
Leaves mostly opposite, those of the uppermost nodes sometimes alternate,
sessile or with short petioles 1–10 mm long. Leaf blades 2–10 cm long, mostly 1–3
times pinnately or ternately deeply and irregularly lobed or dissected into
relatively few long, threadlike segments (the uppermost merely simple and
threadlike), these 0.2–0.5(–1.0) mm wide, the margins more or less entire,
sometimes rolled under, the surfaces glabrous but sparsely to densely
gland-dotted, with 1 main vein. Inflorescences axillary clusters, these
appearing as elongate terminal panicles with racemose branches. Involucre 2.0–3.5
mm long, more or less cup-shaped to nearly cylindrical (sometimes bell-shaped
after pressing), the bracts linear to narrowly lanceolate or narrowly oblong,
sharply pointed at the tip, the narrow margins thin and pale (sometimes only
toward the base), faintly 1- or 3-nerved, glabrous, light green, sometimes
purplish-tinged toward the tip. Disc florets 3–6. Corollas 2.0–2.5 mm long, the
surface often inconspicuously glandular, white, greenish white, or pale
cream-colored. Fruits 1.0–1.5 mm long. 2n=20. August–October.
Introduced,
uncommon, known thus far only from St. Louis City and County (native from New
Jersey to Florida west to Texas; introduced farther north and west). Railroads,
gardens, and open, disturbed areas.
Eupatorium
capillifolium was first
reported by Steyermark (1963) in the supplement to his Flora of Missouri,
based on collections made by Viktor Mühlenbach in the St. Louis railyards. In
parts of the southeastern United States, the species is a problem weed in
pastures. Apparently it contains low levels of the toxin tremetol (see the
treatment of Ageratina for further discussion), which can cause
dehydration and other symptoms in livestock that ingest the plants in quantity.