9. Eupatorium semiserratum DC.
cuneifolium Willd. var. semiserratum
(DC.) Fernald & Griscom
Pl. 266 c, d;
50–120 cm long, not hollow, moderately to densely short-hairy above the
sometimes nearly glabrous basal portion, usually purplish-tinged or purplish
brown, sometimes somewhat glaucous, some 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
poorly differentiated petioles to 8 mm long, twisted at the base so that the
leaves appear nearly vertically oriented. Leaf blades 1–8 cm long,
2–15(–25) mm wide, narrowly oblanceolate to oblanceolate or
less commonly narrowly elliptic, tapered at the base, rounded or angled to a
bluntly pointed tip, the margins sharply toothed mostly above the midpoint, the
surfaces moderately to densely short-hairy, also densely gland-dotted, with 3
main veins, the 2 lateral veins branching from the midvein 2–12 mm
above the blade base. Inflorescences terminal panicles, more or less
flat-topped. Involucre 2.5–4.0 mm long (sometimes appearing longer at
fruiting), more or less cup-shaped, the bracts ovate to narrowly oblong,
rounded to bluntly pointed at the tip, the margins thin and pale, mostly
faintly 3-nerved, densely short-hairy, green. Disc florets 5. Corollas
2.5–3.5 mm long, the surface often somewhat glandular, white. Fruits
1.5–2.0 mm long. 2n=20. August–October.
Uncommon in the
Mississippi Lowlands Division and adjacent portions of the Ozarks; also
historically disjunct in the St. Louis region (southeastern U.S. west to
Missouri and Texas). Edges of mesic upland forests, sand savannas, bottomland
forests, and swamps; also old fields, roadsides, and open, disturbed areas.
For a discussion
of possible hybrids with E. hyssopifolium, see the treatment of that species.
Steyermark (1963) considered E. semiserratum (as E. cuneifolium
var. semiserratum) to inhabit only moist, bottomland habitats, but most
of the collections since 1990 originated from drier, upland sites.