9. Eupatorium semiserratum DC.
E.
cuneifolium Willd. var. semiserratum
(DC.) Fernald & Griscom
Pl. 266 c, d;
Map 1117
Stems 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.