3. Asclepias incarnata L. ssp. incarnata (swamp milkweed)
A. incarnata f. albiflora A. Heller
Pl. 220 g, h;
Map 917
Plants with white
latex and a fibrous rootstock. Stems 50–200 cm long, mostly several-branched,
erect or ascending, glabrous or sparsely and minutely hairy in longitudinal
lines, with 6 to numerous nodes. Leaves opposite, short-petiolate. Leaf blades
4–15 cm long, 0.5–4.5 cm wide, narrowly lanceolate to lanceolate or less
commonly ovate, the base abruptly narrowed or rounded, occasionally shallowly
cordate but not overlapping that of the opposite leaf, the tip gradually
tapered to a sharp point, the margins flat or slightly curled under, glabrous
or more commonly the undersurface sparsely and minutely hairy. Inflorescences 2–12,
terminal or occasionally appearing lateral (at the tips of short branches),
short- to long-stalked, with 10–40 flowers. Calyces reflexed, short-hairy on
the outer surface, the lobes 1.5–2.5 mm long, lanceolate to ovate. Corollas
reflexed, glabrous, pink or rarely white, the lobes 4–6 mm long, elliptic to
oblanceolate. Gynostegium appearing stalked (the column visible below the bases
of the hoods), pale pink, rarely white, the corona slightly shorter than to
about as long as the tip of the anther/stigma head. Corona hoods 2.0–2.7 mm
long, strongly ascending, attached near their bases, oblong-ovate in outline,
the tips broadly rounded, the margins not toothed, the bases not pouched. Horns
attached below the middle of the hoods, extended conspicuously beyond the tips
of the hoods and incurved over the anther/stigma head, linear, not flattened,
tapered to a sharp point at the tip. Fruits 5–9 cm long, erect or ascending
from erect or less commonly deflexed stalks, lanceolate in outline, the surface
smooth, usually minutely hairy. Seeds with the body 6.5–9.0 mm long, the
margins relatively broadly winged, the terminal tuft of hairs white. 2n=22.
June–August.
Scattered nearly
throughout Missouri (eastern U.S. west to North Dakota
and Arizona; Canada). Swamps, sloughs, marshes,
margins of ponds and lakes, banks of streams and rivers, bottomland prairies,
and occasionally bottomland forests; also ditches and railroads.
Swamp milkweed
is gaining popularity as a garden ornamental; however, the young shoots and
leaves are browsed by mammals such as rabbits and deer. Its roots are a food
source for muskrats and other wetland mammals. Woodson (1954) separated a
series of populations in other states to the east and south of Missouri characterized
by less-branched stems, more conspicuous pubescence, and broader, less
gradually tapered leaves as ssp. pulchra (Ehrh.) Woodson.