13. Asclepias tuberosa L. ssp. interior Woodson (butterfly weed, chigger flower,
pleurisy root)
A. tuberosa var. interior (Woodson) Shinners
A. tuberosa f. lutea (Clute) Steyerm.
Pl. 220 c–f; Map
927
Plants with
clear latex and deep-set rhizomes. Stems 20–90 cm long, sometimes few-branched
toward the tip, mostly erect or ascending, densely hairy, with numerous nodes.
Leaves all or mostly alternate, sessile or nearly so. Leaf blades 2–10 cm long,
0.4–2.3 cm wide, linear to broadly lanceolate, the base usually deeply cordate
and clasping the stem, the tip narrowed or tapered gradually to a sharp point,
the margins usually somewhat revolute, the surfaces hairy, especially along the
veins. Inflorescences 1 to several, terminal and in the upper leaf axils,
sessile or short-stalked, with 6–25 flowers. Calyces reflexed, hairy on the
outer (under) surface, the lobes 2–4 mm long, linear to elliptic-lanceolate.
Corollas reflexed, glabrous, bright orangish yellow to reddish orange, the
lobes 6–10 mm long, lanceolate to elliptic. Gynostegium appearing stalked (the
column visible below the bases of the hoods), bright yellowish orange to
reddish orange, the corona conspicuously longer than the tip of the
anther/stigma head. Corona hoods 4.5–6.5 mm long, ascending, attached near
their bases, lanceolate in outline, the tips rounded, the margins with a pair
of short, triangular teeth or lobes below the middle, the bases not pouched.
Horns attached below the middle of the hoods, extended to about the tips of the
hoods and angled or somewhat curved inward over the anther/stigma head, linear,
not flattened, tapered to a sharp point at the tip. Fruits 8–15 cm long, erect
or ascending from ascending or deflexed stalks, narrowly lanceolate in outline,
the surface minutely hairy. Seeds with the body 5–7 mm long, the margins
narrowly winged, the terminal tuft of hairs white. 2n=22. May–September.
Scattered to
common nearly throughout Missouri (eastern U.S. [mostly west of the Appalachian
Mountains] and adjacent Canada west to South Dakota, Utah, and Arizona). Upland
prairies, glades, savannas, and openings of mesic to dry upland forests; also
pastures, roadsides, railroads, and open, disturbed areas.
Woodson (1954)
concluded from his earlier morphometric studies that A. tuberosa could
be classified into four more or less geographically distinct subspecies based
on differences in leaf shapes, these tending to intergrade in distributional
zones of overlap. The widespread eastern ssp. tuberosa and ssp. rolfsii
(Britton) Woodson of the Gulf Coastal Plain are characterized by leaves tending
to be widest above the middle. The ssp. terminalis Woodson of the
southwestern United States and northern Mexico differs in having the leaf bases
mostly rounded or truncate. Later, Woodson (1964) abandoned ssp. terminalis,
placing all western populations into ssp. interior. Woodson (1964) also
reexamined leaf variation along a 1,200-mile roadside transect and found that
during a fourteen-year period between studies ssp. interior and ssp. tuberosa
had expanded their zone of contact westward through the establishment of new
populations along disturbed highway margins, with a resultant breakdown in differentiating
characters in portions of the intervening area.
Woodson (1962)
also studied variation in flower color in the species. The yellow ground color
present in all flowers is caused by the presence of carotenoid compounds, which
are selectively masked by red pigmentation from anthocyanins. Woodson noted
that populations in Missouri and adjacent states tend to have more reddish
orange flowers, but that more distant plants in all directions tend to have
progressively more yellow flowers, with redder flowers also present locally in
some western portions of the range.
Butterfly weed
is not weedy. The bright floral displays it provides along roadsides and in
glades and prairies are among the showiest of Missouri wildflowers. It has
become important enough horticulturally that cultivars are being bred
commercially to accentuate particular flower colors and growth forms. It is an
important plant in wildlife gardening, particularly in butterfly gardens.
However, the deep-set fleshy rhizomes are easily damaged during
transplantation, so plants for the garden should be grown from seeds or
purchased from reputable nurseries.