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.