Whorled Milkweed

This perennial herbaceous plant is ½–2' tall and more or less erect; it is either unbranched or sparingly branched along the upper half of its central stem. The central stem is yellowish green or medium green and terete; it has vertical lines of short pubescence. Whorls of 4-6 leaves occur along the nodes of the central stem; because the internodes of this stem are fairly short, these leaves are produced in abundance. Relative to the orientation of the central stem, the leaves are usually ascending, although sometimes they are widely spreading or drooping. The leaves are 2-3" long, 2-3 mm. across, and linear in shape; their margins are entire (toothless) and strongly revolute (rolled downward). Sometimes whorls of smaller secondary leaves are produced from short lateral stems that develop from the axils of leaves along the central stem. The upper leaf surfaces are yellowish green or medium green and glabrous to sparsely short-pubescent; they are narrowly grooved along the middle where the midribs occur. The lower leaf surfaces are whitish green and short-pubescent; they are partially obscured by the rolled leaf margins. The leaves are sessile or they have very short petioles (less than 2 mm. long). From the axils of middle to upper leaves, umbels of flowers are produced on short peduncles (flowering stalks); there can be 1-4 umbels of flowers at each node. Individual umbels span ¾–1½" across, consisting of 7-20 pedicellate flowers.

Each flower is about 5-6 mm. across and 8-10 mm. long, consisting of 5 sepals, 5 petals, 5 hoods with horns, and a central reproductive column. The sepals are light green, short-pubescent, and lanceolate in shape; sometimes they are tinted purple toward their tips. These sepals are visible at the bases of flower buds, but they are hidden by the petals when the flowers bloom. The petals are white or greenish white, sometimes with pale purplish tints toward their tips; they are oblong-elliptic in shape and strongly declined (bent downward), curving slightly upward toward their tips. The erect white hoods are open-tubular in shape and somewhat oblique, their lower sides facing the center of the flower. The slender white horns are sickle-shaped and inwardly curved; there is one exserted horn per hood. The short reproductive column is white at its apex and light green below. The slender pedicels of the flowers are light green to nearly white, sometimes becoming purplish at their bases; they are 8-12 mm. long, terete, and short-pubescent. The peduncles are ½–1½" long, light to medium green, glabrous to short-pubescent, terete, and ascending. The blooming period occurs from early to late summer, lasting about 1-2 months. There is little or no floral scent. Afterwards, successfully cross-pollinated flowers are replaced by ascending to erect follicles (seedpods that open along one side). These follicles are 3-4" long and about ½" across; they are narrowly lanceoloid in shape and fairly smooth (lacking warts or soft prickles). At maturity during autumn or winter, these follicles split open to release their seeds to the wind. Mature seeds are about 4-5 mm. long, ovate-flattened in shape, brown, and narrowly winged along their margins; their apices have large tufts of white hair. The root system is fleshy-fibrous and long-rhizomatous. Colonies of clonal plants are often produced from the rhizomes.