Asclepias stenophylla Gray

Prairie Milkweed

Asclepias stenophylla plant2

Family - Asclepiadaceae

Habit - Perennial forb from a thickened, somewhat tuberous rootstock.

Stems - Ascending to erect, to 1 m, terete, usually unbranched, green to purple in strong sun, retrorsely pubescent, with white milky sap.

Asclepias stenophylla stemStem.

Asclepias stenophylla leaves2Stem and leaves.

Leaves - Mostly alternate, sessile or very short petiolate, retrorsely pubescent, typically ascending. Blades linear, to 18 cm long, 2-3 mm broad, glabrous or minutely hairy along midvein and margins, green, acute.

Asclepias stenophylla leaf1Leaf.

Asclepias stenophylla leaves

Inflorescence - Axillary umbels, 2-12 per stem, short-stalked, usually appearing sessile, with 10-25 flowers. Peduncles to 2 mm long, densely retrorsely pubescent. Rays of umbels subtended by linear bracts. Bracts densely antrorsely pubescent, to 5 mm long, 1 mm broad at base. Rays to 7 mm long, densely retrorsely pubescent.

Asclepias stenophylla inflorescenceInflorescence.

Flowers - Calyx lobes 5, reflexed, minutely hairy on the outer surface, 2.0-3.3 mm long, lanceolate. Corolla lobes 5, spreading with the tips incurved, glabrous, greenish-white, 4.5-6.0 mm long, elliptic-lanceolate. Gynostegium appearing sessile (the corona base touching the corolla or nearly so), pale green to white, the corona noticeably shorter than to slightly longer than the tip of the anther/stigma head. Corona hoods greenish white, 3-4 mm long, erect, attached toward their bases, narrowly oblong in outline, the tips appearing 3-toothed or shallowly 3-lobed, the margins with a pair of triangular teeth or lobes below the middle, the bases pouched. Horns present but fused nearly the entire length to the inner surface of the hood, visible as a low ridge and extended as the middle tooth or lobe of the hood. Anther column greenish, 4 mm broad, 2-3 mm in diameter. Pistils 2, glabrous. Pollinia 2 mm long (total). Anthers sacs yellow, 1 mm long. Terminator dark purple.

Asclepias stenophylla flowerFlower.

Fruits - Follicles 9-12 cm long, erect or ascending from usually deflexed stalks, narrowly elliptic-lanceolate in outline, the surface smooth, glabrous or minutely hairy. Seeds with the body 5-6 mm long, the margins narrowly winged, the terminal tuft of hairs light cream-colored or tan.

Flowering - May - July.

Habitat - Dry, rocky prairies, glades, ledges of bluffs, usually on calcareous soils.

Origin - Native to U.S.

Other info. - This species is found in scattered locations toward the center of the state. Missouri is near the eastern edge of the plant's U.S. range, which extends mainly northwestward into Montana. It is not as common as other milkweeds in the state but can be found in good quality glade and prairie habitats. The plant can be identified by its extremely narrow leaves and small clusters of white flowers. The leaves can appear either alternate or subopposite, but close examination will usually reveal that there are few if any strictly opposite pairs. The plant could be confused with a similar species, A. verticillata, which also has very narrow leaves and small white inflorescences. In that species, however, the leaf nodes are usually well-defined whorls with several leaves each.

Photographs taken at Busiek State Forest, MO., 6-17-05 (DETenaglia); also at Ha Ha Tonka State Park, Camden County, MO, 6-2-2012, and Danville Conservation Area, Montgomery County, MO, 5-15-2018 (SRTurner).


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