Stems slender, often 2 to 5 feet tall and leafy throughout, more or less branched, smooth or minutely pubescent in two lines along the upper part of the stem. Leaves opposite, oblong-lanceolate, pointed at the apex, narrowed or sometimes slightly heart-shaped at the base, 3 to 7 inches long, one-half to 11/2 inches wide; petioles very short. Flowers numerous in many-flowered terminal, stalked umbels; pedicels of the flowers pubescent, one-half to 1 inch long; corolla red or rose-purple, its lobes oblong; column more than one-half as long as the obtuse pink or purplish hoods; horns slender, incurved, longer than the hoods. Fruit an erect pod (follicle), 2 to 4 inches long.
In marshy or swampy places, New Brunswick to Saskatchewan, south to Tennessee and Colorado. Flowering from July to September.
The Hairy Swamp Milkweed (Asc1epiaspu1chra Ehrhart) is similar to this species, but is softly tomentose-pubescent on the stems, the leaves smooth above and pubescent beneath, the flowers commonly lighter red or pink.
Memoir 15 N. Y. State Museum
Swamp Milkweed - Asclepias incarnata