This plant causes dermititis. It has a short rootstock or caudex with shallow fibrous roots. A plant may have one to several leafy stems. Its lance-shaped, opposite leaves have short stalks. A plant puts up an average of one stem from a short caudex and sprouts each year from this rootstock.
Seeds have long hairs that facilitate wind dispersal in the fall.
It occurs in a range of wet conditions from standing water to saturated soil. Swamp milkweed is a colonizer. The moist habitat of swamp milkweed discourages fire entry. Adjacent communities may serve as seed sources after a fire. Swamp milkweed is a component of prairie wetlands, so it has evolved with some fire exposure.