Smooth, prairie, or Sullivant's milkweed, whatever the name it likes damp. It grows in moist prairies, roadside ditches, or on shores. I suspect it often goes unrecognized because of its general resemblance to common milkweed. Common milkweed grows in drier spots. But in intermediate habitats, you will often see hybrids, adding to the confusion. And if you Google this and look at the images, you will see a number of hybrids passing as their parents. Smooth milkweed is threatened in the northern part of its range, and part of the problem is the contamination of the gene pool by common milkweed. Smooth milkweed grows in AR, IA, IL, IN, KS, MI(T), MN(T), MO, ND, NE, NH, OK, SD, WI(T), and ON. Monroe Co MI, 6/26/11.