Milkweed species in the genus Asclepias contain cardiac glycosides that are poisonous to humans, but they pose the most danger to grazing animals. Humans can eat milkweed, but its toxicity depends on its species, age, how it is prepared and how much is eaten. Milkweed is the plant choice of monarch butterflies, and milkweed can be planted to attract monarchs. Milkweed is hardy in U.S. Department of Agriculture (USDA) plant hardiness zones 3 through 11, depending on the species.