Purple milkweed Purple milkweed is a later-flowering cousin to our common milkweed. The former opens its complicated blossoms in late July and early August while the latter species blooms in the neighborhood of the 4th of July. Milkweeds of all sorts are the favorite plant of the monarch butterfly, whose caterpillars feast on the poisonous leaves and turn their poisons into a kind of armor that tells birds and other would-be predators: Stay Away for Me! The adult orange and black butterflies are just starting to move back into our area—I saw the first one today, but I couldn't get a picture—and in short order, they'll be drinking nectar from this species and laying eggs on any milkweed they can find. In its modest way, Asclepias purpurascens will help make possible the monarch's annual epic migration in the fall to Mexico. The handsome plants provide egg-laying fuel for the adults, whose eggs will mature into a generation of hard-traveling butterflies with amazing stamina and a sense of direction.