The leaves of the common milkweed are poisonous to most animals, but the monarch caterpillars eat nothing else.
The plant contains cardiac glycosides, allied to digitalins used in treating some heart disease. They are absorbed by the monarch butterfly larvae, whose sole source of food is milkweed foliage, make the larvae and adult butterflies toxic to birds and other predators.
The flowers (1/2 inch) are pink to lavender, star-shaped, in dense rounded, two-inch clusters at top of straight stem.
The leaves are light green (4-10 inches long) with gray down underneath and exude a milky juice when bruised. Seedpods are warty, filled with downy fluff. Milkweed grows from tow to four feet tall.
It flowers in June through August.
Common Milkweed is found in old fields, roadsides, and waste places from Saskatchewan to New Brunswick, south to Georgia and west through Tennessee to Kansas and Iowa.
Zones 4 - 9.