I have common milkweed (Asclepias syriaca) growing in my garden -- it planted itself, the seeds blowing in on the wind. I like watching it for monarch eggs and caterpillars as summer continues. Other caterpillars sometimes show up too (see photo below.) The blooms have a lovely scent.
Asclepias, the milkweeds, are native to America. There are over 140 known species. Milkweed is named for its milky sap, which consists of a latex containing alkaloids and some are toxic. The genus is named after Asclepius, the Greek god of healing.
Asclepias species produce some of the most complex flowers in the plant kingdom, Pollen is grouped into complex structures called pollinia (or "pollen sacs"), rather than being individual grains, as is typical for most plants. The feet or mouthparts of flower-visiting insects slip into one of the five slits in each flower formed by adjacent anthers. The bases of the pollinia then mechanically attach to the insect, so that a pair of pollen sacs can be pulled free when the pollinator flies off, assuming the insect is large enough to produce the necessary pulling force (if not, the insect may become trapped and die). Pollination is effected by the reverse procedure in which one of the pollinia becomes trapped within the anther slit.
Asclepias species produce their seeds in follicles. The seeds, which are arranged in overlapping rows, bear a cluster white, silky, filament-like hairs known as the coma (often referred to by other names such as "floss", "plume", or "silk"). The follicles ripen and split open, and the seeds, each carried by its coma, are blown by the wind. American milkweeds are an important nectar source for native bees, wasps and other nectar-seeking insects, though non-native honey bees commonly get trapped in the stigmatic slits and die -- a problem for me as I keep a beehive. Milkweeds are a larval food source for monarch butterflies and their relatives, as well as a variety of other herbivorous insects specialized to feed on the plants. (summarized from Wikipedia)
Other milkweed species in my garden in photo above: A. tuberosa. It's helpful to plant several species of milkweed and in large enough clumps that it attracts the monarch and provides sufficient food for its caterpillars. In the photos below from my garden, spot the viceroy butterfly which mimics the monarch for protection (predators think it is toxic, since it looks so similar to the monarch). The viceroy has an additional stripe along the bottom of its wings. For more info on monarchs see monarchwatch.org. National Geographic as good online information on monarchs as well.