Flora of the week--Asclepias spp.
Anyone familiar with gardening for butterflies will list the various asclepias as one of the top ten necessities for their plant list. The milkweeds, as nectar plants, serve a number of different species of butterflies and also attract hummingbirds. The plant is toxic, so deer aren't likely to browse it, and this toxicity happens to benefit one of the most generally intriguing insects around--the monarch butterfly. (pictured at left on asclepius curassavica [native to Central and South America])
Milkweeds are larval hosts for both monarchs and queen butterflies--and occasionally other species like the grey hairstreak. Since the plant is toxic (unless you happen to be a caterpillar), it serves as protection for caterpillars. Birds will not attempt to eat monarch or queen caterpillars. Last year's crop of asclepias tuberosa got us a half a dozen caterpillars--this year, with more than four square feet of that particular plant in one place and asclepias incarnata in another, we are crawling with the little buggers. We had to stop counting after three dozen because it got too confusing. I can tell you that the tuberosa is about a foot shorter than it was last week!
Most milkweeds will attract aphids, though unless the plant appears to be suffering you can probably ignore them and leave them for your insect predators like ladybugs. The Lady Bird Johnson Wildflower Center lists 42 native species of milkweed, so you should be able to find something tailored to your area of the country, soil and sun conditions.
Tuberosa is native to the majority of the United States--hence its popularity on "top ten" butterfly gardening lists! (Pictured at right with a couple of visible monarch caterpillars.)
Tubersosa, commonly known as "Butterfly Weed" is probably the most popular of the asclepias species grown in home gardens because it is fairly short, even before the caterpillars get hold of it. It generally grows to a height of 12" to 24" inches, though in its favorite conditions (sandy soil, full sun) it can reach 36 inches. This might seem rather tall until you find out that other milkweeds, such as Swamp Milkweed, can easily reach six feet!
While most milkweeds get the common name from a milk-like sap that oozes out when a stem is broken, tuberosa doesn't have this particular trait. It is easily grown from seed or even rooted in water from a cutting.
A great page with images of many species of milkweed is here at Duke: