There are three genera of milkweeds in the gardens. This is the swamp milkweed (Asclepias incarnata), but it is growing well in a sunny dry location. Perhaps that is why it is over 5 feet when the reference books say it is 1-4' in height. The juice is less milky and it is less fragrant than the common milkweed.
This is butterfly weed (Asclepias tuberosa), which attracts many butterflies with its showy bloom and is a favorite of Monarchs. When you visit, see if you can find the caterpillars on the plants.
As I was looking up the botanical names, I found this little factoid about the milkweed family (Asclepiadaceae): "The unusual structure of the flower regulates pollination. Sacs of pollen snag on insects' legs, are pulled from the stamens and then must be precisely inserted in slits behind the crown. If inserted backwards, pollen grains germinate in the wrong direction and are wasted. This may explain why so few pods occur on most plants. Insects too small to pull free die trapped on the flower." (The Audubon Society Field Guide to North American Wildflowers--Eastern Region)