This is Asclepias perennis, also known as Aquatic Milkweed, and White Milkweed. Growing to about 2 feet high, this plant must live in full sun where it blooms repeatedly throughout the year. This plant like the other milkweeds is a very useful plant for the birds, bees, and butterflies. Monarchs love this plant and use it as a larval host plant. It is also the larval host plant for the Queen and Soldier butterflies. Asclepias in general are documented nectar sources for the Monarch, Orange-edged Roadside-Skipper, Dina Yellow, Carus Skipper, Eastern Tiger Swallowtail, and Falcate Metalmark butterflies. It is also the nectar source for the Clarks sphinx, and Milkweed Tussock moths. USDA Hardiness Zones 6 to 9