Aquatic milkweed, Asclepias perennis
10a to 5b
Aquatic milkweed grows in full sun in soil that is continuously wet. It is not rare or threatened but the seeds are difficult to come by since they are seldom collected from their host plants. Seeds and plants are available thru my Amazon affiliate product banners.
View Aquatic milkweed observations on iNaturalist
View Aquatic milkweed plant information at Wildflower.org
View Aquatic milkweed plant information at Plants.USDA.gov