Asclepias

Attractive evergreen or deciduous perennials, with often narrow, whorled leaves, toped by unusual looking, heads of colourful flowers. It's part of the Apocynaceae family, so can produce a milky sap (wear gloves when handling) when stems or leaves are broken. In Asclepias, the monarch butterfly uses this sap, to ensure the larval and adult forms are inedible to birds.