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.
The flowers are attractive to pollinators, especially bumblebees and butterflies, although for smaller insects that attraction may be fatal: the feet or tongues of the visitors are trapped by the flower, and only those that are powerful enough can free themselves after a struggle. The plant is full of toxins, but the leaves are eaten by some butterflies, especially monarchs, making both caterpillars and adults also toxic to predators in the process. While this is not at present something we need to cater for, with further climate change we may well see colonization of the UK, perhaps from the Canary Islands.