Sedum

Sedum is a large genus of flowering plants in the family Crassulaceae, members of which are commonly known as stonecrops. The genus has been described as containing up to 600 species updated to 470. They are leaf succulents found primarily in the Northern Hemisphere, but extending into the southern hemisphere in Africa and South America. The plants vary from annual and creeping herbs to shrubs. The plants have water-storing leaves. The flowers usually have five petals, seldom four or six. There are typically twice as many stamens as petals. Various species formerly classified as Sedum are now in the segregate genera Hylotelephium and Rhodiola. Well-known European species of Sedum are Sedum acre, Sedum album, Sedum dasyphyllum, Sedum reflexum (also known as Sedum rupestre) and Sedum hispanicum. Sedum was first formally described by Linnaeus in 1753, with 16 species, and hence bears his name as the botanical authority (L.). Of the genera encompassed by the Crassulaceae (stonecrop) family, Sedum is the most species rich, the most morphologically diverse and most complex taxonomically. Although molecular phylogeny research has greatly reduced the genus, with about 420 species, it still constitutes a third of the family and is polyphyletic. Therefore the current circumscription must be considered unstable. Of the three subfamilies of the Crassulaceae, Sedum is placed in the subfamily Sempervivoideae. There are now thought to be approximately 55 European species. Sedum demonstrates a wide variation in chromosome numbers, and polyploidy is common. Chromosome number is considered an important taxonomic feature. Sedum species are used as food plants by the larvae of some Lepidoptera species including the grey chi moth. In particular, Sedum spathulifolium is the host plant of the endangered San Bruno elfin butterfly of San Mateo County, California. Sedum lanceolatum is the host plant of the more common Parnassius smintheus found in the Rocky Mountains. As well as Sedum spathulifolium, many other species of Sedum serve the environmental role of host plants for butterflies. For example, the butterfly Callophrys xami uses several species of Sedum, such as Sedum allantoides, for suitable host plants. Many sedums are cultivated as garden plants, due to their interesting and attractive appearance and hardiness. The various species differ in their requirements; some are cold-hardy but do not tolerate heat, some require heat but do not tolerate cold. Numerous hybrid cultivars have been developed, of which the following have gained the Royal Horticultural Society's Award of Garden Merit:-