Rhodiola is a genus of roughly 74–90 perennial flowering plants in the family Crassulaceae (order Saxifragales), closely related to the stonecrops (Sedum) and sometimes merged with that genus by some taxonomic authorities. Plants are native to high-altitude and cold regions of the Northern Hemisphere, with the greatest diversity in China, where about 55 of the roughly 90 species occur, 16 of them endemic. Only three species are recorded in the United States and Canada.
Members of the genus are distinguished by a set of consistent characters: two whorls of stamens totalling twice the number of petals, petals that are free or nearly free (not fused into a tube), and a stout underground rhizome from whose axils the annual flowering stems arise, typically surrounded at the base by a rosette of leaves. Rhodiola is also the only genus in Crassulaceae in which some species bear strictly unisexual flowers — a trait that has evolved independently at least three times within the group, with reversals to hermaphroditism also documented, an unusual pattern among flowering plants.
Taxonomically, the genus was established by Linnaeus on the basis of its dioecious habit, then later collapsed into Sedum, and ultimately reinstated in the 20th century when molecular phylogenetic studies confirmed it as a distinct lineage. It sits in subfamily Sempervivoideae, tribe Umbiliceae, as sister to the small genus Pseudosedum.
Rhodiola species have a long history of use: root extracts have been employed in traditional European and Chinese medicine, notably for fatigue and altitude sickness, and several species are cultivated as ornamentals, though they are demanding to grow outside their native subarctic and alpine conditions. The roots of many species emit a distinctive rose-like fragrance, which is the origin of the genus name.
Etymology
The name Rhodiola combines the Greek rhodon, meaning "rose" and referring to the rose-like scent of the roots, with the Latin diminutive suffix -iola. The fragrant rhizome is thus the direct inspiration for the genus name.
Distribution
Rhodiola grows across high-altitude and cold regions of the Northern Hemisphere. Diversity is greatest in China, which holds approximately 55 of the roughly 90 accepted species, with 16 endemic to that country. Flora of North America records only three species in the United States and Canada.
Ecology
Dioecy — the condition of bearing separate male and female flowers on different plants — has evolved independently at least three times within Rhodiola, and reversals to a hermaphroditic state have also occurred, a pattern that is rare among angiosperms. It has been proposed that dioecy in the genus may be linked to abiotic pollination (by wind rather than insects) in the harsh, cold environments the plants inhabit.
Cultural Uses
Rhodiola root extracts have a documented history of medicinal use across Europe, and in traditional Chinese medicine they are employed for conditions including fatigue and altitude sickness. Several species are also cultivated as ornamental plants, though they prove difficult to establish outside their native subarctic and alpine climates.
Taxonomy Notes
Rhodiola was originally distinguished from Sedum by Linnaeus on the basis of its dioecious flowers, but was subsequently submerged into Sedum. It was reinstated in the 20th century — on morphological grounds (robust rhizomes and annual stems arising from axils of scaly basal leaves) — and that reinstatement was later confirmed by molecular phylogenetic studies. The genus is placed in Crassulaceae subfamily Sempervivoideae, tribe Umbiliceae, where it forms a sister group to Pseudosedum; some authors have suggested Pseudosedum should be synonymised into Rhodiola.
