Rheum is a small genus of clump-forming, herbaceous plants from Asia and eastern Europe. The family Polygonaceae also includes Rumex (dock), Fagopyrum (buckwheat), and Fallopia (Japanese knotweed).
Rheum species typically have large, heart-shaped leaves borne on fleshy petioles arising from substantial rhizomes, and conspicuous inflorescences of small flowers which can be white, pink or green. The distinctive pink-red colour present in these plants is due to the presence of betanins, the same pigments that colour beetroot, bougainvillea and other members of the order Caryophyllales.
Rheum rhizome has been used in Chinese medicine for thousands of years and, by medieval times, was widely traded across Europe and Asia as a purgative. Recent pharmacological research has shown that Rheum rhizomes have antifungal, antimicrobial and antiviral properties, and they are considered candidates for further research on the treatment of sepsis.
Today, the most familiar member of the genus is Rheum x hybridum, the edible rhubarb. The precise origin of this plant is unknown but its parentage is likely to include Rheum rhaponticum and Rheum rhababarum, both of which were grown in Europe by the eighteenth century. The fleshy leaf stalks (petioles) are edible, while the leaf blades are slightly toxic due to high concentrations of oxalic acid, although, during the First World War, the leave blades were eaten as a vegetable in Britain. The petioles contain far smaller amounts of oxalic acid than the leaves. Consequently, rhubarb should be avoided by kidney-sufferers, a condition exacerbated by oxalic acid. Other species of Rheum cannot be eaten safely.
In temperate areas, where rhubarb is harvested annually from March, or February if forced, it is a valuable early-season food source, when few other crops are available. Commercial forcing is carried out by moving mature rhubarb plants from the field into dark, heated sheds in November. The resulting stems are hand-picked by candlelight, as strong light stalls the plants' growth. Traditionally, in England, the centre of production was the 'rhubarb triangle,' an area between Leeds, Wakefield and Morley, which once supplied 90% of the world's forced rhubarb.
Several species of Rheum are grown as ornamentals, providing bold foliage effect in a border. They are generally low-maintenance plants, but may be affected by a leaf-eating beetle which can quickly reduce the leaves to lace. Bears, after they awake from hibernation, will feast on early rhubarb shoots, although such a problem is unlikely to present itself in the UK.
Wang A et al. 2005. Molecular phylogeny, recent radiation and evolution of gross morphology of the rhubarb genus Rheum (Polygonaceae) Inferred from chloroplast DNA trnL-F sequences. Annals of Botany 96: 489-498.
Xiao P et al. 1984. Ethnopharmacologic study of Chinese rhubarb. Journal of Ethnopharmacology 10: 275-293.