Rhubarb (Rheum rhabarbarum or Rheum × hybridum according to the British Royal Horticultural Society) is a species of plant in the family Polygonaceae. It is a herbaceous perennial growing from short, thick rhizomes. The fleshy, edible stalks (petioles), are used in cooking, but the large, triangular leaves contain high levels of oxalic acid, making them inedible. The small flowers are grouped in large compound leafy greenish-white to rose-red inflorescences.
Although rhubarb is a vegetable, it is often put to the same culinary uses as fruits. The leaf stalks can be used raw, when they have a crisp texture (similar to celery, although it is in a different family), but are most commonly cooked with sugar and used in pies, crumbles and other desserts. They have a strong, tart taste. Several varieties have been domesticated for human consumption, most of which are recognised as Rheum x hybridum by the Royal Horticultural Society.
Rhubarb contains anthraquinones including rhein, and emodin and their glycosides (e.g. glucorhein), which impart cathartic and laxative properties. It is hence useful as a cathartic in case of constipation.