A herbaceous perennial, Rheum rhaponticum, of Mediterranean origin, belonging to the plant order Polygonales. Rhubarb is grown for its thick petioles which are used mainly as a cooked dessert (see illus.); it is frequently called the pie-plant. The leaves, which are high in oxalic acid content, are not commonly considered edible. Propagation is by division of root crowns. Victoria, Macdonald, and Valentine are popular varieties (cultivars). Commercial production is generally limited to areas where crowns may become dormant for 2–3 months each year. Outdoor rhubarb is a common garden vegetable in most areas of the United States except the South. Harvesting begins in the spring and continues for 6–10 weeks. Commercial plantings are renewed every 4–8 years. Michigan and Washington are important centers for forced or hothouse rhubarb. Two- or three-year-old field-grown crowns are moved into darkened forcing structures in late winter and forced at 55–60°F (12–15°C) to obtain petioles of a bright-red color.