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RHODODENDRON, ro-do-den'dron, a genus of about 600 species of shrubs and trees of the heath family (Ericaceae), native mainly to cooler parts of the Northern Hemisphere. They are most numerous in the Himala}-an region of Asia; some 25 are indigenous to the United States. Many are popular ornamental plants. Rhododendrons may be evergreen or they may lose their leaves in the fall. The deciduous rhododendrons are known as azaleas. (See AZALEA.) Certain rhododendrons, such as Lapland rosebav.

R. lapponicum. are onlv about a foot tall; others, such as the great laurel, K. maximum, may be 40 feet tall and 12 inches in diameter. In the United States, rhododendrons attain their best development in the mountains of the East and in the Pacific Northwest. Well-known eastern species include great laurel, R. maximum; rhodora, R. canadense; mountain rose-bay, R. catawbiense; flame azalea, R. calendula-cewn; and pink azalea, R. nudiflorum. Coast rhododendron, R. macrophyllum, and western azalea, 7?. occidentale, are common northwestern species.

Rhododendrons are propagated by seeds, cuttings, layers, and grafts. They like light shade to full sun and a moist, well-drained soil which is acid and rich in humus. Because of their shallow roots, no cultivation can be given. The plants should be mulched with pine needles, oak leaves, or other organic material. The occasional addition of aluminum sulphate is beneficial if the soil tends to lose its acidity. Rhododendrons have alternate, usually smooth-edged leaves; flowers borne generally in clusters at the ends of branches; a five-parted calyx; five petals that are united below; mostly five or ten stamens; and a five-or ten-chambered, woody, many-seeded capsule.