Rhododendron Species

Rhododendrons are one of the most popular shrubs that people grow. In the Pacific Northwest of the United States the rhododendron is ubiquitous in the yards of homeowners. The state flower of Washington is the Pacific rhododendron. The botanical name of the Pacific rhododendron is Rhododendron macrophyllum which means "rose tree with large leaves." Rhododendron macrophyllum grows in the wild from British Columbia, Canada to southern California, and the flowers are typically various shades of pink.

There are many other rhododendrons that grow in the wild around the world. These wild types are called species. There are more than 850 species in the Genus (group) Rhododendron and they are native to the temperate regions of North America, Europe, Asia, as well as the tropical regions of southeast Asia and northern Australia. None are indigenous to Africa or South America. There is an enormous diversity of size and shape in this group of plants from prostrate groundcovers growing no more than a few inches high to trees more than 100 feet tall. The wild rhododendrons are found from sea level to 19,000 feet in elevation where they occur in a variety of habitats including alpine regions, coniferous and broadleaved woodlands, temperate rain forests, and even the tropics. Leaf size ranges from less than 1/4 inch to over three feet long; there is also a wide variety of leaf shapes including very rounded, lance-shaped, and elliptical. The flowers may be pink, white, red, purple, yellow, orange, or various shades of each.

Most of the rhododendrons people grow are hybrids which are not the same as species. As stated above, a species Rhododendron is a plant that is found in the wild. The individual plants of each species of rhododendron grow, flower, are pollinated, and set seed. The seedlings generally look similar to the parent plants and thus remain the same species. For example, a plant belonging to the species Rhododendron macrophyllum is pollinated by another plant of Rhododendron macrophyllum and the resulting offspring are also Rhododendron macrophyllum.

Hybrid rhododendrons, on the other hand, are plants resulting from the cross-pollination of one species by a different species and the offspring may look quite different than either parent. For example, when a person takes pollen from a Rhododendron macrophyllum (Pacific rhododendron) and hand pollinates another species of rhododendron such as Rhododendron catawbiense, the Catawba rhododendron native to the east coast of the United States, the seedlings will have characteristics of both parents mixed together. People have been artificially creating hybrids for many years and there are now tens of thousands of hybrid rhododendrons. Some familiar names of hybrids may include, 'The Honorable Jean Marie de Montague,' 'Pink Pearl,' 'Cynthia,' 'Unique,' and 'Purple Splendor.' Additionally, there are some natural hybrids in the wild where different species grow together.

The group of plants commonly called azaleas are actually classified by botanists as belonging to the Genus Rhododendron and the name for each type of azalea has both the word "Rhododendron" and the species name. For example, the common flame azalea of the eastern United States is botanically called Rhododendron calendulaceum. The azaleas are of 2 types: deciduous, or those that lose their leaves in the autumn, and evergreen or semi-evergreen, which lose some but retain most of their leaves throughout the winter. The deciduous azaleas are native the eastern and western areas of the United States and Canada, Japan, China, and scattered locations across eastern Europe. Evergreen azaleas are found only in central-eastern and southeastern Asia. Hybridizers have made innumerable crosses among the azaleas and there are thousands of hybrids including the deciduous 'George Reynolds,' 'Homebush,' and 'Irene Koster' and the evergreen 'Everest,' 'Pink Rosebud,' and 'Hino Crimson.'

Many people grow azaleas and rhododendrons in the Pacific Northwest because the climate is very favorable for their cultivation. Indeed, because the winter weather west of the Cascade Mountains is relatively mild, the blooming season is often quite long. In the Rhododendron Species Botanical Garden flowers often begin to appear in early January on two or three species. The majority bloom from March through May with a smaller number of species flowering in June and July. The last species to flower is called Rhododendron auriculatum and it is especially attractive with its large white flowers which are fragrant!