Rhododendrons (Rhododendron spp.) are flowering shrubs that are hardy in U.S. Department of Agriculture plant hardiness zones 4 through 9, depending on the variety. The fastest-growing rhododendron cultivars grow 7 feet in 10 years, according to the American Rhododendron Society's website.
Range of Growing Rate
The society's website lists 44 named cultivars that grow 1 foot in 10 years, 164 cultivars that grow 2 feet, 366 cultivars that grow 3 feet, 281 cultivars that grow 4 feet, 288 cultivars that grow 5 feet and 186 cultivars that grow 6 feet in 10 years. The cultivars all vary by their time of blooming, bloom color and USDA hardiness zones. Slower growing rhododendrons tend to be smaller cultivars while the fastest growers take up more space. Rhododendrons grow from 1 to more than 20 feet tall, depending on the species, according to a National Gardening Association website article.