With its variety of shapes and flower colors, the rhododendron (Rhododendron spp.) is a staple of many landscapes. Each rhododendron species and variety has its own hardiness rating, but generally grow well in U.S. Department of Agriculture plant hardiness zones 5 through 9. While most home gardeners don't professionally rate rhododendrons, knowing how the experts do it can help you get starting on rating your own plants. Follow some or all of their steps and you may be able to evaluate your own rhododendrons.
In some catalogs or books, the rhododendron name will be followed by the letter "H" and a number, a climate hardiness rating developed by the Royal Horticultural Society and later expanded upon by the American Rhododendron Society (ARS).The ARS has largely abandoned this system in favor of an actual temperature reading. The ratings are the result of analyzing input from growers. H-1 rhododendrons survive to -25 degrees Fahrenheit, H-2 to -15 degrees F, H-3 to -5 degrees F, H-4 to 5 degrees F, H-5 to 15 degrees F, H-6 to 25 degrees F and H-7 to 32 degrees F.
The ARS assigns a number between 1 and 5 to three growing factors. The lowest rating (or evaluation) is 1, the highest 5. The first number determines the flower quality, which may include color, fragrance or shape. The second rates the shrub quality, which may include branching, leaf color and resistance to disease. The third rates performance, which may include growth habit. Many rhododendrons in catalogs might have numbers after the name that look like 3/4/2, for example. A committee determines the ratings by comparing the rhododendron to other rhododendrons, through suggestions of experienced members or through information provided from test gardens.
USDA Hardiness Zones
The U.S. Department of Agriculture assigns a plant hardiness zone to almost all commercially sold plants indicating the lowest temperatures at which the plant normally will not become damaged. Sometimes, hardiness is determined by simply observing where the plant naturally grows. However, new cultivar hardiness ratings are largely determined by nurseries themselves. Through extensive recordkeeping, peer reviews, testing plants at multiple locations and through collaboration with the USDA and universities, a hardiness rating is assigned, according to the Garden Writer's Association.
Sunset Garden Ratings
Sunset magazine uses climate zones to indicate hardiness taking into consideration several other factors beyond just winter temperatures. It evaluates ocean influence and elevation, among other variables. It gathers information from its extensive test plots to determine hardiness. According to Sunset, rhododendrons generally favor Sunset zones 4 through 6 and 15 through 17. However, every rhododendron variety is different.
- Missouri Botanical Garden: Rhododendrons and Azaleas
- The Old Farmer's Almanac: Rhododendrons
- Henning's Rhododendron & Azalea Pages: Hybrid Rhododendrons
- The American Rhododendron Society: Rhododendron Ratings
- American Rhododendron Society: Needed, a More Useful Rating System
- Cloud Mountain Farm Center: Rhododendron "Blaney's Blue"
- Garden Writer's Association: Behind the Scenes of New Plant Introductions
- Sunset: Find Your U.S. Sunset Climate Zone
- The Sunset Western Garden Book: Rhododendron
- American Rhododendron Society: Standards for Judging Rhododendrons (Azaleas)
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