Rhododendron and azalea species are found growing in the wild from the arctic region to the tropics.  Regions suitable for growing rhododendrons and azaleas are those that have naturally acidic soils, adequate water availability, moderate humidity and winds and lack of temperature extremes.  In the U.S. such regions run along the east and west coasts, along the Gulf of Mexico and around the Great Lakes.

The climate of the U.S. Pacific Northwest region, especially between the Cascade Mountain range and the Pacific Ocean, is very accommodating to the growth of many types of rhododendron hybrids and species.  The favorable climate extends down the coast to the San Francisco Bay area, which is about the southern limit for growing the large-leaved elepidote Large leafed, large-sized plants at maturity, that do not have scales on the underside of the leaves.varieties.  Many varieties of small-leaf lepidote Small leafed, usually low growing plants, often bloom earlier than elepidotes, with small scales on the leaf undersides.rhododendrons also thrive in gardens along the U.S. west coast states.

With attention to suitable hardiness, large-leaf rhododendron varieties do well in the eastern United States, generally along the Atlantic coast from Nova Scotia to the upper Carolinas, along both sides of the Appalachian Mountains and in southern Ontario, Michigan, Illinois, Wisconsin, Ohio, Pennsylvania, New York, New Jersey and Connecticut.

Evergreen and deciduous azaleas do well on the west coast from British Columbia to the Mexican border, along the Gulf Coast and throughout the southeastern U.S. and along the east coast up into the Boston area.  Some deciduous azaleas are very cold hardy and some tolerate heat and humidity.   At least two species of deciduous azaleas are native to eastern Canada and several more species are found in New England.  Many species of deciduous azaleas are native to the east, southeast and south and one species grows wild in the west.  Hybrids derived from the native species thrive over large areas of the United States.

Throughout most of the Middle West and the Great Plains rhododendrons can be grown with special attention to growing conditions, and selecting very hardy varieties.  When placing plants choose locations that mitigate extreme temperature changes, especially in spring and fall, and provide protection from drying summer or winter winds.

Some of the maddenii-type rhododendrons grow quite readily in southern California. The tropical vireya rhododendrons do well in the Los Angeles basin, and can be grown outdoors in Florida, Hawaii and in parts of Australia and New Zealand.

For many decades rhododendron and azalea hybridizers have been busy creating new cultivars that have extended the range of flower color, bloom time, plant size, and hardiness, resistance to disease, etc.  This has greatly expanded the regions where rhododendrons and azaleas can be successfully grown.

More information about rhododendron and azalea suitability in the landscape can be found in the following Journal ARS articles:

  Growing Azaleas In New England by Charles W. Findlay, Jr.
  Summer-blooming Weston Hybrids by C. J. Patterson
  Growing Rhododendrons and Azaleas in the Middle Atlantic States by Sandra McDonald
  Rhododendrons in Middle America by Donald W. Paden
  Reliable Rhododendron Species for Pacific Northwest Woodland Gardens by Rudolph Henny
  Growing Rhododendrons in Warm Climates by Willis Harden
  Evergreen Azaleas Up From Inferiority by Franklin H. West