Species are distinct groups of plants that are naturally found in the wild. When natural variation to a species occurs in the wild, it is indicated in botanical nomenclature as a subspecies (a major variation often found in a distinct geographical population) or as a variety (a lesser variation).
A hybrid is a cross of two different species, varieties, or hybrid groups, often to select for qualities such as hardiness, flowering time, flower size or color. The great diversity seen on the market is the result of patient hybridizing work and excellent record-keeping by nurserymen and plant breeders.
These hybrids are the work of Joseph Gable of Stewartstown, Pennsylvania, begun in the late 1920s. The crosses started with Rhododendron poukhanense and R. kaempferi and incorporate many other species and hybrids such as R. indicum and Kurume hybrids. Gable hybrids are evergreen azaleas hardy to Zone 6; they generally grow 4 to 6 feet tall.
Some cultivars: R. ‘Ben Morrison’, R. ‘Big Joe’, R. ‘Boudoir’, R. ‘Carol’, R. ‘Louise Gable’
Some of the most important work in hybridizing in the United States began in the 1930's at the U.S. Department of Agriculture’s Plant Introduction Station in Glenn Dale, Maryland. The hybrids were the work of Benjamin Y. Morrison, who successfully married the large flowers and exciting colors of azaleas developed in the southern United States with the hardiness of more northerly species. Glenn Dale hybrids flower from mid-April into June (in Zone 7).
Some cultivars: R. ‘Cadenza’, R. ‘Manhattan’, R. ‘Martha Hitchcock’, R. ‘Polar Sea’
These evergreen azaleas were developed by Harry Weiskittel at Marshy Point Nursery in Maryland. Many of these hybrids have large, showy, double flowers and are so floriferous the shrub seems like it’s engulfed in blooms.
Some cultivars: R. ‘Carol Kittel’, R. ‘Lady Baltimore’, R. ‘Marshy Point’s Heart Throb’
These hybrids come from Weston Nurseries in Hopkinton, Massachusetts. The hybridizing work was started by Ed Mezitt in the early 1930's. They have been selected and bred for their hardiness and wide range in flower color and flowering time; many are mildew resistant.
Some cultivars: R. ‘Framingham’, R. ‘Landmark’, R. ‘Lemon Drop’, R. ‘Lollipop’, R. ‘Millennium’, R. ‘Parade’
As their name suggests, these hybrids of Robert E. (Buddy) Lee from Independence, Louisiana, flower in spring and again from midsummer through fall (although not as prolifically). These azaleas do well in sun but they are not particularly hardy: a few are hardy to Zone 6 but most are hardy to Zones 7 to 9.
Some cultivars: R. ‘Autumn Belle’, R. ‘Autumn Chiffon’, R. ‘Autumn Sangria’
These hybrids were developed in Holland in the early 1900's. The primary parent, R. kaempferi, was crossed with a selection of other species and hybrids to produce semi-evergreen azaleas that are known for their cold hardiness. They flower in May to June.
Some cultivars: R. ‘Annamaria’, R. ‘White Lady’
These hybrids come from crossing evergreen azaleas found in the southern part of the Japanese island of Kyushu, predominately R. kaempferi, R. kiusianum, and R. sataense. The flowers range in color from white to pink, salmon, scarlet, crimson, magenta and purple. Semi-evergreen and incredibly floriferous, these are the azaleas often seen sheered into shapes.
Some cultivars: R. (Obtusum Group) ‘Amoenum’, R. ‘Hinomayo’, R. ‘Koromo Shikibu’
Begun in the 1960's by Polly Hill of Martha’s Vineyard, Massachusetts, many of these hybrids come from R. nakaharae. They have a low-growing, mounding habit.
Some cultivars: R. ‘Choptank Rose’, R. ‘Joseph Hill’ ‘R. ‘Michael Hill’
The creation of Robert Gartrell of Wycoff, New Jersey, started in the late 1930's; these late-blooming, evergreen azaleas resemble the showy flowering patterns of Satsuki azaleas. They generally grow 3 to 5 feet tall and are hardy to Zone 6.
Some cultivars: R. ‘Conversation Piece’, R. ‘Dorothy Hayden’, R. ‘Lady Robin’
A favorite in Japan, this group of evergreen azaleas has the widest variation in range of flower color, leaf shape and growth habit. They often include as parents R. indicum (narrow leaves) and R. eriocarpum (broad, elliptical leaves). Many of these azaleas are used for sheering into mounded forms while others are used for container plantings. Satsuki means fifth month and, indeed, these hybrids are relatively late blooming for evergreen azaleas, flowering in mid-May and June; they are generally hardy to Zone 7. Flowers range from less than 1 inch to 5 inches in size; can be solid, dotted or flecked in color, and come in different shapes.
Some cultivars: R. ‘Chinzan’, R. ‘Gumpo White’, R. ‘Gunrei’, R. ‘Miyuno-no-tsuki’
Another excellent nursery that not only sells but also hybridizes azaleas is Rare Find Nursery in Jackson, New Jersey.
Rhododendrons & azaleas : a colour guide by 
Rhododendrons and Azaleas by 
Success with Rhododendrons and Azaleas by 
The Timber Press Encyclopedia of Flowering Shrubs by 
