Azaleas are divided into two subcategories: deciduous and evergreen. Deciduous azaleas are native to North America; evergreens are native to Japan. Azaleas are often mistaken for rhododendrons; they are in the same plant family and look very similar. The most common way to tell the two species apart is to look at the leaves. Azaleas have only five or six stamens, rhododendrons have 10 stamens. The leaves of azaleas are thinner, more pointed and have hairs on the underside. Thousands of varieties of azaleas exist; nurseries most frequently carry hybrid evergreen varieties.
Glen Dale varieties of azaleas were developed at the National Arboretum in Glen Dale, Maryland in 1935. They were developed to provide home growers with a cold-tolerant plant. They do well in USDA hardiness zones 6 to 9. Glen Dale azaleas are slow-maturing plants that reach heights between 3-5 feet. Pink to red-orange blooms range in size from 1 1/2 to 3 inches, and appear from late April until June. Currently in existence are over 400 species of Glen Dale azaleas.
Indica evergreen varieties are not native to India as the name would suggest; they originated in China. Indica azaleas are classified into two types: Belgian and Southern. Belgian indica azaleas thrive in warm climates where temperatures rarely get below freezing (zones 8 to 11). Southern indica azaleas will do well in colder climates, tolerating temperatures as low as 15 degrees Fahrenheit. Both varieties of azaleas mature in 5-10 years with heights ranging between 6 and 10 feet. Flowers have double layers of petals ranging in color from white to red and reaching a diameter of 2-3 inches. Belgian hybrids bloom from early spring to late summer; Southern hybrids bloom from mid-summer until late to mid-fall.
Kaempferi hybrid varieties of azaleas originated in Japan. They thrive on the East Coast of the United States in zones 6 to 9. Kaempfrei reach heights of 5-6 feet in approximately 5 years. Blooms appear late in the growing season with some varieties blooming in late fall and winter. The flowers range in size from 1 1/2 to 2 inches. Kaemfreir have a range of colors predominately pink to red with some flowers blooming purple and white. In colder climates, zones 6 to 7, kaemfreir lose their leaves in the winter.
Kurume azaleas are a large group that has been in cultivated in Japan for over 300 years. These hybrids tolerate a small amount of cold, thriving in zones 7 to 8. Flowers bloom in early spring through mid summer. Kurume flowers are small ranging in size from 1/2 to 1 1/2 inches. They offer a wide range of colors including pink, red, purple and white. Kurume are compact bushes ranging in height from 18-30 inches, with small, shiny leaves.
Japanese culture exalts this variety of azaleas, holding festivals and exhibitions in early June. Satsuki azaleas bloom from May until June. Blooms range in colors providing solid and multicolored flowers small flowers; often there is more than one color or color combination on a single plant. Satsuki are small bushes, compact and easily shaped. They are delicate plants that require quite a bit of care and grow best in shaded gardens in zones 7 to 9.