Azaleas belong to the genus Rhododendron. Evergreen azaleas are in the subgenus Pentanthera and deciduous azaleas are in the subgenus Tsutsutsi. In very general terms, azalea flowers usually have 5 stamens, while rhododendrons usually have 10 or more stamens. Azalea leaves have hairs parallel to the leaf surface, usually along the midrib on the underside of the leaf, and tend to be thinner, softer and more pointed than rhododendron leaves. Azaleas flower along the sides of the stems as well as at their ends, while rhododendrons usually flower only at the ends.
The different varieties of azaleas are classified as being species or hybrids. Species grow true from seed (unless they are crossed with other species or hybrids). Hybrids are crosses between other species or hybrids, and may be faithfully reproduced only from cuttings, which are clones of the mother plant. Azaleas have been hybridized for hundreds of years, and almost 10,000 different varieties have been named, although far fewer are in the trade. Azaleas provide a wide variety of plant habits, sizes, colors and bloom times to meet almost every landscaping need or personal taste.
All native American azaleas are deciduous (drop their leaves in the fall), with flower colors ranging from white to purple, pink, red, orange and yellow. Evergreen azaleas, native primarily to Japan, have flower colors including white and various shades of purple, pink, red and reddish orange, but not yellow. Color patterns include single colors and bicolors as well as sectors, stripes and flecks, sometimes on the same plant.
Bloom sizes vary greatly on different varieties of azaleas. Compare the half-inch blossom on R. serpyllifolium with the 4 to 5 inch blooms on ‘Higasa’. There are many different bloom types, ranging from “single” flowers with five (rarely six) petals, to “hose-in-hose” flowers with 10 to 12 petals, to “double” flowers with a variable number of petals, to “double hose-in-hose” flowers with thirty or more petals (‘Balsaminiflorum’). Petal shapes vary greatly, from thin strap-like petals (‘Koromo Shikibu’) to completely overlapping and rounded petals. Petal edges may be flat, recurved, wavy or ruffled. Leaves vary from as little as 1/4 inch to well over 6 inches in length, commonly 1 to 2 inches for evergreen azaleas. While normally green, some plants have leaves with white or yellowish mottling (‘Keiretsu’) or edges (‘Silver Sword’).
Plant habits of the different varieties range from stiffly upright, to broad spreading, to irregular. Plant height can range from under a foot to well over 15 feet. A few evergreen varieties (‘Pink Cascade’) are weeping and may be grown as a hanging basket. Many varieties are dense and compact, others are quite open, and some almost tree-like.
Most azalea varieties bloom in the spring, around May in the mid-Atlantic area of the United States, with some blooming a month or so earlier, and some as late as August. Blooms typically last for one or two weeks. In warm climates such as the deep south, some azalea varieties bloom again in the fall.
Azaleas are relatively pest-free, forgiving and easy to grow plants. Their cultural needs include:
- High shade is preferable but some varieties do well in full sun, especially deciduous types. While more sun typically produces more compact plants with more blooms, the blooms will not last as long.
- Slightly acid soil (pH 5.5-6) is best and is usually found under oak, pine and holly trees.
- A mulch of pine bark, pine needles or wood chips helps to keep moisture in the ground, even out changes in the soil temperature, and keep weeds out. An inch or so around the root zone and a thicker layer between the plants is
- Azaleas do not like “wet feet”. Good drainage is most easily provided by planting azaleas with the tops of their root balls a few inches above ground level and mounding the soil up to the plants. This is particularly important with heavy clay soil.
- Azaleas like moist soil at their roots. This may require supplemental watering through early fall, at least until plants are established in the ground for a few years. Adequate water after bloom helps to produce more flower buds for next year. An infrequent deep soaking is more effective than superficial sprinkling. The amount of water needed depends on the soil, temperature, humidity, wind and sunlight. In a dry fall, water heavily after a good frost, before cold weather sets in.
- Established azaleas do not need fertilizer. To avoid inducing new growth which may be killed in the winter, do any fertilizing in late winter or early spring; never after July 1.
- To avoid cutting off next year’s flower buds, do major pruning of azaleas soon after they bloom. Shortening or removal of long slender stems with no side shoots and cutting out dead wood may be done at any time.
- As needed, a fungicidal spray in the spring as the buds show color will control “petal blight”, a fungal disease that appears as discolored dots on the petals and quickly discolors and collapses the blossoms.
For information on which varieties are best suited to growing conditions in your area and meet your personal preferences, we recommend your talking with mebers of your local Azalea Society chapter. They may also suggest sources of unusual varieties. You will find them to be quite friendly and generous in sharing information–sometimes even plant material.
An alternative source of information is the online azalea discussion forum (see the Ask Us page). When requesting azalea information, be sure to describe your landscape objectives and your local growing conditions in some detail.