Culture
Best grown in light, acidic, sandy, well-drained soils in part shade. Tolerates well-drained humusy loams. Although winter hardy to USDA Zone 4, it does not prosper in areas with high summer temperatures, and is not recommended for planting south of USDA Zone 7. Prefers a sun dappled shade or high open part shade. Foliage may scorch in full sun unless soils are kept uniformly moist. Consistent moisture is best, but soils must drain well (doesn’t like “wet feet”). Poor drainage inevitably leads to root rot, therefore raised beds/plantings should be considered in heavy clay soils such as those present in much of the St. Louis area. Roots must never be allowed to dry out. Acidify soils prior to planting and thereafter as needed. Shallow, fibrous root systems (do not cultivate around shrubs) will benefit from a good mulch (wood chips, bark or pine needles) for retention of moisture, stabilization of soil temperatures and winter protection. Clip off spent flower clusters immediately after bloom as practicable.
Noteworthy Characteristics
Rhododendron arborescens, commonly known as sweet azalea (fragrant flowers), smooth azalea (glabrous twigs and leaves) or tree azalea (occasional tree-like height), is an upright, loosely-branched deciduous shrub that typically matures to 8-12’ (occasionally to 20’) tall. It features smooth, elliptic to obovate, often blunt-tipped, dark green leaves (1-3” long) with whitened undersides. Leaves turn red to purple in fall. Fragrant, sticky-hairy, funnel-shaped, white (occasionally pale pink) flowers (to 2” across) bloom in clusters of 3-6 from late May to July after the leaves have appeared. Each white flower contains a red style and exserted dark pink to red stamens which protrude beyond the mouth of the tubular corolla. This azalea is native to areas along streams on woodland slopes and moist shrubby balds in the Appalachian Mountains from southern Pennsylvania and eastern Kentucky to Georgia and Alabama. It is also sometimes found near bogs and swampy woodland areas. Fruits are narrow, egg-shaped capsules (to 3/4” long).
Genus name comes from the Greek words rhodo meaning rose and dendron meaning tree. Transferred from the Greek name for Nerium oleander.
Specific epithet comes from Latin meaning more or less treelike.
Azalea means dry in reference to a reported preference some azaleas have for dryish soils.
Problems
Azaleas are susceptible to many insect and disease problems, including but not limited to canker, crown rot, root rot, leaf spot, rust, powdery mildew, aphids, borers, lacebugs, leafhoppers, mealybugs, mites, nematodes, scale, thrips and whitefly. A healthy plant in the proper environment with proper care should have limited problems, however.
Uses
Group or specimen for shrub borders, mixed borders, open woodland gardens, native plant gardens and open shade gardens. Effective near patios and as a specimen around the home.