Culture
This azalea is reportedly winter hardy to 0 degrees F. It is not reliably winter hardy in USDA Zone 6 (including St. Louis) where it must be well mulched and sited in a protected location insulated from winter winds. In general, it is best grown in acidic, humusy, organically rich, medium moisture, moisture-retentive but well-drained soils in part shade. It prefers a sun dappled or high open shade. Good soil drainage is essential (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. Shallow, fibrous root systems (do not cultivate around plants) will benefit greatly from a mulch (e.g., wood chips, bark or pine needles) to help retain moisture and stabilize soil temperatures. Roots must never be allowed to dry out. Acidify soils as needed. Clip off spent flower clusters immediately after bloom as practicable.
Noteworthy Characteristics
Rhododendron indicum is native to Japan. It is a dense, semi-evergreen azalea that typically grows to 2-3’ tall. Funnel-shaped, red to scarlet flowers (to 2.5” diameter) bloom in trusses in late (May). Lanceolate to oblanceolate, slightly toothed leaves (to 1.5” long) are glossy dark green above and paler beneath. Leaves acquire reddish tints in winter.
Genus name comes from the Greek words rhodo meaning rose and dendron meaning tree. Transferred from the Greek name for Nerium oleander.
Specific epithet means of India but this plant is native to Japan.
R. ‘Macrantha’ is a commonly sold cultivar of this species.
Problems
Rhododendrons and 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
Where winter hardy, grow in mass, in groups or as specimens in shrub borders, mixed borders, woodland gardens and shade gardens. In St. Louis, site this shrub in protected locations, including along walls and foundations that face south.