Culture
Best grown in acidic, organically rich, humusy, medium moisture, moisture-retentive but well-drained soils in part shade. Prefers a sun dappled or high open shade. Morning sun with afternoon shade is also acceptable. Tolerates a fair amount of sun in cool northern summers, but leaves may scorch in hot afternoon sun in the St. Louis area. Plant in a location protected from strong winds. Plants perform well on north or east facing slopes. Do not site plants within or near the drip line of trees in the walnut family (most rhododendrons/azaleas are sensitive to toxic juglones produced by roots of walnuts, butternuts, pecans and hickories). Good soil drainage is essential (plants do not 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 (plants generally like soil pH in the range of 5.0 to 5.5). Add sulfur or iron sulfate to soils to lower the pH. Add limestone or lime to soils to raise the pH. Clip off spent flower clusters immediately after bloom as practicable.
Noteworthy Characteristics
Rhododendron is a genus of 500 to 900 species and includes both of what we commonly call rhododendrons and azaleas. Most are evergreen but some are deciduous. They originate mostly from the Northern Hemisphere with high concentrations in western China, the Himalayas and Myanmar (Burma). They are grown for their showy spring flowers and in the case of evergreen types for their attractive winter foliage. True rhododendrons have 10 stamens in a flower and azaleas have only 5. Much hybridization has resulted in a great number of hybrid cultivars. Of note to gardeners in cool temperate areas are the large and small leaved evergreen rhododendrons and the evergreen and deciduous azaleas.
The evergreen rhododendrons produce large to small, linear leaves that are stiff and evergreen. They usually branch from a central trunk unlike the evergreen and deciduous azalea that are multi-stemmed from the ground. Large trusses of flowers are produced at the tips of branches followed by a flush of new leaves. Colors range from white to pink, red, and purple. Size can range from 1-2 feet to over 15 feet tall depending upon the cultivar.
Genus name comes from the Greek words rhodo meaning rose and dendron meaning tree. Transferred from the Greek name for Nerium oleander.
'Marydel' was found by the horticulturalist, Polly Hill, in the wild in Delaware near the Choptank River and the town of Marydel. It is believed to be a natural hybrid of Rhododendron atlanticum and Rhododendron periclymenoides. It has deep pink buds that open into 1 3/4 in., very fragrant, white flowers that have a deep pink tube and pink at the margins of the petals. 'Marydel' is a semi-dwarf shrub that grows 3 to 4 ft. tall and 5 to 6 ft. wide.
Problems
Rhododendrons and azaleas are susceptible to many insect and disease problems. Insect pests include aphids, borers, lacebugs, caterpillars, leafhoppers, mealybugs, nematodes, scale, thrips and whitefly. Mites may also appear. Disease pests include blights, canker, crown rot, leaf gall, root rot, leaf spot, rust and powdery mildew. Chlorosis (leaves turn yellow) often indicates an iron deficiency in the soil that is often caused when the soil pH becomes too high. A healthy plant in the proper environment with proper care should have limited problems, however.
Uses
Mass, group or specimen. Shrub borders, mixed borders, woodland gardens and shade gardens. Also effective in foundation plantings or as a hedge. Woodland margins.