Azaleas are the defining shrubs of the spring garden: massed flowering in pink, red, orange, white and purple from mid-spring, on plants from 2 to 8 feet depending on the type. They are rhododendrons (the line between the two is botanical, not horticultural) and share the same needs: acid soil, good drainage and dappled shade. The evergreen azaleas are the Japanese and Southern Indian types of mild climates; the deciduous azaleas, many of them native to the eastern United States, are the hardy, fragrant ones for cold winters.
Plant them 4 to 8 feet apart in acid soil (pH 4.5 to 6) rich in organic matter, high rather than deep, and mulch with bark or pine needles; the roots are shallow and fine, so they need steady water in dry spells and never lime. Prune only right after flowering, since the buds for next year form in summer. Lace bugs stipple the leaves of evergreen azaleas in full sun; shade is the cure.
They are not grown as cut flowers, since the branches are short-lived in water and the plants too valuable to harvest, but a few forced branches in late winter are a treat. Bumblebees and the early swallowtails work the flowers, and the native deciduous azaleas (pinxterbloom, flame azalea, swamp azalea) feed far more wildlife than the Asian evergreens. Deer browse azaleas hard. The whole plant is poisonous if eaten.
Powdery mildew
White or grey powdery coating on leaves - usually starting on older growth in humid conditions or when nights cool.
Improve air circulation by thinning plants. Apply neem oil or potassium bicarbonate spray at first sign. Avoid overhead watering.
Botrytis (grey mould)
Grey fuzzy mould on petals and stems, worst in cool wet conditions.
Remove affected parts immediately. Improve air circulation. Avoid overhead watering. Apply copper fungicide if severe.
Crown Rot
Lower leaves yellow and wilt; the base of the rosette turns brown and soft, sometimes with white fungal threads at the soil line. Plants collapse in wet conditions.
Remove and destroy affected plants. Improve drainage and avoid overhead watering. Do not mulch directly against the crown. Space plants for airflow and avoid replanting in the same wet spot.
Petal blight
Flowers develop small watery spots that rapidly enlarge into soft brown blotches, collapsing the bloom into a slimy mass that clings to the plant instead of dropping cleanly.
Remove and destroy infected blooms and fallen petals, and clear old mulch beneath the plant where the fungus overwinters. Avoid overhead watering; in wet springs a preventive fungicide at bud break helps where it recurs.
Leaf gall
Leaves, buds, or young shoots thicken into pale green, reddish, or whitish fleshy galls that later turn white with a powdery spore bloom before browning.
Pick off and destroy galls before they turn white and shed spores. Improve air circulation and avoid overhead watering in spring. The damage is largely cosmetic and rarely needs fungicide.
Azalea Lace Bug
Stephanitis pyrioides →Leaf upper surfaces show stippled, silvery-white speckling; undersides carry dark spots of excrement and small clear-winged insects. Heavy feeding bronzes foliage.
Spray leaf undersides with insecticidal soap or horticultural oil when nymphs appear in late spring. Repeat through the season. Site plants in shade, since stressed sun-grown shrubs are hit hardest.