Catawba rhododendron is the large-leaved, evergreen cousin of the azaleas, a native of the southern Appalachians that carries enormous trusses of lilac-purple bloom in late spring and reaches 6 to 12 feet. It and its hybrids ('Roseum Elegans', 'Nova Zembla', 'English Roseum') are the hardiest of the big-leaved rhododendrons, reliable to zone 4, and the backbone of the old shrub borders of the Northeast.
It wants what a mountain woodland gives: acid soil (pH 4.5 to 6) rich in organic matter, dappled shade, shelter from winter wind and afternoon sun, and steady moisture. The roots are shallow and fine, so plant it high, never deeper than it grew in the pot, mulch it with bark or pine needles and keep it watered in dry spells. Deadhead the trusses after bloom if you can reach them, and prune only to shape, right after flowering.
It is a landscape shrub rather than a cutting-garden plant: its value is scale and structure, evergreen all winter and spectacular for three weeks in spring. Bumblebees and the first swallowtails work the flowers. Deer browse it hard in winter where they are hungry, and the whole plant is poisonous if eaten. Leaves that curl and droop on a cold morning are normal; they open again as the day warms.
Ecoregions:Eastern Temperate Forests
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.
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.
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.
Fungal leaf spot
Brown or purplish spots on the leaves, often ringed with a yellow halo, that spread and merge in warm, humid, or wet weather; heavily spotted leaves yellow and drop early.
Pick off spotted leaves and clear fallen debris to remove overwintering spores. Improve air circulation by thinning, and water at the base so foliage dries quickly. If it persists, apply a copper- or sulfur-based fungicide at the first sign.
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.