Rhododendron care: acidic soil, drainage, and azalea lacebug

Q: When should I prune rhododendrons?

Per Clemson HGIC and NC State Extension Plant Toolbox, prune immediately after flowering is complete. Rhododendrons set next year's flower buds by mid to late summer. Any pruning after that point removes the following year's bloom. Deadhead spent flower trusses by snapping them off carefully, avoiding the new growth buds immediately below. Dead or damaged wood can be removed at any time. Do not prune in fall or winter.

Q: What is the white or gray coating on my rhododendron leaves?

There are two likely candidates, and they require different responses. If the coating is on the upper leaf surface in a patchy, dusted pattern, it is powdery mildew -- a fungal disease favored by humid, shaded conditions. If the leaves are stippled gray with dark tarry spots on the undersides, the cause is azalea lacebug. Per Missouri Botanical Garden, check the undersides for dark excrement and spiny nymphs to confirm lacebug. insecticidal soap on the leaf undersides controls lacebug; improving air circulation and reducing humidity addresses powdery mildew.

Q: Are rhododendrons deer-resistant?

Largely yes, but with exceptions. Rhododendrons are generally rated as resistant by most extension sources because of their toxicity -- the leaves contain grayanotoxins that are poisonous to many animals. In high deer-pressure environments, hungry deer may browse rhododendron foliage despite the toxins, though they are far less likely targets than hostas, daylilies, or tulips. In my zone 7a yard, the peonies and hostas would disappear long before deer turned to rhododendrons. The bigger deer threat to rhododendrons is antler rubbing, which damages stems mechanically. Wire cage protection around young shrubs during the rut reduces this risk.

Rhododendrons fail most often because of wrong soil pH or poor drainage. The soil must be pH 4.5-5.5, well-drained, and rich in organic matter. Azalea lacebug is the primary pest and is far worse on plants in full sun.

Pink rhododendron shrub with dense clusters of flowers in full spring bloom — Photo: Unsplash on Unsplash

By Thomas Joseph Published 2026-06-10 Updated 2026-06-13 Reviewed by Thomas Joseph 15 min read

I don't grow rhododendrons. My Long Island yard is sandy loam in full sun for most of the day, and rhododendrons require dappled shade, consistent moisture, and acidic well-drained soil rich in organic matter. The full-sun, sandy-soil combination that works well for my peonies and ornamental grasses is exactly what rhododendrons struggle in. On Long Island, the most successful rhododendrons I've seen grow on the north or east sides of houses — never in open, south-facing beds. This guide is built from Clemson HGIC, NC State Extension Plant Toolbox, Missouri Botanical Garden, RHS, and UC IPM for lacebug control.

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Rhododendron vs. azalea: what's the difference?

All azaleas are rhododendrons, but not all rhododendrons are azaleas. Botanically, azaleas are in the genus Rhododendron — the distinction is informal and based on growth habit and flower structure. For practical care purposes:

Rhododendrons (in the narrow sense): typically large-leaved, evergreen, with large trusses of 6 or more flowers per cluster. Flowers have 10 or more stamens. Mature size ranges from 3-20+ feet depending on species and cultivar.
Evergreen azaleas: smaller leaves, typically evergreen, flowers in smaller clusters with 5-6 stamens. Compact to medium size, 2-8 feet.
Deciduous azaleas: lose leaves in winter. More cold-hardy than most evergreen azaleas. Native species include flame azalea (R. calendulaceum) and smooth azalea (R. arborescens).

Care requirements overlap substantially — both need acidic soil, good drainage, and protection from hot afternoon sun. This guide covers both, with notes on where they differ.

Reader questions

Questions from other readers about this guide. Ask your own — if it's a good question, the answer goes here for the next reader.

What soil pH do rhododendrons need?

Per Clemson HGIC, rhododendrons require a soil pH between 4.5 and 6.0. Per NC State Extension Plant Toolbox, the soil pH must be "Acid (<6.0)." Per RHS, the optimal range is pH 5.0-6.0. If your soil is naturally above 7.0, consider container growing in an acidic potting mix rather than trying to durably acidify in-ground soil, which is difficult in soils with free lime. Per Penn State Extension, "lime can severely hamper the health of plants like blueberry, azalea, and rhododendron" -- never apply lime within the root zone of established rhododendrons.

When should I prune rhododendrons?

Per Clemson HGIC and NC State Extension Plant Toolbox, prune immediately after flowering is complete. Rhododendrons set next year's flower buds by mid to late summer. Any pruning after that point removes the following year's bloom. Deadhead spent flower trusses by snapping them off carefully, avoiding the new growth buds immediately below. Dead or damaged wood can be removed at any time. Do not prune in fall or winter.

What is the white or gray coating on my rhododendron leaves?

There are two likely candidates, and they require different responses. If the coating is on the upper leaf surface in a patchy, dusted pattern, it is powdery mildew -- a fungal disease favored by humid, shaded conditions. If the leaves are stippled gray with dark tarry spots on the undersides, the cause is azalea lacebug. Per Missouri Botanical Garden, check the undersides for dark excrement and spiny nymphs to confirm lacebug. insecticidal soap on the leaf undersides controls lacebug; improving air circulation and reducing humidity addresses powdery mildew.

Are rhododendrons deer-resistant?

Largely yes, but with exceptions. Rhododendrons are generally rated as resistant by most extension sources because of their toxicity -- the leaves contain grayanotoxins that are poisonous to many animals. In high deer-pressure environments, hungry deer may browse rhododendron foliage despite the toxins, though they are far less likely targets than hostas, daylilies, or tulips. In my zone 7a yard, the peonies and hostas would disappear long before deer turned to rhododendrons. The bigger deer threat to rhododendrons is antler rubbing, which damages stems mechanically. Wire cage protection around young shrubs during the rut reduces this risk.

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Rhododendron care: acidic soil, drainage, and azalea lacebug

Rhododendron vs. azalea: what's the difference?

Rhododendron care: acidic soil, drainage, and azalea lacebug

Rhododendron vs. azalea: what's the difference?

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