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At a glance
| Botanical name | Rhododendron spp. (azalea groups) |
|---|---|
| Family | Ericaceae (heath family) |
| Type | Evergreen or deciduous flowering shrub |
| USDA zones | Varies by type; native pinxterbloom azalea 4–9; choose cultivars rated for your zone |
| Mature size | Evergreen groups about 2–10 ft (Gable 2–4, Southern Indica 7–10+); native deciduous species 3–15 ft |
| Bloom | Spring (rebloomers into fall; some natives in summer) |
| Light | Filtered shade, or morning sun and afternoon shade |
| Soil | Acidic (pH 4.5–6.0), moist, well-drained, high organic matter |
| Native range | Asia (most evergreen types); eastern North America (many deciduous species) |
| Toxicity | Toxic to dogs, cats and horses (ASPCA) |
Azalea vs rhododendron, and the two main types
All azaleas belong to the genus Rhododendron. Clemson Extension separates them from "true" rhododendrons by a few traits: azaleas have five stamens, leaves without scales and often hairy, and mostly funnel-shaped flowers, while rhododendrons have ten or more stamens, are usually evergreen and tend to have bell-shaped flowers. Most azalea species are deciduous, though most azaleas used in home landscapes are evergreen. Care is similar; the large-leaved types are covered in rhododendron care.
| Evergreen azaleas | Deciduous azaleas | |
|---|---|---|
| Origin | Asian species and hybrid groups (Southern Indica, Kurume, Kaempferi, Satsuki, Gable, Glen Dale, Encore) | Many North American natives (flame, pinxterbloom, swamp azalea) and hybrids such as Exbury and Knap Hill |
| Winter | Keep leaves | Drop leaves; bare stems in winter |
| Hardiness | Varies by group; Clemson Extension calls Kurume cold-hardy and Kaempferi, Gable and Glen Dale very cold-hardy | Varies; native pinxterbloom is hardy in zones 4–9 (Missouri Botanical Garden) |
| Flowers | Kurumes can be completely covered in small flowers in spring | Many natives are fragrant; pinxterbloom flowers just before the leaves emerge |
| Light | Filtered light, afternoon shade | Tolerate more sun where summers are cool |
Reblooming evergreen azaleas such as the Encore series flower in spring and again in late summer or fall (Clemson Extension). For the full rundown of groups, see types of azaleas; for specific recommendations by zone and bloom time, see best azalea cultivars.
Zones and where azaleas grow
Hardiness depends on the type. Clemson Extension describes Kurume azaleas as cold-hardy and Kaempferi, Gable and Glen Dale hybrids as very cold-hardy, and Missouri Botanical Garden lists the native pinxterbloom azalea for zones 4–9. Clemson advises planting only varieties known to be hardy in your area: cold snaps can split bark, especially when early morning sun hits plants after a hard freeze, and azaleas in partial shade usually suffer less cold injury.
In hot, humid areas the limits are heat and root disease. Clemson Extension notes that some natives, such as Florida flame and Oconee azalea, are very heat tolerant, while Knap Hill and Exbury hybrids are not recommended for the Coastal Plain. Drainage and mulch matter even more there, because root rot thrives in wet soil.
Espoma Holly-tone 4-3-4
Azaleas use the same acid-forming fertilizers as blueberries; the comparison covers which ones Extension recommends. See how we chose it.
How much sun do azaleas need?
Clemson Extension describes azaleas as preferring light to moderate shade, such as the filtered shade of tall pines. If no filtered shade is available, choose a site with morning sun and afternoon shade.
- Too much sun: flowers fade faster and plants suffer more water stress and lace bug damage. Clemson Extension notes azaleas in full sun are more susceptible to lace bugs than those grown in partial shade, and flowers last longer in filtered sun.
- Too much shade: Clemson Extension warns heavy shade causes weak growth and fewer flowers.
- Exceptions: some deciduous natives tolerate sun in cool-summer climates but scorch in hot afternoon sun (Missouri Botanical Garden).
Soil: acidic, loose and well-drained
Most azalea problems start in the soil. Clemson Extension says azaleas grow best in acid soil, pH 4.5–6.0, that is well drained and high in organic matter. They are shallow-rooted, most roots sit in the top foot of soil, and they are easily damaged by excess soil moisture.
- Test first. When pH is too high, iron becomes unavailable and the youngest leaves turn yellow between darker green veins (iron chlorosis), per Clemson Extension. Testing options are in how to test soil pH at home.
- Adjust pH before planting according to the soil test. Acidifying soil takes months, not days; rates and expectations are in lowering soil pH for blueberries and azaleas.
- Amend the whole bed, not the hole. Clemson Extension recommends mixing organic matter, ideally composted pine bark, through the entire rooting area, about 1–2 inches worked into a bed 6–8 inches deep, rather than filling one planting hole with peat that holds water like a sponge.
- Heavy clay: plant in a raised bed or amend as large an area as possible and set plants high. Wet soil is what triggers Phytophthora root rot.
How to plant an azalea bush
Container-grown azaleas can be planted any time of year with careful watering, but Clemson Extension notes that fall planting lets the plant establish before the next summer's heat, while spring planting needs extra attention to watering.
- Prepare a wide bed rather than a single hole, amended with organic matter if the soil is poor, and acidified if a test calls for it.
- Loosen the root ball. If roots circle the outside of the container ball, Clemson Extension suggests three or four evenly spaced, quarter-inch-deep cuts from top to bottom so roots grow out into the soil.
- Plant high. Dig only as deep as the root ball and set its top level with or slightly higher than the ground, never lower (Clemson Extension). Clemson notes crown rot is favored when plants are set too deeply.
- Backfill and water thoroughly to settle the soil.
- Mulch 2–3 inches of pine straw, composted pine bark or leaves, kept a couple of inches off the main stem (Clemson Extension).
Spacing: space plants by their expected mature spread. Clemson Extension's method for mixed plantings is to add the two mature spreads and divide by two. Check the cultivar's tag.
Watering, mulching and fertilizing
Water: shallow roots dry out quickly, and Clemson Extension says azaleas need irrigation during dry periods, especially those planted in spring. To check, pull back the mulch near the base: if the top few inches of soil feel dry, soak the soil to at least 6–8 inches deep. Use soaker hoses or drip lines, since overhead watering may promote disease. Do not keep the soil soggy.
Mulch: a permanent 2–3 inch layer of pine straw, composted pine bark or leaves conserves moisture, moderates soil temperature and adds organic matter as it breaks down (Clemson Extension).
Fertilizer: Clemson Extension notes azaleas have low nutritional needs, and well-amended, mulched soil often supplies enough. Fertilize only for a reason, such as a soil test result or pale, slow growth. The best time is late spring (after new growth emerges) to early summer, using a complete, extended-release, acid-forming azalea fertilizer; never fertilize in late fall, which can delay dormancy and cause injury.
When to prune azaleas
Prune right after flowering. Azaleas form next year's flower buds in midsummer; UGA Extension notes many azaleas in Georgia begin setting buds in July, so pruning after early July reduces the next spring's flowers. Clemson Extension also recommends pruning just after spring flowering to protect the following year's bloom.
- Thinning cuts: remove leggy branches back to a main stem or another branch. Clemson Extension and UGA Extension both note thinning can be done any time of year without significant impact on flowering.
- Young plants: UGA Extension suggests tipping new shoots after 5 or 6 inches of growth to build a compact, well-branched plant with more blooms.
- Overgrown shrubs: renew them by cutting back to within 6–12 inches of the ground before spring growth begins (Clemson Extension; UGA Extension gives 12 inches in early spring). That season's flowers are sacrificed, and the soil should be kept moist afterward.
Azaleas follow the "prune after bloom" rule for spring-flowering shrubs; see how to prune flowering shrubs.
When do azaleas bloom?
Most azaleas bloom in spring, and Clemson Extension notes bloom time depends on the variety, the weather, the planting zone and the microclimate. In South Carolina, for example, Southern Indica azaleas bloom from late March into early April, Kurumes from late March to mid-April, Kaempferis in late April and early May, and Satsukis from May to June. Some native deciduous species bloom later: swamp azalea from mid-May into June and plumleaf azalea in July and August. Reblooming series such as Encore flower again in late summer or fall. Mixing early, midseason and late cultivars stretches the season; the options are compared in best azalea cultivars.
Common azalea problems
| Symptom | Likely cause | Fix |
|---|---|---|
| Yellow leaves with green veins, newest growth first | Iron chlorosis from high soil pH | Test pH and adjust; a foliar iron spray gives a quick, temporary green-up |
| Pale, stippled leaves | Azalea lace bug | More shade; see lace bug on azalea |
| Wilting that water doesn't fix; soggy, blackened roots | Phytophthora root rot | No cure once symptoms appear; remove the plant and don't replant azaleas in that spot |
| Freckled spots on petals that turn soft and watery; flowers rot | Petal blight (Ovulinia azaleae) | Pick and destroy infected flowers; avoid overhead watering; replace old mulch |
| Thick, fleshy, pale leaf galls in April and May | Leaf gall (Exobasidium vaccinii) | Pick off and destroy galls |
| No flowers on a healthy plant | Pruned after buds formed, or too much shade | Prune only right after bloom; give filtered sun or morning sun |
Clemson Extension calls Phytophthora root and crown rot one of the most common disease problems of azaleas and rhododendrons and notes chemical controls are ineffective once symptoms appear, so prevention through drainage and shallow planting is the only effective strategy. Clemson also lists lace bugs and spider mites as the most common insect pests.
Deer, pets and other questions
Deer: azaleas are not generally rated as deer-resistant shrubs. Where deer are common, use fencing or repellents from the start; see starting a deer-resistant garden.
Pets: the ASPCA lists azaleas as toxic to dogs, cats and horses because of grayanotoxins; clinical signs include vomiting (not in horses), diarrhea, weakness and cardiac failure. If a pet eats azalea leaves, contact a veterinarian or ASPCA Animal Poison Control.
Common mistakes
| Mistake | Why it matters | Do this instead |
|---|---|---|
| Planting with the root ball below grade | Deep planting favors crown rot and smothers shallow roots | Set the top of the root ball level with or slightly above grade |
| Pruning in late summer, fall or winter | Removes buds that formed in midsummer | Prune right after bloom, and not after early July |
| Skipping the soil test | High pH causes chlorosis that fertilizer cannot fix | Test first and adjust pH to 4.5–6.0 |
| Full afternoon sun on traditional evergreen types | Scorch and heavy lace bug damage | Filtered light or morning sun, afternoon shade |
| Fertilizing in late fall | Can delay dormancy and cause winter injury | Feed lightly from late spring to early summer only |
| Planting in poorly drained clay | Triggers Phytophthora root rot | Raised bed or berm with organic matter |
Frequently asked
When should you prune azalea bushes?
Prune right after flowering ends. Azaleas set next year's flower buds in midsummer, so later pruning removes the coming spring's blooms; UGA Extension advises not pruning after early July. Clemson Extension and UGA Extension both recommend pruning right after bloom.
Do azaleas need full sun?
No. Clemson Extension says azaleas prefer light to moderate shade, such as the filtered shade of pines, or a site with morning sun and afternoon shade. Full sun means faster-fading flowers and more lace bugs, while heavy shade means weak growth and few flowers.
When do azaleas bloom?
Most azaleas bloom in spring, with timing set by variety, weather and location; in South Carolina, Clemson Extension lists Southern Indica and Kurume azaleas from late March, and Satsukis in May and June. Some native deciduous azaleas bloom into summer, and reblooming types flower again in late summer or fall.
How big do azaleas get?
It depends on the group. Clemson Extension lists Gable hybrids at about 2 to 4 feet, Glen Dale hybrids at 4 to 6 feet, and Southern Indica azaleas at 7 to 10 feet or more. Native deciduous azaleas range from about 3 to 15 feet. Check the cultivar tag and space plants for their mature width.
Are azaleas perennials?
Yes. Azaleas are woody perennial shrubs that live for many years. Evergreen azaleas keep their leaves through winter, while deciduous azaleas drop them in fall and leaf out again in spring.
When should you fertilize azaleas?
Clemson Extension recommends fertilizing, if needed, from late spring (after new growth emerges) to early summer with a complete, extended-release, acid-forming azalea fertilizer, and never in late fall. Azaleas have low nutritional needs, so healthy, dark-green established plants often need little fertilizer.
Why do azalea leaves turn yellow with green veins?
That pattern is iron chlorosis, usually caused by soil pH that is too high for the azalea to absorb iron, per Clemson Extension. Test the soil; if pH is above the 4.5 to 6.0 range, lower it as the test recommends, and use a foliar iron spray for a quick, temporary green-up.
Are azaleas poisonous to dogs and cats?
Yes. The ASPCA lists azaleas as toxic to dogs, cats and horses, with signs that include vomiting (not in horses), diarrhea, weakness and cardiac failure. Contact a veterinarian or ASPCA Animal Poison Control right away if a pet eats azalea.
Sources
- Azalea care (HGIC 1059), Clemson Cooperative Extension HGIC
- Azalea planting (HGIC 1058), Clemson Cooperative Extension HGIC
- Azalea & rhododendron diseases in South Carolina (HGIC 2050), Clemson Cooperative Extension HGIC
- Azalea bloom show is over so it's time to prune, University of Georgia Extension
- Rhododendron periclymenoides (pinxterbloom azalea), Missouri Botanical Garden Plant Finder
- Azalea, ASPCA Animal Poison Control