How to Grow Azaleas

Pick an azalea suited to your region

Azaleas are rhododendrons, part of a genus with more than 900 species. Most garden azaleas are evergreen hybrids of Asian origin, while many native azaleas of the Southeast are deciduous. Mississippi State rates azaleas as a group for zones 5 to 9, but hardiness varies widely between types, so choose plants known to do well where you live.

In South Carolina, Clemson recommends evergreen groups such as Southern Indica, Kurume, Satsuki and the reblooming Encore series. It notes that Kaempferi, Gable and Glen Dale hybrids are very cold-hardy but generally not suited to the coast. For heat on the Coastal Plain, Clemson points to native Florida flame azalea and the Confederate series hybrids bred from it. In Houston, Harris County Extension names large Indica varieties such as ‘Formosa’, ‘George Tabor’, ‘G. G. Gerbing’ and ‘Judge Solomon’ as the ones that tolerate fairly sunny sites.

In the far North, choice is the main hurdle. Iowa State notes that most rhododendrons and azaleas lack the cold hardiness for Iowa. For Iowa landscapes it recommends the University of Minnesota’s Northern Lights series of azaleas, such as ‘Northern Lights’, ‘Pink Lights’, ‘Rosy Lights’ and ‘White Lights’, whose flower buds are hardy to -30 to -45°F.

Light: filtered shade

Azaleas prefer light to moderate shade, and the high, filtered shade of pine trees is close to ideal. Morning sun with afternoon shade also works. Flowers last longer out of the hot sun, and plants in full sun suffer more from lace bugs. Heavy shade swings too far the other way, giving weak growth and few flowers.

Avoid planting under shallow-rooted trees such as maples, which compete hard for water and nutrients.

Soil: acid and well drained

Azaleas need acid soil. Clemson and Mississippi State give a range of about 4.5 to 6.0, Harris County aims for 4.5 to 5.5, and Iowa State 4.0 to 5.5. Test before planting. Much of the Southeast is already acid enough, but coastal sand, soil near new foundations where mortar was dumped, and the alkaline soils of the Houston area need work. Iowa State notes that most Iowa garden soils are well above the range azaleas want. Amend with organic matter and sulfur as the test recommends.

Drainage matters just as much. Azalea roots are fine and shallow, and in wet soil they rot. On heavy clay or low ground, build a raised bed. Harris County raises beds 4 to 6 inches on well-drained sites and 10 to 12 inches where drainage is poor, and Mississippi State suggests mounding soil 8 to 12 inches high.

Clemson recommends amending the whole bed rather than just the planting hole. Mix 1 to 2 inches of composted pine bark or other organic matter into the top 8 inches of soil. Avoid fresh, uncomposted bark, which ties up nitrogen as it breaks down.

How to plant azaleas

1. Dig a hole only as deep as the root ball and two to three times as wide.
2. If roots circle the outside of the root ball, make three or four shallow vertical cuts down its sides.
3. Set the plant so the top of the root ball is level with the ground or slightly above it, never below.
4. Backfill, firm gently and water thoroughly to settle the soil.
5. Spread 2 to 3 inches of pine straw, pine bark or shredded leaves over the root zone, keeping it a couple of inches from the stems.

Do not mix commercial fertilizer into the planting hole. For timing, see when to plant azaleas.

Watering

Shallow roots dry out fast. Pull back the mulch and check: when the top few inches feel dry, soak the soil 6 to 8 inches deep. Clemson notes that spring-planted azaleas in sandy soil may need water twice a week during their first year. Use soaker hoses or drip lines rather than overhead sprinklers, which encourage disease. The goal is steady moisture without waterlogging.

Fertilizing

Azaleas are light feeders, and a mulch of organic matter often supplies enough. If growth is poor, test the soil first. When you do feed, use an acid-forming azalea fertilizer from late spring, after new growth appears, to early summer. Never feed in late fall: nitrogen then can delay dormancy and lead to cold injury.

Pruning

Prune right after spring bloom. Azaleas set next year’s flower buds over the summer, so cuts made later remove flowers. Thin leggy shoots back to a branch to keep a natural shape. To renew a badly overgrown shrub, Clemson cuts it back to 6 to 12 inches before spring growth begins, giving up that year’s flowers.

Common problems

• Lace bugs: pale stippling on the tops of leaves, dark spots underneath. Worse in full sun. Spray horticultural oil or insecticidal soap on the leaf undersides.
• Iron chlorosis: yellow leaves with green veins, usually from high soil pH. Test and lower the pH.
• Petal blight: flowers spot, go soft and cling to the plant. Clean up fallen petals and replace infected mulch.
• Root and crown rot: wilting and dieback in wet soil. Plant high and improve drainage.
• Cold injury: bark splitting after sudden freezes or early sun on frozen stems. Plant varieties hardy for your area.
• Spider mites, leaf gall and leaf spot: usually minor with good siting and care.