What’s So Special About Azaleas and Rhododendrons?

By Pat Chadwick
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May 2015 - Vol. 1 No. 5

Pink-Blooming Azalea

Few landscape shrubs are as widely celebrated and anticipated in the spring garden as azaleas and rhododendrons. Their showy, exuberant displays of color excite the senses and have a way of taking center stage in just about any landscape. If you watched the Masters Golf Tournament at the Augusta National Golf Club in Augusta, Georgia, you’ll know what I’m talking about. The billowing masses of colorful, mature azaleas and rhododendrons in the background clearly competed with the golfers for the viewer’s attention.

These plants are admired and revered in many countries and across many cultures. In the United States and in Asia, countless azalea festivals are timed to occur when the shrubs are at their showiest. At least two major horticultural societies are devoted to this family of plants. Just about every public botanical garden this side of the Mississippi and even beyond has an azalea and rhododendron collection.

So what’s so special about azaleas and rhododendrons? The answer is simple. They are the epitome of spring. A specimen in full bloom has no equal in the spring landscape. When viewed in the garden, they are drop-dead gorgeous in all their various forms and colors. A single specimen is enchanting when viewed close up. A grouping of them viewed at a distance or encountered in a woodland setting is nothing short of glorious. Their form, habit, and amazing colors dominate the spring landscape.

DIFFERENCES BETWEEN AZALEAS AND RHODODENDRONS

Azaleas and rhododendrons are native to Asia, Europe and North America and may be either deciduous or evergreen. All evergreen species originally came from Asia and generally tend to be hardy only to Zone 6. The deciduous species are native to this country and tend to be hardy to Zone 4. Of 17 native species, all but two are native to the southeastern part of the United States. They have been extensively hybridized for the past couple of hundred years, resulting in thousands of hybrids, both named and unnamed, with characteristics that don’t always clearly identify them as either azalea or rhododendron.

Botanically speaking, azaleas and rhododendrons belong to the genus Rhododendron, with azaleas identified as a subspecies of the rhododendron family. According to botanists, there are not enough botanical differences between the two to classify them as two separate species. The American Rhododendron Society describes the plants this way: “All azaleas are rhododendrons but not all rhododendrons are azaleas.”

To a novice, it can be difficult to tell the difference between some azaleas and rhododendrons. A case in point is the P.J.M. rhododendron, a hardy variety which is smallish (three-to-five feet) like a typical azalea. It blooms early in spring like an azalea and has small leaves, but the foliage is leathery and evergreen, like rhododendron foliage. The shrub is completely covered with blossoms like an azalea, but it also bears clusters of blossoms in terminal groups like a rhododendron. The number of stamens, however, help identify the shrub as a rhododendron. The characteristics listed below may help you distinguish between the two plants. Bear in mind, however, that these are GENERAL characteristics only and may not hold true for every hybrid or species:

Bloom time — Excluding the repeat-blooming species, azaleas bloom beginning in April whereas rhododendrons usually bloom later in the spring.
Flowers — Azaleas have tubular or funnel-shaped flowers. Rhododendron flowers tend to be bell-shaped. Azaleas have one flower per stem but the shrub produces so many stems that the shrub appears covered in blossoms. Rhododendron flowers grow in round clusters at the ends of branches. Both azaleas and rhododendrons have five lobes per flower.
Stamens — True rhododendrons have ten or more stamens, which amounts to two per lobe. Azaleas have five stamens, or one per lobe.
Color of flowers – Azaleas come in many shades of white, cream, pink, red, lavender, purple, orange and yellow Their color palette is much broader than that of rhododendrons, which tend to be restricted to white, orchid pink, purple, red and occasionally yellow.
Foliage – Azalea foliage tends to be elliptical shaped, thin, small and pliable. Most azaleas are deciduous or partly deciduous but many are evergreen, depending on the cultivar and the climate in which the plant is growing. Rhododendron foliage is large, paddle shaped, thick, and evergreen. The underside of the leaves may be scaly and may have small dots.

Thanks to the efforts of both American and Asian hybridizers, thousands of azalea and rhododendron species are available commercially. One garden center in Northern Virginia, for example, carries more than one hundred azalea species in their inventory and about fifty rhododendron species. The choices can be overwhelming. Before buying a plant, consider color, foliage, size and planting site. The bloom time in spring is generally only three-to-four weeks. After that, think about what the foliage will contribute to your landscape. This will help you decide whether you want a deciduous or an evergreen plant. Azaleas tend to average about three-to-five feet in height and width, but bear in mind while some cultivars may be smaller, a lot of them can grow quite large. So keep the final size in mind. Sunlight requirements vary from species to species, but morning sunlight and dappled afternoon shade normally work well for these plants.

There has been a resurgence of interest in azaleas over the past decade or so due to the introduction of the Encore series of re-blooming species. This collection includes more than two dozen choices of single and double blooming cultivars. The initial bloom period occurs in spring with a follow-on show of blooms in summer and yet again in fall. Bloom-a-thon is another series of reblooming azaleas available on the market with reblooming characteristics similar to the Encore series. Should you choose a reblooming azalea for your garden, keep in mind how the color will work (or not) with your landscape throughout the entire growing season.

CULTURAL REQUIREMENTS

Soil pH – Both azaleas and rhododendrons thrive best in acidic soil with a pH of 4.5 to 6.0. If in doubt, a soil test should be done before planting. If the pH is too high, apply a small amount of agricultural sulfur or iron sulfate. Conversely, if the pH is too low, apply ground limestone. See Virginia Cooperative Extension Publication 426-602, “Growing Azaleas and Rhododendrons,” for guidance on fertilization (https://pubs.ext.vt.edu/426-602/426-602.html).
Sunlight – Azaleas and rhododendrons prefer a cool site with filtered sunlight. They can tolerate morning sun but will fare best if protected from strong afternoon sun. If the only site available receives full sun, then choose a deciduous azalea, which can tolerate more sun than its evergreen cousins.
Water – Azaleas have shallow roots which should be irrigated during dry periods. A soaker hose or drip irrigation is the best choice to slowly water the plants. Overhead irrigation is not a good idea because it can promote foliar diseases. Don’t over water the plant. Too much moisture can cause the plant to be susceptible to root rot diseases.
Mulch — Layer two-to-three inches of organic mulch over the root ball. Use pine straw, composted pine bark or decomposed oak leaves. All will enrich the soil as they decompose. Be careful to keep the mulch a couple of inches away from the main stem.
Fertilizer – Azaleas and rhododendrons generally require little nutritional supplementation. But if a soil test indicates the need for fertilizer, use one that is especially formulated for azaleas and rhododendrons and apply carefully to avoid damaging the fine roots. A fertilizer that supplies ammonium nitrogen is a good choice. Cottonseed meal is also recommended as a suitable fertilizer. Fertilize in early spring when moisture is plentiful.
Pests – The most common pests affecting azaleas are lacebugs and spider mites. In my experience, lacebugs tend to attack azaleas that are planted in full sun. Good cultural practices are the best defense against these problems. Careful plant location, good soil aeration, good drainage, careful mulching and watering habits all collectively help fight pests and disease.
Diseases — Azaleas are subject to leafy gall, which can be destructive to the leaves in the early spring. The best method of controlling the problem is to hand pick infected leaves. Azaleas and rhododendrons are also subject to phytophthora root rot in moist, hot conditions.
Deadheading — Most azaleas are self-cleaning. The blossoms turn brown, which can look unattractive for a few days, but then they drop off and disappear into the mulch.
Pruning — Prune azaleas after they have finished flowering. New flower buds for next year’s blooms are set by midsummer. If pruned after say mid-June, the plant may not produce any blooms next spring. Prune individual branches back to a spot where they join a larger branch. If the plant is overgrown and needs to be reduced in size, cut large branches back. New growth will spring from the stubs that remain.

HOW TO PLANT

Soil — Azaleas and rhododendrons thrive best in moist but well-drained soil that is acidic and rich in organic matter.
Location – choose a site sloping to the north or to the east in order to protect the plant from drying south or west winds. Winter winds tend to evaporate water from the leaves of evergreen azaleas and rhododendrons. If the ground is frozen, the water cannot be replaced. As a result, the leaves may be damaged and may turn brown.
Soil Preparation — Azaleas and rhododendrons have very delicate, shallow root systems and struggle to penetrate our heavy Virginia red clay. Dig the planting hole at least twice the width of the root ball but no deeper than the root ball. Position the plant either at the same depth as it was in the container or one-to-two inches higher than the surrounding soil.
Drainage – Good drainage is vital to the health of these plants. Since this is particularly important for shallow-rooted plants, it is best to amend heavy clay with good quality, loamy topsoil throughout the entire planting site.
Mulch – Two-to-three inches of mulch will keep the root ball cool and moist. Just make sure the mulch is pulled away from the main stem by at least a couple of inches. Replace or replenish mulch in the fall before a hard freeze.

AZALEA AND RHODODENDRON COLLECTIONS IN BOTANICAL GARDENS

Azaleas and rhododendrons are so popular on the east coast that it is impossible to name all the collections that are available without expanding this article to an unmanageable length. Of the many options available, a few that are within driving distance of Charlottesville include:

Norfolk Botanical Garden – Their collection of azaleas and rhododendrons numbers approximately 3,800 plants in the rhododendron genus representing 558 different species and cultivars. Their collection is scattered throughout the garden, so wear comfortable walking shoes if you visit.
The United States National Arboretum in Washington, D.C. — This arboretum is one of the capital’s must-see attractions at any time of year but particularly when the azaleas and rhododendrons are in full bloom. Former arboretum Director Benjamin Y. Morrison is personally responsible for developing hundreds of hybrids and you will not be disappointed if you decide to tour their collection. Check out their website for a weekly update on what’s in bloom.
Lewis Ginter Botanical Garden — This nearby botanical garden in Richmond features members of the rhododendron genus throughout the gardens, along paths, and near water, where they are displayed to best advantage.

The west coast also offers a number of botanical gardens with impressive collections of rhododendrons. Many of the specimens on display are at least 40 feet tall – a monstrous size compared to their East Coast cousins. The interesting thing about these plant specimens is that some of them are endangered while others are extinct in their native habitats.

RESOURCES

Azalea Society of America (http://azaleas.org)

American Rhododendron Society (http://www.rhododendron.org)

The United States National Arboretum (www.usna.usda.gov)

The Mendocino Coast Botanical Gardens (www.gardenbythesea/org)

Virginia Cooperative Extension (www.ext.vt.edu)

Clemson University (http://www.clemson.edu)