Varieties of Azaleas and Rhododendrons

Catawba Rhododendron, PJM Rhododendron and a Stunning Azalea Plant

Rhododendrons flowers
Kapa65/Pixabay/CC By 0

Ever wondered what the difference is between azaleas and rhododendrons? Landscaping enthusiasts seem content to group all the varieties together -- both in their vocabulary and in their yards -- as long as they get to enjoy the beautiful pictures these long-time gardening favorites paint on their landscapes! But for those curious about what distinguishes the one from the other, the information will be provided on Page 2.

The present page focuses on introducing a few varieties of these spectacular flowering bushes -- through pictures and brief descriptions. Information will be provided regarding the care for azaleas and rhododendrons on Pages 3 and 4.

Two Popular Varieties of Rhododendron Bush

The Catawba rhododendron variety (Rhododendron catawbiense) is a broadleaf evergreen bush that has dark green, leathery foliage, and spring flowers that are great for attracting hummingbirds. This rhododendron shrub is easy to transplant, but it does require an acidic soil and prefers a shady location. Flowers can be white, lavender, rose or the red that hummingbirds so love.

Catawba rhododendron bushes can reach a height of 6'-8' with a spread of 4'-6'. Displays are most effective when rhododendron bushes are massed together. These are poisonous plants -- do not allow children to ingest any of the azaleas or rhododendrons.

Listed for zones 4-8, they are native to the southern part of the Appalachians ("Virginia through Georgia," according to the NC State Extension). New Englanders looking for a type native to their region will be interested in Rhododendron canadense, the so-called "Rhodora azalea." 

P.J.M. rhododendron (Rhododendron x ‘P.J.M.’) is considered perhaps the hardiest of the rhododendrons, in terms of withstanding not only cold, but also heat and sun.

Varieties of P.J.M. are available with either pinkish-lavender or white flowers. Because it flowers early, blooms are susceptible to frost damage. The green of its summer foliage yields to a mahogany color in winter -- a plus for those seeking year-round interest on the landscape.

The flowers of P.J.M. rhododendrons are smaller than those of Catawba rhododendron. P.J.M. rhododendron’s overall size is also smaller, as it reaches approximately 4’ x 4’ at maturity. Zones 4-8. Good choice for foundation plantings and rock gardens

A Pretty Picture -- Whatever You Call This Azalea....

The Stewartsonian azalea (Rhododendron x Gable 'Stewartsonian') is an evergreen suitable for growing in zones 5-8. Dimensions approximately 5' x 5'. Grows best in part shade and, like most azaleas and rhododendrons, acidic, well-drained soil. Apply an organic mulch to protect its shallow roots from water-loss and extremes in soil temperature.

Developed by Joseph Gable of Stewartstown, Pennsylvania, people can't seem to make up their minds on the spelling of its name. I've seen it listed both as "Stewartstonian" and "Stewartsonian." This is another bush whose green summer foliage yields to a mahogany color in winter, making it an ideal candidate to provide year-round interest on the landscape.

Also another good choice for foundation plantings, since it remains a relatively compact plant.

On Page 2 we'll explore the difference between azaleas and rhododendrons. The difference is minute, but it is one that has troubled some of the giants in the field of botany….

Beginners at landscaping and gardening often confuse azalea plants and rhododendrons, as stated on Page 1. The confusion is understandable; for azalea plants and rhododendrons are related. All azaleas are Rhododendrons (note the capital "R"), but not all Rhododendrons are azaleas. Still confused? Well, you're in good company, company that includes the famous Linnaeus. Linnaeus' taxonomy is the standard in botanical classification.

Let's have a closer look at the challenging taxonomy of azaleas and rhododendrons, before proceeding to the instructions for caring for them (Pages 3 and 4).

Throughout the following explanation of scientific names versus common names, I will be reminding you to take note of whether I say "Rhododendron" or "rhododendron." With a capital "R," I'm referring to a plant genus. With a lower-case "r," I'm referring to a subset of that genus.

The genus Rhododendron is in the heath family, which also includes its namesakes, the heathers and heaths; blueberries and cranberries; and mountain laurels. Most members of the heath family need an acidic soil in which to grow. When you have a naturally acidic soil in your yard, it's a lot easier simply to grow acid-loving plants rather than trying to change the pH level of your soil.

Scientific Names: Linnaeus' Taxonomy

The Swedish naturalist, Carolus (Carl) Linnaeus established the genus, Rhododendron in 1753.

However -- and here's where some of the confusion started -- Linnaeus' taxonomy established "Azalea" as a separate genus. But it was soon pointed out that azalea plants should be considered a subset of the Rhododendron genus, rather than a genus all unto themselves.

Then in 1834 another naturalist corrected Linnaeus' taxonomy on this point.

An Englishman, George Don, broke down the genus, Rhododendron into eight sub-categories, composed of numerous species. The azaleas comprise two of these sub-categories (evergreen azalea bushes and deciduous azalea bushes).

Consequently, if you read the scientific name of an azalea, you'll see the word, Rhododendron. That's because azalea plants belong to the genus, Rhododendron -- and the word "azalea" has essentially become this bush's common name. But there are also members of this genus that are just plain "rhododendrons" (note the lower-case "r"). In recent years "rhododendron" has come to be used by gardeners essentially as a common name for those plants in the genus, Rhododendron that have large, leathery, evergreen leaves. The leaves on azalea plants tend to be smaller, by comparison. Within the rhododendrons themselves, leaf-size comparisons are used to make a further division: namely, between elepidotes (large-leaf rhododendrons) and lepidotes (small-leaf rhododendrons).

So how would you be able to identify an azalea, as distinct from a rhododendron? In general, rhododendrons are larger shrubs than are azalea plants, and, as just said, they have larger leaves. Also, in general, azalea flowers have five stamens, while the rhododendron flowers have ten stamens.

Finally, unlike rhododendrons, many azalea plants are deciduous.

Now that you've heard about popular varieties of azaleas and rhododendrons -- and have been introduced to the challenge of classifying them in Linnaeus' taxonomy -- it's time to look at how to care for rhododendron and azalea plants. That's the subject of Page 3....

Although plant care for azaleas and rhododendrons isn't as challenging as their taxonomy (see Page 2), you still need to know what steps to take to promote optimal health. On Page 4 we'll consider such matters as pruning azaleas and rhododendrons, but I'll begin here with tips on planting azaleas and rhododendrons.

First Rule in Care for Azaleas and Rhododendrons: Location, Location, Location

Planting azaleas and rhododendrons in a spot cooled by partial shade, where the soil is acidic and well-drained, is a step in the right direction in the proper care for azaleas and rhododendrons.

Despite being known as shrubs that grow in shade (dappled shade, preferably), some varieties will tolerate (or even prefer) full sun if sufficient water is provided. For example, I grow the Stewartstonian type in a spot where it receives a lot of sun in the morning, midday and late afternoon in my zone-5 garden, yet with adequate water it performs just fine.

Soil pH should be about 5.5; have your garden soil tested first before planting azaleas and rhododendrons. An overly alkaline soil can be corrected by applying fertilizers for "acid-loving" plants, such as blueberries and azaleas. These specialty fertilizers will contain ammonium-N, which will lower soil pH.

Basics of Planting Azaleas and Rhododendrons: Soil

A good tactic for providing the necessary drainage is planting azaleas and rhododendrons in raised garden beds. Amend the soil with decomposed sawdust or pine bark (both are acidic), or apply garden compost.

The recommended planting time for balled-and-burlapped plants is late fall or early spring. In either case, water well after planting azaleas and rhododendrons. Don't apply fertilizer at the time of planting; new foliage and roots aren't yet ready to handle the high salt content of fertilizer, and they can be burnt.

Shady Companion for Azaleas and Rhododendrons: Red Oak Trees

Since azaleas and rhododendrons prefer shade, it behooves landscapers to choose a good shade tree to have growing near them. Will just any shade tree do? No! Azalea and rhododendron plants like acid soil and have shallow roots. A shade tree compatible with them will not mind acid soil and will not have shallow roots. For if the shade tree, like the azaleas and rhododendrons, has shallow roots, it will be in competition with your bushes.

Henning's Rhododendron and Azalea Pages, an excellent information source on these plants, suggests a few shade trees compatible with azaleas and rhododendrons, among them being the red oak. The Henning’s site reports that the red oak is a fast grower, eventually reaching a height of 60’-75’ with a spread of 40’-50’. The red color of its autumn leaves makes it a fine fall foliage plant. In addition, the red oak is one of the pollution-tolerant trees, an important consideration in urban and suburban landscaping.

On Page 4 we consider more issues regarding the care of azalea and rhododendron plants: fertilizing, mulching and pruning azaleas and rhododendrons....

After azaleas and rhododendron plants have had time to settle in where you've planted them (see Page 3), the next step in caring for these bushes is fertilizing. But even then, be careful not to over-fertilize. Stay away from the mentality that says, "If some fertilizing is good, then more must be better." There are standard fertilizers to use on azalea and rhododendrons bushes -- mixes that can be purchased at nurseries and major hardware chains.

Follow the instructions on the fertilizer's label. Except perhaps for one instruction -- the amount of fertilizer to apply. Cut that in half. It is usually better to be conservative about applying fertilizer.

A good time to fertilize azaleas and rhododendrons is right after they have finished blooming.


Ally #1 in Azalea, Rhododendron Care: Mulch

Mulching is an essential part of proper care for azalea bushes and rhododendrons. The roots of these shallow-rooted plants need the protection that mulch affords against extremes of heat and cold -- and against drying out. Remember, the fact that these plants like a well-drained soil doesn't mean they like to be dry. Azaleas and rhododendrons are not desert plants; they like water. They just don't like to be sitting in it for long periods of time, which would cause their roots to rot.

The best mulches for azaleas and rhododendrons are acidic mulches, such as pine straw (although, as I report in the following article, some experts now dispute some aspects of the concept "acidic mulches").

For information on other acidic mulch choices, please consult Choosing the Proper Mulch. Since mulch eventually does break down and become a component of the underlying soil, you might as well go with an acidic mulch. There's no sense in fighting the acid-loving tendencies of azalea and rhododendron plants.



Pruning Azaleas, Rhododendrons

Pruning azaleas and rhododendrons should be undertaken immediately after they finish blooming (usually June or July). Pruning the bushes later than that risks interfering with the development of next year's buds. Begin by pruning off dead or injured branches, which could cause disease and insect problems in the future. Then prune back tall, gangly limbs shooting out of the top of the bush. This will promote a more attractive, compact shape.

A proper regimen of pruning azaleas and rhododendrons, in conjunction with the other care tips offered above, will help these flowering shrubs provide your landscape with eye-opening hedges or specimen plants for years to come.