Varieties of Azaleas and Rhododendrons

Catawba Rhododendron, PJM Rhododendron, and Some Stunning Azalea Plants

Rhododendrons flowers
Large, leathery, evergreen leaves are one of a rhododendron's best features. Kapa65/Pixabay/CC By 0

Have you ever wondered what the difference is between azaleas and rhododendrons? That information will be provided below so that you will be able to "get it right" when shopping at the garden center. But let's begin by discussing some great varieties of these spectacular flowering bushes; we'll conclude with information on how to grow them.

Two Popular Varieties of Rhododendron

The Catawba rhododendron variety (Rhododendron catawbiense) is a broadleafevergreen bush that has dark green, leathery foliage, as well as 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 feet, with a spread of 4-6 feet. Displays are most effective when rhododendron bushes are massed together. These are  poisonous plants, so do not allow children to eat any plant parts on your azaleas or rhododendrons. Listed for  USDA planting 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 one of the toughest of the rhododendrons, in terms of holding up not only to the cold but also to heat and sunlight.

It is also one of the most widely grown. Varieties of P.J.M. are available with either pinkish-lavender or white flowers. Because it flowers early, blooms can sometimes be harmed by frost. The leaves of this bush are evergreen. The green of its summer foliage yields to a mahogany color in winter, which is 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 about 4 feet by 4 feet at maturity. Grow it in zones 4-8. P.J.M. (which are the initials of one of this hybrid's developers, Peter J. Mezitt) is a good choice for foundation plantings and large rock gardens

Azaleas With Red, Orange, and Gold Flowers

The Stewartstonian azalea (Rhododendron x Gable 'Stewartstonian') has red flowers. It is an evergreen suitable for growing in zones 5-8. Its size is about 5 feet by 5 feet. It grows best in part shade and, like most azaleas and rhododendrons, in 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. You will see 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 choice for four-season interest in the yard. It is also another good selection for foundation plantings since it remains a relatively compact plant.

Unlike Stewartstonian azaleas and the two rhododendrons considered above, the next two examples of azaleas are deciduous shrubs:

  1. Gibraltar
  2. Golden Oriole

Gibraltar puts out masses of flowers of a rich orange color. It can grow to be 5-6 feet tall, with a spread somewhat less than that. Both Gibraltar (Rhododendron 'Gibraltar') and Golden Oriole (Rhododendron 'Golden Oriole') want to be grown:

  1. In partial sun.
  2. In an acidic, well-drained soil.
  3. In zones 5-8.

Golden Oriole can attain a height of 6 feet and a width of 4-6 feet. Its flower buds are orange, but, when they open, they display a bright, golden color.

Rhododendrons vs. Azaleas

Now let's 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.

It may trouble you too if you are the curious type because the wording can be confusing.

And since you will come across this wording while shopping at plant nurseries, it would be helpful to have the confusion cleared up before you buy a plant.

The confusion is understandable since azalea plants and rhododendrons are related. All azaleas are Rhododendrons (note the capital R), but not all Rhododendrons are azaleas. Still confused? Well, you are in good company, company that includes the famous scientist, Linnaeus. Linnaeus is the man who gave us the scientific system for naming plants that we still use to this day.

Throughout the following explanation of scientific names versus common names, remember to take note of whether "Rhododendron" or "rhododendron" is used. One uses the capital R to refer to the plant genus and the lower case to refer to a subset of that genus.

The genus, Rhododendron is in the heath family, which also includes its namesakes, the heathers and heaths, plus blueberries, 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 is a lot easier simply to grow acid-loving plants rather than trying to change the pH level of your soil.

Blame the Naming Confusion on Linnaeus

Linnaeus established the genus, Rhododendron in 1753. Linnaeus' naming system established "Azalea" as a separate genus. But it was soon pointed out by other scientists that azalea plants should be considered a subset of the Rhododendron genus, rather than a genus all unto themselves.

In 1834 another scientist, George Don corrected Linnaeus on this point. He broke the genus, Rhododendron down into eight sub-categories, made up of numerous species. The azaleas make up two of these sub-categories (evergreen azalea bushes and deciduous azalea bushes).

So if you read the scientific name of an azalea (on a plant label at the nursery, for example), you will see the word, Rhododendron. That is 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 (such as the Catawba and PJM already mentioned). 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 large-leaf and small-leaf types.

So how would you be able to identify an azalea, as distinct from a rhododendron? On average (but there are exceptions), rhododendrons are larger shrubs than are azalea plants, and they have larger leaves. Also, in general, azalea flowers have five stamens, while the rhododendron flowers have ten stamens. The "stamens" of a flower are those thin stems sticking out (they are male flower parts and produce pollen). Finally, unlike rhododendrons, many azalea plants are deciduous.

Location

Planting azaleas and rhododendrons in a spot cooled by at least a bit of shade, where the soil is acidic and well-drained, is a step in the right direction in their proper care. Despite being known as shrubs that grow in shade (dappled shade, preferably), some varieties will tolerate (or even prefer) full sun if given enough water. For example, in zones 5 and 6, you can grow the Stewartstonian type in a spot where it receives a lot of sun in the morning and midday, with a little shade in late afternoon, as long as you give it enough water.

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 listed as being for "acid-loving" plants. These specialty fertilizers will contain ammonium-N, which will lower soil pH.

Soil Needs

These shrubs need good drainage. If your land lacks good drainage, try planting azaleas and rhododendrons in raised garden beds. Amend the soil with organic matter, 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. Do not apply fertilizer at the time of planting; new foliage and roots are not yet ready to handle the high salt content of fertilizer, and they can be burnt.

Red Oak Trees

Since azaleas and rhododendrons prefer some shade, you should choose a good shade tree to have to grow 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 suggests a few shade trees compatible with azaleas and rhododendrons among them being the red oak (Quercus rubra). The red oak is a fast grower, eventually reaching a height of 60-75 feet, with a spread of 40-50 feet. The red color of its autumn leaves makes it a fine fall foliage plant for zones 4-8. In addition, the red oak is one of the pollution-tolerant trees, an important factor in urban and suburban landscaping.

After azaleas and rhododendron plants have had time to settle in where you have planted them, the next step in caring for these bushes is fertilizing. But even then, be careful not to over-fertilize. Do not subscribe to the idea 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 so that you do not risk burning your plants.

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

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 gives against extremes of heat and cold, and against drying out. Remember, the fact that these plants like a well-drained soil does not mean they like to be dry. Azaleas and rhododendrons are not desert plants; they like water. They just do not 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 generally thought to be acidic mulches, such as pine straw (although some experts now dispute the idea that pine needles make the ground more acidic). 

Pruning

Prune azaleas and rhododendrons immediately after they finish blooming (often in June in zone 5). 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, gangling limbs shooting out of the top of the bush. This will promote a more attractive, compact shape.

A proper routine for pruning azaleas and rhododendrons, along 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.