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.

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There's a difference between azaleas and rhododendrons, although what the two share dwarfs what separates them, including a preference for a well-drained, acidic soil. Still, it's a handy distinction to grasp when you're shopping at the garden center. There are great varieties of both of these spectacular flowering bushes, equally useful as specimens and in hedges. Let's learn about some of the best choices and 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 prefer 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 to 8 feet, with a spread of 4 to 6 feet. Displays are most effective when rhododendron bushes are massed together. These are poisonous plants, so don't allow children to eat any plant parts on your azaleas or rhododendrons.

Listed for USDA planting zones 4 to 8, they're native to the southern part of the Appalachians. 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's 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 to 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, Pink, Orange, and Gold Flowers

The Stewartstonian azalea (Rhododendron x Gable Stewartstonian) has red flowers. It's an evergreen suited to zones 5 to 8. Its size is about 5 feet by 5 feet. It grows best in part shade. Apply an organic mulch to protect its shallow roots from water loss and extremes in soil temperature.

Developed by Joseph Gable of Stewartstown, Pennsylvania, this is another bush whose green summer foliage yields to a mahogany color in winter, making it an ideal choice for four-season interest. It's also another good selection for foundation plantings since it remains a relatively compact plant.

Blaauw's Pink azalea (Rhododendron Blaauw's Pink) has salmon-pink flowers. It becomes a maximum of 5 feet high and 6 feet wide. This evergreen can be grown zones 6 to 9. The shrub is best-suited to partial shade.

Unlike Stewartstonian and Blaauw's Pink azaleas, the next three examples of azaleas are deciduous shrubs:

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

  • Partial sun
  • Zones 5 to 8

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

Rhododendron Bloom-a-Thon is one of the very best choices if you're looking for a showy bush because it produces double, pink flowers. Another selling point is that it reblooms. The bush becomes about 4 feet tall, with a spread of slightly less than that. Grow it in full to partial sun in zones 6 to 9.

Rhododendrons vs. Azaleas

Now let's explore the difference between azaleas and rhododendrons. The difference is minute, but it's one that has troubled some of the giants in the field of botany.

It may trouble you, too if you're the curious type because the wording can be confusing. 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. You're in good company if you're still confused, 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, take note of whether "Rhododendron" or "rhododendron" is used. You use the capital R to refer to the plant genus and the lower case to refer to a subset of that genus.

The genus of Rhododendron is in the heath family, which also includes its namesakes, the heathers (Calluna vulgaris), plus Andromeda (Pieris japonica), and mountain laurels (Kalmia latifolia). Most members of the heath family require acidic soil. 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.

Blame the Naming Confusion on Linnaeus

Linnaeus established the Rhododendron genus 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, George Don corrected Linnaeus on this point. He broke the genus of 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, you'll see the word, Rhododendron. That's because azalea plants belong to the genus, Rhododendron, and "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 of Rhododendron that have large, leathery, evergreen leaves (such as the Catawba and PJM ). 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 between large-leaf and small-leaf types.

There's a general guideline to identify an azalea, as distinct from a rhododendron (but there are exceptions). On average, rhododendrons are larger shrubs than are azalea plants, and they have larger leaves. Also, azalea flowers usually have five stamens, while the rhododendron flowers have ten. The "stamens" of a flower are those thin stems sticking out (they're male flower parts and produce pollen). Finally, unlike rhododendrons, many azalea plants are deciduous.

Sunlight Requirements

Planting azaleas and rhododendrons in a spot cooled by at least a bit of shade 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 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 Needs

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 contain ammonium-N, which lowers soil pH.

If your land lacks good drainage, try planting azaleas and rhododendrons in raised garden beds. Amend the soil with compost. Install balled-and-burlapped plants in late fall or early spring. In either case, water well after planting azaleas and rhododendrons. Don't apply fertilizer at planting time: New foliage and roots aren't yet ready to handle the high salt content of fertilizer, and they can be burnt.

Red Oaks as Companion Plants

Since azaleas and rhododendrons prefer some shade, you should choose a compatible shade tree to grow near them. Because these shrubs like acid soil and have shallow roots, a compatible shade tree won't mind acid soil and won't have shallow roots. For, if the shade tree, like the azaleas and rhododendrons, has shallow roots, it'll be in competition with your bushes.

A few shade trees are compatible with azaleas and rhododendrons, among them being red oak (Quercus rubra). Red oak's a fast grower, eventually reaching a height of 60 to 75 feet, with a spread of 40 to 50 feet. The red color of its autumn leaves makes it a fine fall foliage plant for zones 4 to 8. Red oak's also a pollution-tolerant tree, an important factor in urban and suburban landscaping.

After azaleas and rhododendron plants have had time to settle in where you've planted them, the next step in caring for these bushes is fertilizing. But, even then, be careful not to over-fertilize. Don't 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's usually better to be conservative about applying fertilizer so that you don't risk burning your plants.

A good time to fertilize azaleas and rhododendrons is right after they've 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). The fact that they like a well-drained soil doesn't mean they like to be dry. Azaleas and rhododendrons aren't 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 thought to be acidic mulches, such as pine straw (although the idea that pine needles make the ground more acidic is now disputed). 

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'll promote a more attractive, compact shape.