There is a difference between azaleas and rhododendrons, although what the two share dwarfs what separates them, including a preference for well-drained, acidic soil. There are great varieties of both of these spectacular flowering bushes, equally useful as specimens and in hedges. Learn about some of the best choices and how to grow them.
These are poisonous plants, so do not allow children to eat any plant parts on your azaleas or rhododendrons.
Rhododendrons vs. Azaleas
How to tell the difference between azaleas and rhododendrons? Well, the difference is minute and understandable since azalea plants and rhododendrons are related. All azaleas belong to the Rhododendron genus, but not all Rhododendrons are azaleas. So if you read the botanical name of a common azalea on a plant label at the nursery, you will likely see the word, "Rhododendron."
The genus of Rhododendron is in the heath family, which also includes its namesakes, the heathers (Calluna vulgaris), Andromeda (Pieris japonica), and mountain laurels (Kalmia latifolia). Most members of the heath family require acidic soil. When you have 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.
Keep in mind that there are also members of this genus that are just plain "rhododendrons." 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 P.J.M. ). 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.
On average, rhododendrons are larger shrubs than 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 are male flower parts and produce pollen). Finally, unlike rhododendrons, many azalea plants are deciduous.
This rhododendron shrub is easy to transplant, but it does prefer a shady location. Flowers can be white, lavender, rose, or red, a favorite of hummingbirds.
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.
Listed for USDA planting zones 4 to 8, these plants are native to the southern part of the Appalachians. New Englanders looking for a type native to their region will be interested in Rhododendron canadense or "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. P.J.M. are the initials of one of this hybrid's developers, Peter J. Mezitt. 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. It is a good choice for foundation plantings and large rock gardens.
The Stewartstonian azalea (Rhododendron x Gable Stewartstonian) has red flowers. It is 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 with green summer foliage that yields to a mahogany color in winter, making it an ideal choice for four-season interest. It is also another good selection for foundation plantings since it remains a relatively compact plant.
Blaauw's Pink Azalea
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 in zones 6 to 9. The shrub is best-suited to partial shade.
Deciduous Shrub Azaleas
Unlike Stewartstonian and Blaauw's Pink azaleas, the next three examples of azaleas are deciduous shrubs:
- Gibraltar: 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 and in zones 5 to 8.
- Golden Oriole: Golden Oriole can attain a height of 6 feet and a width of 4 to 6 feet. Its flower buds are orange, but, when the buds open, the display is a bright, golden color.
- Bloom-a-Thon: Rhododendron Bloom-a-Thon is one of the very best choices if you are 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.
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 the 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. 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.
Do not apply fertilizer at planting time. New foliage and roots are not yet ready to handle the high salt content of fertilizer, and they can be burned by the fertilizer. A good time to fertilize azaleas and rhododendrons is right after the plants have finished blooming.
There are standard fertilizers to use on azalea and rhododendrons bushes, mixes that can be purchased at nurseries and major hardware chains. Be careful not to overfertilize. Follow the instructions on the fertilizer's label, except for 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.
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). Azaleas and rhododendrons like well-drained soil, not dry soil. Azaleas and rhododendrons are not desert plants; they like water. The plants 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 acidic mulches, such as pine straw (although the idea that pine needles make the ground more acidic is now disputed).
Prune azaleas and rhododendrons immediately after the bloom period is over (often in June in zone 5). Pruning the bushes later can interfere 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.
Red Oaks as Companion Plants
Since azaleas and rhododendrons prefer some shade, choose a compatible shade tree to grow nearby. Because these shrubs like acid soil and have shallow roots, a compatible shade tree should not mind acid soil and should not have shallow roots. You do not want the tree to be in competition with your azaleas or rhododendrons.
A few shade trees are compatible with azaleas and rhododendrons, most notably the red oak (Quercus rubra). Red oak is 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. This tree is also a pollution-tolerant tree, an important factor in urban and suburban landscaping.