How To Plant and Care for Rhododendrons
It's hard to resist growing flowering rhododendrons when you see them in bloom in the spring. But do you have a good spot for them?
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Two states on opposite sides of the country have a rhododendron as their state flower. Washington claims Rhododendron macrophyllum, while West Virginia recognizes Rhododendron maximum. Both are native to those areas. From east to west, we love our rhododendrons.
What Are Rhododendrons?
Rhododendrons are flowering shrubs in the plant family Ericaceae, known for growing best in acidic soils.
Native rhododendrons grow in North America, Europe and Asia, generally along the edges of wooded areas where the soil and shade are to their liking. The name rhododendron comes from the Greek words for “rose” and “tree.” Many grow to be quite large, often six to 10 feet tall, although some hybrids stay much smaller.
Types of Rhododendrons
The two main types of shrubs in the genus Rhododendrons are rhododendrons and azaleas.
One of the most popular groups of hybrid rhododendrons are the PJM group, named for the breeder, Peter J. Mezitt, who developed them in the 1930s. They are known to be exceptionally hardy, up to U.S. Department of Agriculture Plant Hardiness Zone 4. They generally grow three to five feet tall.
Specialty nurseries also sell native rhododendrons, which vary in size and hardiness depending on where they grow naturally.
As with rhododendrons, you can also find native azaleas at specialty nurseries.
Other types of azaleas include many hybrids and named varieties. Some, like ‘Perfecto Mundo Double White’ Azalea, will re-bloom throughout summer and fall. Others, like ‘Bollywood,’ have variegated foliage.
Whether you buy an azalea or a rhododendron, always check the label for specific information on mature size and USDA Zones to be sure you’re getting a shrub suitable for your garden.
What is the Difference Between Rhododendrons and Azaleas?
If you ask a botanist the difference, they’ll talk about the flowers and whether the shrubs are deciduous (drop their leaves in the fall) or evergreen (hold on to leaves until spring).
Azaleas have thinner leaves. They can often be deciduous, and their funnel-shaped flowers have five or six stamens. Rhododendrons have thicker, leathery leaves and tend to be evergreen, with bell-shaped flowers containing 10 stamens.
When To Plant Rhododendrons
You’ll most often find rhododendrons for sale in the spring when they’re in flower. The advantage of buying and planting one then is, you’ll see what flower you’re getting. The disadvantage? You’ll need to water it throughout the summer, especially if it is hotter or drier than usual.
However, you can also plant rhododendrons in the early fall, often a better time to plant shrubs in general. Give them at least six weeks or more to become established before winter.
Where To Plant Rhododendrons
Rhododendrons grow best in well-drained, acidic soil with some shade. According to Natalie Carmolli from Proven Winners Color Choice Shrubs, “If you see the foliage is yellowing between the veins, then you probably have a pH problem. Conduct a soil test to see if your pH is between their preferred levels of 4.5 to 6.0.”
Carmolli notes if your soil isn’t acidic enough, amend it with wettable sulfur or ferrous sulfate. She also warns not to use aluminum sulfate because it’s toxic to rhododendron roots.
How To Plant Rhododendrons
Once you’ve chosen a good spot for your rhododendron, plant as you would any container-grown shrub.
- Dig a hole as deep as the container is tall, and about twice as wide as the container.
- Before removing the rhododendron from the container, place it in the hole to verify the depth. The shrub shouldn’t be planted deeper than it is in the container.
- Remove the rhododendron and gently loosen the soil around the roots.
- Place the rhododendron in the hole and backfill with the same soil you dug out.
- Water the rhododendron and cover the root area with mulch. If using leaf mold (i.e. mulch made from decomposing leaves), Carmolli says to avoid kinds with walnut leaves, which are toxic to rhododendrons. You should also avoid planting them near walnut trees.
How To Care for Rhododendrons
Stacey Hirvela from Proven Winners Color Choice Shrubs offers two pieces of advice.
- Mulch: “Mulch is imperative for healthy rhododendrons,” she says. “It helps keep the shallow roots cool and moist and more closely mimics growing conditions in rhododendrons’ native areas. Opt for an organic mulch, like shredded bark or pine straw, over rock or other inorganic options.”
- Deadhead with care: “Many people deadhead, or remove the spent flowers, from rhododendrons,” she says. “While this does help the plant put its energies into making more flowers for the following season, it isn’t strictly necessary. If you decide to deadhead your rhododendron, work carefully so you avoid damaging any buds that have begun to form.” I would also add, do this as soon as the flowers have faded.