Rhododendron and azalea pruning basics | Garden Notes

Barbara Faurot
Posted 2/17/22

“May, and after a rainy spring

We walk streets gallant with rhododendrons.”  

– Poet Alicia Suskin Ostriker

The name Rhododendron derives from the Greek rhodon (rose) …

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Rhododendron and azalea pruning basics | Garden Notes

How rhododendron grow in the wild. No pruning needed.
How rhododendron grow in the wild. No pruning needed.
Photo courtesy of Tina Sāra
Posted

“May, and after a rainy spring

We walk streets gallant with rhododendrons.”  

– Poet Alicia Suskin Ostriker

The name Rhododendron derives from the Greek rhodon (rose) and dendron (tree). Azalea, a member of the Rhododendron genus, comes from the Greek azaleos (dry). Despite the name, most thrive in humus-rich, moisture-retaining soil. 

Both are members of the Ericaceae family, sharing characteristics with heathers, salal, manzanita, kinnikinnick, and madrona. They often have evergreen leaves, four or five united sepals, and urn- or bell-shaped flowers. 

Several species are native to the Pacific Northwest, notably our state flower, Rhododendron macrophyllum or Pacific rhododendron, and Rhododendron occidentale or Western azalea. Worldwide, there are nearly 1,200 rhododendron (including azalea) species that occur in the wild, and 28,000 rhododendron and 10,000 azalea cultivars developed or hybridized in nurseries. 

The Pacific Northwest coast has close to ideal conditions for most varieties: moderate temperatures and rainfall, and slightly acidic soil. Their roots need oxygen, so good drainage is key, along with plenty of organic matter to feed the soil.

Most rhododendron and azalea are easy to care for, and require minimal pruning if the right plant is in the right place. Steve Hootman, executive director and curator of the Rhododendron Species Botanical Garden in Federal Way, concurs. “We don’t prune very much. We like to show species as they occur in the wild,” Steve shares. “If a plant isn’t in the right place, it will never have the right form.”

Occasional maintenance pruning can promote good health and structure. If needed, remove the Four Ds (dead, damaged, diseased, or deranged material) at any time of year. Prune dead material back to a branch source, and cut broken, crossing, or wrong-way branches to a branch source, or just above a dormant bud. Remove any diseased branches well below the damage and discard them in the trash. Make cuts outside the branch collar (the small bump at the base of each branch).

Flower buds form on the terminal ends of branches in the summer or early fall. “If something needs to be pruned, it’s best to do it just after blooming,” advises Steve. 

Some gardeners prefer to deadhead the spent blooms, but it’s not necessary. The plant will continue to produce new buds next season. 

For all varieties, thinning cuts, or branch removal, can be used to reduce density and increase light penetration and air circulation. Cut smaller lateral branches off the main stems for more open, filtered light.

For large-leaf evergreen rhododendron, branches can be pruned back to a bud or to a whorl of leaves. If single, slender vegetative buds are removed from the terminal tip, the “subtending” or nearby buds will be forced to grow, causing more branching and potentially more flowering. 

Smaller-leaf rhododendron and evergreen azalea have leaves and buds up and down the stem. Most do not require pruning, but may be pruned after flowering if a looser habit or certain form is desired. Cuts may be made to a leaf or bud anywhere along the stem. 

Deciduous rhododendron and azalea bloom on their terminal tips and have varying degrees of basal bud activity. Thinning cuts can be made to reduce the size or density. Some older canes can be removed to stimulate new shoot formation.

Steve mentions that at the Rhododendron Species Botanical Garden, older “bushy” small-leaf plants, whether evergreen or deciduous, can be rejuvenated by cutting them to the ground after flowering. They’ll respond more quickly than large-leaf plants.

In dire situations where a large-leaf plant has been mal-pruned or damaged, radical renovation is an approach to consider. February or early March is usually the best time for this approach.

Radical renovation is not a halfway measure. There is a risk that plants will not recover from the stress, so candidates should be well-established. If a plant has outgrown its space, moving it and replacing it with a smaller variety is a better bet. Rhododendron and azalea are good candidates for transplanting with their broad, flat, fibrous root systems.

If you decide radical renovation is called for, you’ll cut the entire plant to a low framework, say 6 to 12 inches from the ground, or 3 to 7 inches for a smaller plant. Each primary branch can be cut to a slightly different height to help produce a natural form. The adventitious buds hidden in the stem, bark, and roots will be stimulated to emerge and form new branches.

The renovated plant may need supplemental water for the next few seasons. Layering compost and coarse organic mulch around the root zone is beneficial.  Don’t add fertilizer to avoid stimulating excessive growth. 

The plant won’t produce flowers for the first year or two. Once the new branches emerge, you can selectively prune them to create a new framework. Within three to five years, the plant should re-grow to its original size. 

A note of caution: Some rhododendron species and their hybrids have smooth or peeling bark and may not produce growth from latent buds. It’s worth doing some research on your variety if you don’t see small “plumping” buds on the lower stems. 

The Rhododendron Species Botanical Garden (rhodygarden.org) is home to the largest collection of rhododendron species in the world. Steve suggests it’s a great time to visit: “Things are already starting to bloom.”

Master Gardeners are available to address your gardening questions at the online plant clinic: jefferson.wsu.edu/plant-clinic.

(Barbara Faurot is a Jefferson County Master Gardener and Master Pruner, working with other volunteers who serve as community educators in gardening and environmental stewardship.)

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