Inside The New York Botanical Garden

Azaleas: Keeping Them Healthy

Posted in Gardening Tips on May 26 2011, by Sonia Uyterhoeven

Sonia Uyterhoeven is Gardener for Public Education.
Rhododendron 'P.J.M.'
Rhododendron 'P.J.M.'


The best pruning jobs begin with a goal, a determination of what needs to be accomplished–reducing size, controlling shape, rejuvenating growth. As with all pruning jobs, removing the dead and diseased wood is the first call of order. Then take a step back and look at the framework before moving ahead with your mission.

See also: Native Azaleas

The best time to prune azaleas and rhododendrons is immediately after bloom into mid-July. If you wait too long, buds will already have formed for the following year. In general, azaleas and rhododendrons need minimal pruning.

Reducing Size

To reduce the height of an azalea while allowing it to retain its natural appearance, follow the branch targeted for removal down to a lower lateral branch, and make a cut just above the point of intersection. The cut should be slightly above where the two branches intersect so as not to cut into the tissue of the branch that will remain.

Another way of reducing the size of an azalea is to cut it back to just above a whorls of leaves. Also look for circular scars around the stem, where leaves once were. Cutting just above these areas should create good bud break. In either case, make the cut just above new buds, whether they are visible or latent.

Controlling Shape

Evergreen azaleas can be sheared back to form nice mounds if desired. If you are pruning the azalea into a mound, remember to occasionally open up the plant and let some light penetrate into the dense mass of foliage so that it doesn’t get too congested.

See also: Azalea Planting Tips

If you’d like to make a young azalea or rhododendron more compact and well-branched, the easiest thing to do is to pinch off the vegetative buds in spring. First take a look at the plant to familiarize yourself with the bud system. The fat swollen buds are flower buds. These form in the previous season and overwinter, and you’ll want to leave these alone. They are generally twice the size of the vegetative buds, which are the narrower, pencil-like, smaller buds.

The number of vegetative buds at the tip of a stem determine the number of new stems that will be produced. Sometimes you will find 2-4 vegetative buds, but more often, there will be just one. If you snap off this single vegetative bud (it will be about a half-inch long) with your thumb and index finger early in the season, immediately after flowering, you will induce the plant to produce more buds. Generally, it produces 2-4 buds at the same location, which will turn into 2-4 new shoots. In this way you can influence the branching structure of the plant.

Rejuvenating Growth

Some azaleas and rhododendrons get leggy over time. If the plant is misshapen or too large, you can prune it drastically. Such a rejuvenation pruning should be done early in the spring, mid-March to early April, in the New York area (usually 2-3 weeks before new growth starts). Cut the plant back hard to about 8-10 inches from the ground. You can cut the entire plant back or leave one or two smaller stems as a source of energy (these are cut back later once growth resumes). Remember to water the rejuvenated plant well during its first season. New suckers may need to be thinned mid-season.

Some rhododendrons, particularly the Dexter hybrids such as ‘Scintillation’, do not rejuvenate well. Many other rhododendrons and azaleas do. When rejuvenating an azalea or rhododendron, fertilize the year before to prepare the plant and then fertilize again in spring (end of April) after the hard pruning to stimulate growth.

See also: Azaleas A to Z: Know Them to Grow Them

For smaller azaleas, use bypass pruners to make the cuts. For larger jobs, you may need a folding saw or a pair of loppers. Work with sharp, clean tools. Tools can be sanitized with isopropyl alcohol.

It would be tedious and time consuming to deadhead azaleas and small flowering rhododendrons. But since seed production does take energy away from the plant, large flowering rhododendrons can benefit from deadheading.

Hold on to the stem and grab hold of the spent flower head. Snap it off being careful not to damage the new buds forming on the sides of the stem. Your hands will get sticky from this task; you can clean them with Tecnu® or some oil-based hand cleaner.

Rhododendron canescens 'Varnadoe's Phlox Pink'
Rhododendron canescens 'Varnadoe's Phlox Pink'

Pests and Problems

In general, azaleas and rhododendrons are fairly problem free. Below are the most common problems encountered.

Lace Bugs

Lace bugs are rarely a problem for azaleas grown in the shade but can be a problem for those grown in full sun. The underside of the leaves will have stippling or small yellow spots, which are created when the small, mottled-wing insect pierces the leaves and sucks out fluid.

See also: Top Tips for Designing with Azaleas in the Home Garden

There are several ways to combat these pests: Spray the underside of the leaves with water in mid to late May to dislodge the nymphs; release natural predators such as green lacewings; or spray the foliage with insecticidal soap or horticultural oil in mid to late May and again in mid to late July, making sure to cover the underside of the leaves. Spraying works best with 2-3 applications separated by 5-7 days. Always read the instructions when applying pesticides.

Black Vine Weevils

Black vine weevils are small beetles with a long snout that eat c-shaped notches into the leaves of rhododendrons and azaleas. These beetles are nocturnal feeders that hatch in mid-June. Go out at night with a flashlight and either pick them off by hand or place a white sheet under the plant and shake them off.

The grubs cause the real damage, hatching at the base of the plant and eating away at the root system. They can be treated with nematodes in April-May and again in August. Nematodes that specifically treat black vine weevil grubs are available. Do not apply in direct sunlight, keep the soil moist, and always read application instructions.

Powdery Mildew

Powdery mildew affects many decidious azaleas. Good air circulation and increased levels of light help avoid this problem. Options for control include products derived from neem oil, potassium bicarbonate (GreenCure®), Bacillus subtilis (Plant Guardian™), and selective pruning.

Wind Burn

Wind burn on foliage is a common problem and happens when azaleas and rhododendrons are exposed to sun and drying winds in winter. Mulch and water the plants well in fall. A burlap windbreak also helps as can anti-desiccants: apply once in late November-December and again in late January. Prune damaged foliage in spring.

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