Pruning Azaleas and other Shrubs

Don't prune for the sake of pruning

Brian Jervis: Ask a Master Gardener

Saturday, May 28, 2016

Q: How should I prune my azaleas? — Tim, Tulsa

A: Before you prune azaleas, or any plant, ask yourself what the reason is for pruning. If there is no good reason, don’t prune just for the sake of pruning. Inappropriate pruning abounds in area landscapes, especially in crape myrtles.

Pruning trees and shrubs is best done in winter, especially late winter. The exception to this is spring-blooming shrubs, which should be pruned after blooming is completed. The only reason to prune at that time, rather than winter, is to preserve the buds that form blossoms in spring. Like most spring-blooming plants, azaleas produce flower buds from mid-summer into fall of the previous year. If pruned any time after buds are formed, during summer, fall or winter, spring blossoms will be lost.

There are several legitimate indications for pruning azaleas or any shrub. One is to control size or alter shape and appearance. However, if the shrub is too large for an area and must be cut back yearly, thought should be given to removing the plant and getting a smaller cultivar.

Another reason to prune is to remove any dead or damaged limbs; this should be done anytime they are noticed. Also, removal of any overcrowded, crossing or rubbing limbs, especially those inside the shrub, will improve air flow, improve health and lessen the likelihood of diseases.

For azaleas that are old and overgrown, “rejuvenation pruning” may be the way to go. This process may need to be done over a few years, but it can rehabilitate an old favorite shrub. It may be done before new growth starts in spring with the understanding blossoms will be lost. The process is to remove about a third of the oldest largest stems back to a few inches from the ground. The younger stems will then have more growth and eventually more blossoms.

Then remove another third of the older stems yearly over the following two years and the shrub will then be “rejuvenated.”

One thing that is best avoided when pruning azaleas is using the hedge trimmers to try to shape them into a boxy hedge. Far better results are obtained by using hand clippers and pruning one branch at a time, creating a form that seems natural, as well as achieving your pruning goals.

All of the deciduous summer-blooming shrubs such as abelia, Rose-of-Sharon and crape myrtles are best pruned in late winter or early spring before summer buds form. The broadleaved evergreen shrubs like camellia, hollies, boxwoods, nandina and photinas should also be pruned in late winter and early spring.

Needled evergreens like pines and arbor vitae usually do not need pruning. Be careful about pruning these plants. You might refer to the OSU fact sheet HLA-6409, “Pruning Ornamental Trees and Shrubs,” for information about pruning these and all plants. It is available from OSU or from the Tulsa Master Gardener’s website, tulsamastergardeners.org.

For more information or to ask a question about gardening, contact the Master Gardeners at 918-746-3701 from 9 a.m.-4 p.m.

Garden tips

Powdery mildew Alert

With springtime temperatures and wet weather, this fungus infection is widespread now. Master Gardeners are getting daily calls about the disease on a host of plants.

The infection can involve trees and shrubs, such as crapemyrtles and lilacs, as well as many ornamental flowers. Most garden vegetables are susceptible to it, especially cucumbers.

The disease infects leaves, where it typically produces a whitish powdery coating. In some plants, the fungus causes only yellow spots, rather than powdery discoloration. The infected leaves usually die when infected.

To treat the infection, OSU recommends a fungicide, chlorothalanil, found in Ortho Garden Disease control, as well as other brands. This fungicide is effective, but it only prevents new infection and cannot eliminate established disease. Leaves with infection should be removed, if possible, and placed in the trash.

Organic treatments may include horticultural oils, fungicides based on copper or sulfur compounds and potassium bicarbonate solutions. These are not as effective as chlorothalanil and are mainly used as a preventative in susceptible plants.

Also, if you are able to increase air circulation and sun exposure of the plants, this significantly reduces the incidence of the disease. The ultimate control is to plant those cultivars of plants that have shown resistance to the disease.