Pruning Deciduous Shrubs

Determining what time of year to prune your ornamental deciduous shrubs can be tricky and confusing for the average home gardener. For example, four common types of hydrangeas require several different pruning approaches!

We hope to clear up some misconceptions and provide clear guidance for several common landscape shrubs.

The first and most important step to pruning is properly identifying the species or variety. Since many plants have more than one common name (some which can be misleading or overlap with other plants) we always recommend determining pruning recommendations based on the plant’s botanica Latin name (for example, smooth hydrangeas' botanical name is Hydrangea arborescens).

You should also understand your plants' growth habits – Are they fast or slow growing? What is their natural height, width and form? Does your shrub flower, and if so, does it bloom on old wood or new wood? Knowing your plant's botanical name can help with understanding its specific growth habit, and the reverse is true, too – knowing your plant's growth habits can help you properly identify it.

We often think of late winter and early spring (February-March) as the optimum pruning season because plants are still dormant, allowing you to see the plant's natural shape and branching, as well as any old or damaged wood. Pruning during late dormancy also allows plants to heal cut wounds before the high energy demands of growing new branches and leaves. This is why trees are most often pruned during this late dormancy window.

With many shrubs grown for their beautiful flowers, though, choosing when to prune them can be a little more complicated than pruning trees. Shrubs that flower on new wood – that is, the branches they grow this year – should be pruned during late winter and early spring to encourage new branch growth and consequently abundant flowering. These species include Rose of Sharon (Hibiscus syriacus), Japanese spirea (Spirea japonica), purple leaf sand cherry (Prunus x cisterna), crape myrtle (Lagerstroemia indica) and hardy hibiscus (Hibiscus moscheutos).

However, many other shrubs should actually be pruned after their flowering season, occurring anywhere between May and August, depending on the species' flowering window.

Many well-known landscape plants, such as forsythia (Forsythia x intermedia), lilac (Syringa spp.), flowering quince (Chaenomeles speciosa), rhododendron and azalea (Rhododendron spp.), viburnum (Viburnum spp.), weigela (Weigela florida), and Virginia sweetspire (Itea virginica) all bloom on old wood, so pruning off branches in late winter – before they have a chance to grow new wood – effectively removes this year's flower buds and will result in a disappointing flower show. For these types, it is best to wait until after they bloom to clean them up.

Hydrangeas are a bit of a mixed bag – some flower on new wood, and others on old. It is important to know the species and varieties in your landscape so you can prune at the appropriate time. Smooth hydrangea (H. arborescens) flowers on new wood, and so should be pruned in February/March.

Many smooth leaf varieties can tolerate being cut back to the ground. Panicle or "peegee" hydrangea (H. paniculata) – including popular varieties like 'Limelight,' 'Bobo,' and 'Moon Dance' – flowers on new wood and should also be pruned in late winter/early spring. Oakleaf hydrangea (H. quercifolia) flowers on old wood, and so should be pruned after flowering is over, if at all! Bigleaf hydrangea (H. macrophylla) flowers mainly on old wood, but some newer cultivars flower on new wood; pruning after flowering is generally recommended for bigleaf hydrangeas.

Regardless of what time of year is the right, the fundamentals for how to prune are essentially the same. First, do you have the right tools for trimming? Pruners and loppers should be large enough to handle the size of the branches being cut and they should be sharp. Undersized pruners and dull blades will not make a sharp clean cut, potentially leading to disease or insect infestation.

Also, be sure to clean your pruners after each plant trimming so as not to transfer any disease between plants. Hand tools can be cleaned with a 70% alcohol solution, a 10% bleach solution, or a hydrogen peroxide solution. Allow them to air dry completely after cleaning.

Next, consider why you are pruning. What is your objective? Is your plant overgrown and in need of significant rejuvenation? Maybe it is not producing large or numerous blooms anymore? Or maybe it is just in need of a clean-up for plant health and flower vigor?

Selective pruning will keep the plant's natural appearance, growth habit, shape, and size. Start with an initial health assessment by inspecting your plant. Remove any dead, broken, diseased, or pest-infected canes, branches, and twigs. If the cane or branch is dead (dry and gray in color), cut it back to the base. Look for any insect infestation signs in the branches and remove those sections.

Dispose of any diseased plant debris in the garbage – diseases can survive composting process and cause problems later on. Remove branches that are rubbing against each other as well as suckers, which grow straight up from the base of the trunk, and water sprouts, which grow straight upward from branches. You can recognize these growths by their different bark color and texture from the rest of the plant.

Once you've completed this initial pruning, the next step is to get your plant into shape. In general, do not remove more than 1/3 of your plant’s branches or height at one time. There are some exceptions to this rule, such as when a complete rejuvenation pruning is needed or for certain fast-growing species. For example, multi-stem shrubs such as forsythia and red stem/redosier dogwood (Cornus sericea) can withstand and benefit from a drastic cut back. Slow-growing shrubs, however, usually cannot and therefore should only be lightly pruned.

There are three general pruning methods: thinning out, renewal pruning, and complete rejuvenation. New growth may need to be thinned out and properly trimmed to allow for branching with all three methods.

If thinning out the plant's growth is your objective, remove the tallest and oldest stems first back to ground level to allow more sun and airflow to the plant and encourage new growth. Trim smaller branches next by making 45 degree cuts 1/8 to 3/8 inch above the bud. The angle of your cut should be in the same direction the bud will continue to grow. This helps to maintain your plant's natural shape and size.

Renewal pruning of an overgrown shrub occurs over a three-year period and requires the gradual removal (15%-20%) of the oldest and tallest of branches each year leaving only younger branches. This method is best used on older shrubs that may not be in the best of health.

Complete rejuvenation pruning involves cutting all branches and stems back to two inches from the ground, as in the case of forsythia and redosier dogwood described above.

Except in the case of more formal hedge rows with species such as yew (Taxus spp.), boxwood (Buxus spp.) and privet (Ligustrum amurense), avoid "heading back" well-established shrubs to try to control size. Branch removal cuts are more effective for controlling size than heading cuts for slow-growing, established shrubs.

Shearing them into artificially round or square shapes results in an unnatural appearance and can cause excessive growth and water sprouts, undoing the work toward reducing size and impacting bloom quantity.

Proper maintenance of your ornamental shrubs can seem daunting and can be time-consuming initially, but after the first year or two, you will be a pro and the task will become easier. The rewards of a healthy, beautiful landscape will be worth the toil!

Pruning and many other horticultural questions can be answered by Master Gardeners at the Beaver County Extension office. Call the Garden Hotline at 724-774-3003 or email beavermg@psu.edu.

This article originally appeared in the Beaver County Times, in the Over the Garden Gate column on February 28th, 2024.