When to prune your shrubs for healthier growth this year

• Pruning decisions should be based on local weather and plant dormancy, not the calendar.
• Identify if a shrub blooms on old or new wood to determine the correct pruning time.
• Shrubs that bloom on new wood can be pruned in late winter, while those blooming on old wood should be pruned after flowering.

Determining what time of year to prune your ornamental deciduous shrubs can be tricky and confusing, but we hope to clear up some misconceptions and provide clear guidance for several common landscape shrubs.

Late winter and early spring (February-March) is often the optimum pruning season because plants are still dormant. Pruning during late dormancy allows plants to heal cut wounds before the high energy demands of growing their new branches and leaves commence. Late winter pruning also allows you to see the plant’s natural shape and branching, as well as identify any old or damaged wood.

Start with your pruning goal

Start with why you are pruning. What is your objective? Is your plant overgrown and in need of significant rejuvenation? Selective pruning will keep the plant’s natural appearance, growth habit, shape, and size. Maybe it is not producing large or numerous blooms anymore? Or maybe it’s just in need of a clean-up for plant health and flower vigor.

Assess your plant’s growth habits first. Is it fast or slow growing? What is its natural height, width, and form? 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 often assist you in properly identifying it.

Proper plant identification determines pruning success

Therefore, the most important step to pruning ornamental shrubs is to properly identify the species or variety. Since many plants have more than one common name (some of which can be misleading or overlap with other plants), we always recommend determining pruning recommendations based on the plant’s botanical or Latin name.

Before you do any cutting, 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 (bore holes) and remove those sections. Dispose of any diseased plant debris in the garbage – diseases can survive the 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.

The 3 pruning methods every gardener should know

There are three general pruning methods: thinning out, renewal pruning, and complete rejuvenation. All three methods may require new growth to be thinned out and properly trimmed to allow for good branching.

If thinning out the plant’s growth is your objective, first remove the tallest and oldest stems 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 three years 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. For example, multi-stem shrubs such as forsythia and red stem/red osier dogwood (Cornus sericea) can withstand and actually benefit from a drastic cut back like this. Slow-growing shrubs, however, usually cannot and therefore should only be lightly pruned.

Avoid shearing most ornamental shrubs

Avoid shearing your ornamental shrubs into artificially round or square shapes, as it will result in an unnatural appearance and can cause excessive growth and water sprouts, undoing the work toward reducing size and impacting bloom quantity. 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.

Last but not least, does your shrub flower, and if so, does it bloom on old wood or new wood?

With many shrubs grown for their beautiful flowers, choosing when to prune them can be a little more complicated.

Shrubs that bloom on new wood

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 during the same year, and consequently more abundant flowering. These species bloom later in the growing season. Some examples include:

• Rose of Sharon (Hibiscus syriacus)
• Purple leaf sand cherry (Prunus x cisterna)
• Crape myrtle (Lagerstroemia indica)
• Hardy hibiscus (Hibiscus moscheutos).

Shrubs that bloom on old wood

Other well-known early season blooming landscape plants 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. Those shrubs include:

• Forsythia (Forsythia x intermedia)
• Lilac (Syringa spp.)
• Flowering quince (Chaenomeles speciosa)
• Rhododendron and azalea (Rhododendron spp.)
• Viburnum (Viburnum spp.)
• Weigela (Weigela florida)
• Virginia sweetspire (Itea virginica)

For these types of shrubs, it is best to wait until after they bloom to clean them up.

Tools and cleaning steps that prevent plant disease

Regardless of what time of year is the right time, 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. 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.

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 724-371-2062 or email [email protected].

Maureen McCulloch is a Penn State Master Gardener in Beaver County.