When and How to Prune New vs. Old Growth Perennials for Beautiful Blooms
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Once winter starts to wane and days grow longer, it's time to tidy up the yard and prepare for spring. One of the most confusing tasks is when to prune woody flowering perennials. Cut back for new wood and preserve old growth - what does it mean?
In simplest terms, not all woody perennials follow the same schedule for blooming, which impacts when they can be pruned. So, let's break down how to prune new vs. old growth perennials with quick and easy tips.
Old Growth Perennials
“Old growth” or “old wood” flowering plants bloom on the year’s previous growth. These perennials develop their flower buds for the next year shortly after blooming in the current year. In even simpler terms, a plant produces beautiful blooms, they fade and new ones begin developing but don’t bloom until the growing season rolls around again next year.
For this reason, any pruning of an old growth shrub in the late winter or early spring will cut off the buds for the current season. That equals no spring or summer flowers. Pruning these plants for reshaping and/or controlling growth needs to happen immediately after blooms fade.
Examples of old growth plants include azalea, forsythia, oakleaf hydrangea, and big leaf hydrangea such as Endless Summer.
In addition to poorly timed pruning, buds on old growth plants are also susceptible to damage during extreme winter weather. Even after the weather warms, those buds aren’t necessarily safe. A false spring can lure a plant to push out its buds, which will suffer if a late cold snap comes along.
Keep in mind, pruning old growth plants isn’t required for its vitality and health. The plant will continue to grow and bloom just fine without pruning. If space isn’t an issue, then let the plant be as it is and enjoy its natural growth pattern.
Pruning is necessary if you want to maintain a certain shape, however. Removing any dead and broken branches can happen at any time. Nothing will bloom on a dead branch, anyway.
New Growth Perennials
“New growth” or “new wood” plants form their buds in the spring, the current growing season. These plants can be safely pruned anytime in the fall, winter or early spring without risk to any flowering.
Generally, it's just simpler to annually prune new growth plants in late February and early March alongside other yard tasks in prep for spring. Plus, leaving new wood plants as they are during the fall and winter provides nice structure and visual interest to a yard during the winter.
After pruning, new branches will push out in the spring. Flower buds grow on those new branches (new wood) and then bloom during the summer and into the fall.
Some flowering new wood plants include butterfly bush; sage varieties, such as black and blue salvia and radio red autumn sage; limelight and other smooth hydrangeas; and panicle hydrangeas such as Annabelle.
These hardy plants thrive after pruning and are reliable, colorful additions to any yard. Many, such as butterfly bushes, are such prolific growers they can handle major pruning.
In general and especially for hydrangea, aim for pruning no more than about 30% of the plant’s total height in order to maintain structure and stability. This is commonly called the “one-third” rule.
Work in small increments as you prune so you don’t cut too much off at a time. You can always cut more, but you can’t instantly grow more!
Although it's not generally recommended to prune midseason, in hot planting zones, such as 9A - 8A in the southeastern U.S., limelight hydrangeas may benefit from a mid-to-late summer trim.
The plant’s early summer blooms grow so big they eventually flop over from their own weight. Additionally, the petals burn in the hot July and August sun. With conservative pruning comes another round of fresh, white upright blooms. If anything, it's a nicer aesthetic to have fresh blooms vs scorched brown ones.
What are Reblooomers
Not to throw a wrench in things, but some perennials bloom on both new and old wood. These “rebloomers” are actually old wood plants that have been selectively bred to bloom twice in a growing season.
Blooms emerge in the spring and again later in the summer. Proven Winners, a major plant supplier found at practically every garden center, sells varieties of azalea, lilac, indian hawthorne, clematis, weigela and even a hybrid hydrangea that will bloom twice in a season. Another popular garden center brand, Monrovia, also offers reblooming hydrangeas.
The Proven Winners and Monrovia websites both detail their rebloomers, as well as other plants they sell.
According to Proven Winners, rebloomers should be pruned the same as old wood hydrangeas. Monrovia advises deadheading spent flowers and pruning out any dead or weak stems in the early spring.
Pruning 101
For a deep dive on pruning tips, such as shearing, thinning, handling a hedge, preventing disease and more, check out this article by the University of Maryland Extension Service.
Land grant universities, such as the University of Maryland, are home to extension offices that focus on research and education on just about any topic related to gardening, agriculture and conservation.
Specifically, these offices are part of the Cooperative Extension System, an agency of the U.S. Department of Agriculture. Extension offices are a wealth of free information and services for everyone from farmers to homeowners.
Websites for various extension offices often feature encyclopedia-like details on regional plants. For example, the N.C. State University Extension has a Gardener Plant Toolbox featuring pictures and details for more than 4,600 plants that grow in and around the state.
If you can’t find the answer to your gardening question online, call or email your local extension agent.
Now that you know how to prune new vs. old growth perennials with confidence and where to find even more information when questions arise, happy gardening!
