By Heather Kirk-Ballard
LSU AgCenter Horticulturist
(01/22/21) Pruning intimidates some gardeners. When to prune and when not to prune? It can be scary.
For trees and shrubs that are grown for their flowers, you must consider when they bloom before you decide when to prune them. Prune at the wrong time, and many of our spring-blooming shrubs and trees will not put on their much-anticipated spring flower show.
There is a large group of plants that can be pruned during the winter and in the early spring, but there are many flowering shrubs and trees that you want to avoid pruning during this time. Let’s distinguish what to prune and what not to prune in the winter and early spring.
What not to prune? Spring-flowering trees and shrubs that bloom from January through April. If you were to prune them now, you prune the stems along with the soon-to-be flower buds. This means no spring flower show, or at least a very non-exciting flower show. Buds are set on last year’s growth on what’s dubbed the “old wood.”
Some of the most common spring-flowering trees and shrubs in Louisiana are azalea, banana shrub, camellia, deutzia, Indian hawthorn, Japanese magnolia, star magnolia, Taiwan flowering cherry, spirea, quince and wisteria.
There is that tricky group out there that always causes confusion and lots of phone calls and emails. These include the flowering shrubs that bloom on old wood much like our spring bloomers, but instead are late bloomers. By late bloomers, I mean those that wait until early summer rather than spring. These late bloomers will form next year’s flower buds in late summer or early fall when the days become shorter and the weather cools.
Some examples of shrubs that bloom on last year’s growth that are later bloomers are climbing roses, gardenias, hydrangeas (bigleaf and oakleaf) and garden roses (not the ever-blooming types). All of these shrubs bloom in the early summer and should be pruned shortly after they have completed their bloom in the summertime.
There is another group of bloomers to discuss. The repeat bloomers, such as Encore azaleas and Knock Out roses, bloom in the spring, again in the fall and sometimes in between. What do we do with these?
Well, here is a good rule of thumb to go by for all flowering plants: Prune shortly after they have completed their bloom. Whether the buds are set on new growth or old growth, if you prune right after the bloom and before new blooms set, you will be just fine.
This leaves another question: What to prune? Some plants that can be pruned now through early spring are non-flowering evergreen and deciduous trees and shrubs. These can include foundation plantings as well as screens and hedges such as boxwoods, yaupons, yews, distylium, euonymus, junipers, hollies, cleyera, photinia, wax myrtle and the list goes on.
You may also prune ornamental grasses and tropical plants. If you are like me, you might feel an urge to prune dead things. However, I ask you to resist for two reasons. Hear me out.
First, tropical plants will undoubtedly be fooled by our weather. One week we are in the 40s; the next, we are in the 80s. You can trim now, but be aware that the next cold snap will zap the new tender growth encouraged by a brief warm spell. This can stress the plants, but they will survive.
Second, birds that once found protection in trees full of leaves now bare from winter dormancy need a place to nest. Birds and animals love dormant clumps of grass for eating, nesting and hiding. I understand that the idea of small animals hiding in your landscape is less than appealing to some — but that is better than in your house, right?
You also can prune summer-flowering shrubs and trees that set their flower buds on new plant growth that resumes in the spring and early summer. These plants can be pruned in the wintertime. Some summer-blooming plants are abelia, althea, cassia, crape myrtle, oleander and vitex. I recommend trimming these this time of year because the form is open and you can easily see the big picture.
Now that you know what you can prune, you need to consider the best way to prune. Pruning can simply be removing crossing branches. Some folks use the “heading back” technique — shortening shoots or branches to stimulate growth, control size, encourage fullness and maintain specific shapes. You also can thin out select branches at their point of origin.
Pruning at any time of year — whether it is the “best” time or not — likely will not kill or permanently damage your plants under most circumstances, even if you do something wrong when you prune.
Buds set on last year’s growth on flowering trees such as this Japanese magnolia should not be pruned until after they finish blooming in the spring. Photo by Heather Kirk-Ballard/LSU AgCenter
Dormant ornamental grasses can be trimmed in wintertime but make for great food and nesting areas for birds. It’s recommended to wait until spring. Photo by Heather Kirk-Ballard/LSU AgCenter
Summer-blooming plants such as Lime Sizzler firebush can be trimmed anytime to control growth or keep a particular shape. Flowers form on new growth. Photo by Heather Kirk-Ballard/LSU AgCenter
Spring-blooming azaleas should be pruned right after blooming before new buds set for next year’s show. Photo by Heather Kirk-Ballard/LSU AgCenter