We usually think of pruning our fruit, nut and ornamental trees and shrubs as a job for the dormant season, when the sap has retreated and the plants have gone into their winter swoon.
That’s true for many kinds of woody plants. But not all. And there are good reasons why some plants that you might prune in winter can also be pruned in summer. The bottom line is that you need to know both the purpose of pruning and the effect of pruning if you’re going to achieve the results you want.
Let’s start with the understanding that in spring, the sap rises, growth hormones flood the tissues, and growth spurts follow. But spring also is the time of wet, cool weather that favors fungus and disease. And fungus favors lush, tender new growth. So on trees and shrubs that may be prone to fungal and bacterial infections, it might be wise to wait until the hot dry weather of summer before you prune.
Asian pears, apples, flowering crab apples, quince, pears and firethorn (pyracantha) are susceptible to fire blight, which is a bacterial infection that can wipe out an entire orchard if not checked. Fire blight makes the leaves and stems look as though they were burned or scorched. They dry, turn black and wilt.
If you haven’t had any problems with it, you can prune during dormancy, but if you’ve seen fire blight at all, wait until July to mid-August to prune. Keep a jar of bleach handy and dip your shear blades into it between each cut. This will sterilize your equipment so you don’t spread the bacteria from branch to branch.
Fungus such as rusts, black knot of plums or cherries and evergreen blights should be pruned out in July. Late season pruning encourages tender new growth that doesn’t have the time to harden off before cold weather and rains return, increasing the chance of disease. By pruning in July, new growth will have time to harden off and become woody and resistant to infection in the fall and winter.
Take note of when and how your ornamental flowering trees and shrubs bloom. You’ll find two types of bloom strategies among your ornamentals. First, some will bloom on new wood that starts growing the in the spring. That’s why we prune repeat-blooming roses to just a few canes in the dormant season. Those few canes will produce a lot of new growth that will produce a lot of flowers in spring and then sporadically through summer into fall.
Second, roses that bloom just once (in May or June) and many flowering trees and shrubs bloom on year-old wood. English hawthorn is an example of such a tree. Among flowering shrubs, forsythia, mock orange (Philadelphus spp.), flowering quince (Chaenomeles spp.), rhododendrons including azaleas, spring-blooming spirea (Spiraea prunifolia and S. x vanhouttei), lilacs and viburnums all bloom on year-old wood.
These year-old bloomers should be pruned pretty much as soon as they finish their flower display. This means they’ll grow new wood during the summer, which will become next year’s year-old wood. This new wood will be exposed to lots of sunshine, the sunshine will stimulate the plants to make buds that flower next year, and you’ll be set for next spring’s displays. You can see that if you wait to prune until after the plants have already grown new wood with its nascent flower buds, you’ll be pruning away next year’s flowers.