In a recent column, I discussed pruning both deciduous and evergreen landscape plants. I mentioned that if you have shrubs that need pruning, know what you wish to accomplish before you begin.
Whatever the time of year, plants respond to pruning in very predictable ways. If you want to do the best possible job of pruning, it's necessary to fully understand those responses.
For instance, if you are pruning a flowering shrub, be sure you know if the shrub blooms on new growth (current season's growth) or last year's growth. If you prune spring-flowering shrubs such as flowering quince or dogwood in winter, you will remove some of your spring flower display.
On the other hand, the crape myrtle produces flowers on current season's growth, so you can prune that species during the winter months without sacrificing bloom.
In most cases, you'll want to selectively prune branches without changing the plant's natural form using "thinning" cuts. A thinning cut removes a branch completely back to its point of origin from the parent stem, or back to a large diameter side branch.
This method of pruning results in a more open plant, and does not stimulate excessive new growth. You can remove a considerable number of branches without changing your plant's natural appearance or growth habit. You should also be able to maintain the plant's size for many years using thinning cuts.
This type of pruning is best done with hand pruners, loppers or a pruning saw, but not with pruning shears.
Shrubs pruned or sheared to unnatural shapes, such as in hedges or topiaries, require much more work to maintain their size and shape.
Remember, there is a difference between pruning and shearing. Pruning involves the removal of individual branches or twigs, while shearing involves clipping or shaping by cutting off twig tips. Such pruning cuts are properly called "heading" cuts. Gardeners often wait too long to begin developing a hedge or topiary. To develop such unnatural plant shapes takes a good deal of effort, beginning when the plant or plants are quite small.
For hedges, begin shearing when the plants are young to stimulate low branching, then remove half of the new growth for the next couple of years. By the third or fourth year, you should have achieved the final size and shape that you have in mind. You should never allow the plants to grow untrimmed to their final height before shearing. By that time, it's too late to get maximum low branching.
The best time to prune most shrubs and to shear hedges, except spring blooming shrubs, is just before the start of new growth in spring. The new spring growth quickly softens the plant somewhat and helps to prevent a severe, unnatural plant shape. In most cases you will want to prune spring-blooming shrubs after they have bloomed.
Ed Perry is the county director and farm adviser for the University of California Cooperative Extension and can be reached at email@example.com.