If the thought of cutting into what looks like a perfectly happy plant makes you cringe, you're not alone. Even homeowners who know the benefits of pruning—better health, more pleasing habit, bigger flowers—are often still confused about exactly the right time and right way to make the cuts, fearing they'll lop off next year's flowers, stunt the plant's growth, or kill it outright. But once you understand how plants respond to pruning, you'll realize how many problems a well-placed cut can solve.
The first step to successful pruning is timing it right. Shrubs that flower on new wood, or branches that form in spring and flower in summer—rose-of-Sharon and summersweet are two—should be pruned in late February or early March. This results in fewer but larger flowers the first year. "Pruning distributes the plant's stored energy among fewer flower buds so that the ones left behind get more to eat," explains horticulturalist Lee Reich. Prune spring-flowering shrubs right after they bloom, giving them the rest of the growing season to develop new branches and buds, since these bloom on old wood, or last season's growth. "But if you miss the ideal time to prune, you can always wait until the shrub's flowers brown out," says This Old House landscape contractor Roger Cook.
Successful pruning involves mastering two basic cuts. Follow along to learn how to use them to remedy common problems you encounter.
"To deal with a wayward evergreen branch, be sure to cut it back to the center of the shrub, where it meets another stem. If you just lop off the offending section, the cut stub will be obvious and unsightly."—Roger Cook, TOH landscape contractor
Problem: You've inherited a sloppily chopped shrub.
Do make selective cuts to neaten up a poorly pruned shrub by stimulating new growth where you want it and removing injured, less vigorous wood. Cuts heal more quickly when made in the right spot and at the correct angle with a sharp, clean tool. Find a branch with a bud facing the direction you want new growth to follow. Prune just above that bud at a 45-degree angle, with the lowest point of the cut farthest from the bud.
Don't leave more than ¼ inch of growth above the bud, as this can encourage rot. Cutting too low can cause the bud to dry out, and cutting at an angle greater than 45 degrees can create a large surface area that's slow to heal, inviting disease.
Problem: Your shrub has dense foliage at the top but looks lifeless inside.
Do open up the plant with thinning cuts. Just trimming back branch tips, either with manual pruners or electric shears, results in dense foliage at the top of a shrub and a tangle of weak, leafless branches at the center. Thinning cuts remove whole branches down to the base or take
off large sections of branches back to a main stem, allowing light and air to reach the center of the plant and encouraging healthy new growth throughout. Remove the thickest, oldest wood first before moving to younger stems.
Don't remove more than one-third of a plant's mass in a year, to keep it vigorous and looking good.
Problem: You've got a shrub that's lopsided.
Do use heading cuts to spur growth in the right direction. Unlike thinning cuts, which remove a branch, heading cuts shorten a branch down to a bud you want to encourage to branch out. Though it seems counterintuitive, you need to prune the shorter side of a lopsided shrub to stimulate growth and even it out. Position the pruner on the part of the stem you want to remove, just above a bud that will grow in the direction you want to encourage.
Don't remove more than one-quarter of a stem's overall length in any single cut. For shrubs that are dramatically lopsided, use thinning cuts to remove older wood from the longer side as well as heading cuts on the shorter side.
Problem: You've got an old shrub that's a woody, tangled mess.
Do give the plant an overhaul by thinning out the old wood over the course of three years, making room for all new growth. Starting at the base of the shrub, eliminate the centermost branches, taking out no more than one-third of the shrub's total mass. New growth from the base should follow the next growing season. Remove another third of the old wood at the base in each of years two and three. By the end of year three, the shrub should be made up of entirely new, vigorous growth.
Don't remove more than one-third of the shrub's branches at any one time. This preserves enough foliage that the plant can make sufficient food (through photosynthesis) to stay robust and generate new growth quickly.
Problem: You have a flowering evergreen shrub that's leggy at the bottom.
Do use your index finger and thumb to pinch off the end (or terminal) buds of new branches on rhododendron, azalea, Japanese pieris, mountain laurel, and other broad-leaved evergreens to encourage side branching on the lower part of the shrub. As with a heading cut, manually removing the terminal leaf bud signals a dormant bud below it to grow, stimulating lush side branching. This is also a way to control the shrub's overall size.
Don't pinch off the flower buds in the process. These are the bigger, fatter buds at the ends of branches.