Be kind to your shrubs when you pick up pruners
April 21, 2022 | View PDF
You've got clippers in hand, a shrub in mind and a gleam in your eye. It's pruning time.
But do you have a plan?
Before you clip a stem, know your shrub, said Neil Bell, horticulturist for Oregon State University's Extension Service.
"What you want to focus on is flowering time and growth habit," he said. "Certain shrubs you can prune right about now; others you should wait until after they flower."
Spring-blooming shrubs like forsythia, mock orange, flowering quince, deutzia and lilac, should be left alone until flowering is over. These bloom on last year's stems, known as old wood, from buds that form in summer or fall. Prune now and you sacrifice flowers.
Summer- and fall-blooming shrubs, on the other hand, flower on new wood. They're fair game to prune now. Some include rose of Sharon (Hibiscus syriacus), PeeGee hydrangea (Hydrangea paniculata) winterberry (Ilex verticillata), Japanese spiraea (Spiraea japonica) and a lot of the subshrubs like Russian sage (Perovskia atriplicifolia), hardy fuchsia (Fuchsia magellanica), cape fuchsia (Phygelius) and bluebeard (Caryopteris).
There are some exceptions, Bell said. If the goal is to remove some older canes to even out the canopy or thin out the center to facilitate air circulation, pruning a spring-blooming shrub during the dormant season is acceptable.
"It's easier since you can see the plant's form," he said. "Just be judicious. Thin out only the oldest canes, leaving the height and width the same to assure you maintain growth habit and you're not cutting off too many flowers."
Keep in mind, Bell said, that many shrubs need only light pruning, especially if given the room to grow to their mature size. Rhododendron, azalea, rockrose (Cistus) and ceanothus are a few of these. They're better off with a light trim every couple of years. Pieris, rosemary, lavender and Daphne odora and other winter daphne are also low-maintenance. Just cutting off the flowers does the trick.
If Bell has one piece of advice he's most adamant about, it's avoid shearing or topping a shrub.
"The biggest sin is taking a pair of hedge clippers and chopping off the top of a shrub or shaping it into a geometric shape," Bell said. "Plants have a natural growth habit. Shearing ruins the form, cuts off the flowers if done before flowering on spring-blooming shrubs and may even weaken the plant."
Instead, if you want a rounded bush, seek out those that grow that way naturally, he said, pointing to rockrose (Cistus), hebe and some daphne and spirea as examples. Or use plants like boxwood, privet, laurel and Japanese holly (Ilex crenata) that respond well to shearing into spheres, squares or other shapes.
Most of the time, well-considered pruning begins with determining the shape of the plant. Is it vase-shaped? Upright? Weeping? You'll want to keep that form in mind as you proceed. When it comes time to pick up the clippers, start by cutting out dead or dying canes (stems that grow from the ground). If the shrub needs thinning, cut out the old, less-attractive canes first, Bell said. They'll often be the tallest, so pruning them will keep the plant at a more manageable size. Crossing branches within the canopy can also be clipped out if they're causing too much congestion. Cut some of those stems back to a main branch. But don't go wild. Less is more.
"You prune mainly because if left to its own devices, a plant looks unkempt," Bell said. "The canopy becomes crowded, it becomes larger than we want. Pruning is a way to maintain size and accentuate its ornamental attributes."
Judicious clipping may also help ward off bacterial and fungal diseases by opening the interior of the plant to light and air circulation, he noted.
If a plant has gotten out of control, it can be cut to the ground and allowed to grow back, Bell said. However, that applies only to vigorous plants. If a plant is declining because of too much shade, poor soil or a disease, no amount of pruning will rejuvenate it. Even for vigorous plants, constant pruning is not ideal.
"It's possible, but if a plant is too big for its space, you'll be pruning it down all the time and it will never have the chance to reach its potential," Bell said. "You sacrifice the ornamental appeal of the plant and it's a lot of trouble. It's better to move it to a different space; or to take cuttings and plant those in a better spot."
Unless you inherit an overgrown plant, he said, it's always better to start with the right plant for the right place.
Look for more information on pruning, including illustrations, in the OSU Master Gardener handout Pruning Trees and Shrubs.