CORVALLIS, Ore. – Every gardener owns a pair of clippers, but not every one knows how to use them for the daunting job of pruning shrubs.
"Pruning is both art and science and not something most of us get training in," said Steve Renquist, a horticulturist with Oregon State University’s Extension Service. "Pruning should both enhance a plant's natural beauty and form, and keep the plant vigorous and productive.”
People unsure of how to approach pruning tend to think of it as a way to cut a shrub down to size, he said. But there is more than one reason to pick up the pruners.
To get started, learn the two types of cuts used on shrubs – heading cuts remove ends of branches to make the plant denser; thinning cuts remove entire branches or canes to give the plant a more open form. It’s also important to know the natural shape or habit for each shrub in your garden.
Mound-forming shrubs, such as abelia and escallonia, need thinning cuts near the ground level. Remove tall shoots that tower above the mound form. Don't shear mound-forming shrubs or they will become too dense.
Cane-forming shrubs that send up new growth from the base of the plant, including forsythia and lilac, should be allowed to reach their natural height. To keep them looking their best prune once a year using thinning cuts to take out one-eighth to one-fifth of the canes, preferably the oldest.
Upright or tree-like shrubs like rhododendron usually need little pruning and will look best when thinned slightly every few years.
Save heading cuts for hedges, where tight compact growth is desirable.
Plants with colorful twigs in winter – such as red-twig dogwood and purple osier willow – can be cut back to the ground to encourage brightest color.
Flowering shrubs require a little more thought before pruning if you want them to bloom nicely each year. Most importantly, Renquist said, consider when they bloom. If a shrub flowers in late winter or spring, such as azalea, mock orange and flowering quince, prune after bloom. If they bloom in summer or fall, prune during dormancy in winter.
Don’t just chop the top off of a shrub, he said. Topping destroys the natural beauty and weakens it structurally. Instead, think about a shrub’s mature size before buying it and invest in one that will fit the space when mature. Otherwise, you’ll be tempted to prune too severely and chance damaging or even killing the shrub.
If you get stuck with pruners in hand and no idea what to do, turn to your local OSU Extension Master Gardeners.