Note: The following article is reprinted from the St. Paul Pioneer Press, Feb. 5, 2005, issue, with
permission from the author, Marge Hols, Pioneer Press garden columnist and a Ramsey County Master
Gardener. The article is based partly on a talk to Ramsey and Washington County Master Gardeners by
David Hanson, a research specialist in urban and community forestry at the University of Minnesota.
Nip & Tuck
Here's how to prune trees at the right time to promote the best growth.
Thanks to tree researcher Dave Hanson, I knew just what to say when my tree service proposed trimming a wayward honeylocust in my backyard last summer:
“Nope, the best time to prune honeylocust—and most other trees--is while they’re dormant in late February or early March.”
"Pruning cuts made in late winter will seal quickly once new growth begins," says Hanson, a research specialist in urban forestry at the University of Minnesota. “The worst time to prune trees and shrubs is in spring when they’re trying to break bud. You’re taking all that energy they’ve saved up and throwing it away.”
Late winter pruning is essential for some trees that are susceptible to disease: honeylocust to Nectria canker, oak to oak wilt, and crabapple, mountain ash and others to fireblight. Never trim these trees in April, May or June unless a branch breaks in a storm, Hanson says. Then cut off the branch and spray the wound immediately with a clear varnish or water-based paint, not a black tar tree sealer. For other timing details, see “What to Prune When”.
"This is the best time of year to assess your trees and shrubs while there are no leaves so you can see the limbs,” Hanson says. “Grab a clipboard, take a walk around your yard and check your trees and shrubs, one by one. It’s cold out, but we’re in Minnesota. Just put on boots and a good pair of gloves.”
The main reasons to prune are to remove dead or broken branches, improve structure and control size and shape.
If you have a young tree, deciduous or evergreen, the aim is to prevent problems by pruning during formative years, Hanson says. Look for three things: broken branches, co-dominant leaders and other structural problems such as crossed limbs rubbing together.
In a tree a few years older, the main scaffold branches should be spaced out, so note any branches too closely spaced on the trunk. If you intend to walk under the tree make a note to remove lower branches up to six to eight feet.
On big, mature trees, the main concern is safety. Check for broken branches hanging in the tree and any major openings in the trunk or mushrooms growing on the tree, which indicate weak, decayed wood. If those branches are hanging over your house, driveway, garden or play area, they need attention.
Assess each shrub, too. Are there dead, broken or overcrowded branches? Is the shrub getting too big for its space? Did it grow well last year?
You can probably prune small trees and shrubs yourself. For safety reasons, Hanson recommends you hire an arborist to prune large trees. He says you should never try to work near a power line. Let an arborist deal with it.
For most home pruning, you’ll need only four tools, Hanson says, and not one of them is a chain saw. Use a bypass hand pruner to cut branches under one inch in diameter and a long-handled lopper for branches up to one-and-one-half inches. To cut larger branches, use a curved blade folding saw or an arborist’s saw with a curved blade 21 to 26 inches long.
Hanson wants to make sure you know where and how to cut off a branch. You should cut just beyond the branch collar, the rough, swollen part of the branch joining the trunk (see photo). The wound will be smaller and heal faster than if you used the old method of cutting flush with the trunk.
When cutting off a sizable branch, make three cuts to avoid having bark rip down the trunk. (See diagram and photo.) Two feet from the trunk, make your first cut on the underside of the branch, cutting one-third of the way through. Make the second cut about a foot farther from the trunk, cutting from the top all the way through so the branch drops off. Third, make a proper pruning cut just beyond the branch collar.
Most trees should have one central branch or leader. If two branches are competing for the honor, keep the branch you think will grow straightest and cut off the other one. If the leader needs help growing straight up this spring, tie a bamboo stick to it with masking or duck tape or a light cord. Remove the stick in a few months.
Hanson recommends three techniques for pruning deciduous shrubs. To renew a shrub, cut the oldest one-third of the canes to three to six inches above ground. To rejuvenate a shrub, cut all branches back to three to six inches. To reduce shrub height, prune taller branches back to a main branch.
To control size and shape, most evergreens including arborvitae, hemlock, Japanese yew, juniper and pine should be pruned in late June and July after new growth appears. Pines, including Mugo pine shrubs, take special treatment. Cut off only three-fourths of each new-growth candle. If you prune into old wood the stem will die back. Other evergreens can be sheared or taller branches can be pruned back to an inner branch union.
Hanson says we’d have to do a lot less pruning if we just bought the right plant for the right spot.
“Pay attention to mature height and width when selecting a tree or shrub,” he says. “Don’t put a spruce right next to the house where eventually you’ll have to shear off one side and take off the top. Choose small trees like a hawthorn, magnolia, serviceberry, crabapple, Japanese tree lilac or Amur maple.”
What to Prune When
Here’s advice from the University of Minnesota Extension Service on the best times to prune trees and shrubs.
Late-February and March
- Most deciduous trees, including ash, aspen, cottonwood, elm, hickory, hackberry, Kentucky coffeetree, linden, poplar and willow.
- Trees susceptible to diseases: apple, crabapple, shrub cotoneaster, hawthorn, pear and mountain ash (fireblight); honeylocust (Nectria canker) and oak (oak wilt). Do not prune these trees in April, May or June.
- Trees with free-flowing sap may be pruned in late winter as excessive sap flow causes little harm. Or, to avoid sap flow, prune when leaves are fully expanded in June or July: birch, blue beech, boxelder, butternut, ironwood, maple and walnut.
- Evergreen trees with lateral buds: spruce, fir and Douglas fir.
- Shrubs grown primarily for foliage: alpine currant, barberry, burning bush (winged euonymus), dogwood, honeysuckle, ninebark, purpleleaf sand cherry, Siberian peashrub, smokebush, sumac, viburnum and winterberry.
- Shrubs that bloom on new growth: ‘Annabelle’ hydrangea and late-blooming spirea.
- Prune shrub roses back to live wood as soon as sprouts grow.
- Trees and shrubs that bloom in spring on last year’s growth should be pruned shortly after blooming: apricot, azalea, chokeberry, chokecherry, clove currant, flowering plum and cherry, forsythia, lilac, magnolia, serviceberry and early-blooming spirea.
- Evergreens including arborvitae, hemlock, Japanese yew, juniper and pine.
For more information:
“Pruning Trees and Shrubs”, University of Minnesota Extension Service
Or, order publication at 612-624-4900.
“How to Prune Trees”, U.S. department of Agriculture
Or, order publication at 651-649-5244.
“Illustrated Guide to Pruning, Second Edition,” E.F. Gilman,