Most deciduous trees are best pruned when dormant in fall or winter. They may also be pruned at other times except in late winter or early spring when many trees will exude sap if cut. Evergreens usually require little or no pruning except the removal of dead or diseased branches in late summer.
The first step in pruning a tree is to remove any dead, diseased or damaged wood and cut out weak or straggly shoots. Once this is done, assess the remaining framework and decide which branches should be pruned back or removed for a well-balanced growth. You don't want to impair the natural growth habit of a tree by pruning.
Hard pruning stimulates vigorous growth; light pruning produced only limited growth.
It is important to make pruning cuts accurately to minimize damage to the tree. If cutting back a stem, cut just above a healthy bud, pair of buds, or side shoot pointing in the required direction of growth. For example, if thinning out congested stems, cut back to a bud or shoot that is growing outward so it will not rub against another stem as it grows. Cut neither too far from the bud, which leaves a stub that provides an entry point for disease, nor too close which could damage the bud itself.
When pruning trees with opposite buds, make a straight cut with sharp pruners directly above a pair of buds. For trees with with alternate buds, make a sloping cut 1/8" - 1/4" above a bud so that the base of the cut is just level with the top of the bud on the opposite side of the stem.
If pruning back a branch completely, cut just outside the branch collar (this is the slight swelling on the branch where it joins the trunk.) This is where the callus is formed that will eventually cover the wound. NEVER cut flush with the main stem since this damages the tree's natural protective zone, making it more vulnerable to disease infestations. The branch collar on dead branches may extend some way along the branch, but it is still important to make any cut outside it.
Removing entire branches that are less than 1" in diameter, make a single cut with a pruning saw or pruners. For larger branches, remove the bulk of the weight leaving a short stub of about 12" or so in length. To remove the remaining stub, undercut it just outside the branch collar, then cut through from above to finish the removal.
DON'T apply any type of wound paint or dressing after removing the branch. There is no evidence that this speeds up the healing process or prevents disease infestations and may actually slow the healing process.
Conifers may be pruned any time of year, but pruning during the dormant season may minimize sap and resin flow from cut branches.
Hardwood trees and shrubs without showy flowers: prune in the dormant season to easily visualize the structure of the tree, to maximize wound closure in the growing season after pruning, to reduce the chance of transmitting disease, and to discourage excessive sap flow from wounds. Recent wounds and the chemical scents they emit can actually attract insects that spread tree disease. In particular, wounded elm wood is known to attract bark beetles that harbor spores of the Dutch elm disease fungus, and open wounds on oaks are known to attract beetles that spread the oak wilt fungus. Take care to prune these trees during the correct time of year to prevent spread of these fatal diseases. Contact your local tree disease specialist to find out when to prune these tree species in your area. Usually, the best time is during the late fall and winter.
Flowering trees and shrubs: these should also be pruned during the dormant season for the same reasons stated above; however, to preserve the current year's flower crop, prune according to the following schedule:
Trees and shrubs that flower in early spring (redbud, dogwood, etc.) should be pruned immediately after flowering (flower buds arise the year before they flush, and will form on the new growth).
Many flowering trees are susceptible to fireblight, a bacterial disease that can be spread by pruning. These trees, including many varieties of crabapple, hawthorn, pear, mountain ash, flowering quince and pyracantha, should be pruned during the dormant season. Check with your county extension agent or a horticulturist for additional information.
Trees and shrubs that flower in the summer or fall always should be pruned during the dormant season (flower buds will form on new twigs during the next growing season, and the flowers will flush normally).
To encourage the development of a strong, healthy tree, consider the following guidelines when pruning.
Prune first for safety, next for health, and finally for aesthetics.
Never prune trees that are touching or near utility lines; instead consult your local utility company.
Avoid pruning trees when you might increase susceptibility to important pests (e.g. in areas where oak wilt exists, avoid pruning oaks in the spring and early summer; prune trees susceptible to fireblight only during the dormant season).
Use the following decision guide for size of branches to be removed:
Under 2" diameter - go ahead,
Between 2" — 4" diameter— think twice, and
Greater than 4" diameter— you better have a real good reason.
Assess how a tree will be pruned from the top down.
Favor branches with strong, U-shaped angles of attachment. Remove branches with weak, V-shaped angles of attachment and/or included bark.
Ideally, lateral branches should be evenly spaced on the main stem of young trees.
Remove any branches that rub or cross another branch.
Make sure that lateral branches are no more than 1/2 — 3/4 of the diameter of the stem to discourage the development of co-dominant stems.
Do not remove more than 1/4 of the living crown of a tree at one time. If it is necessary to remove more, do it over successive years.
Always maintain live branches on at least 2/3 of a tree's total height. Removing too many lower branches will hinder the development of a strong stem.
Remove basal sprouts and vigorous epicormic sprouts.
Use crown reduction pruning only when absolutely necessary. Make the pruning cut at a lateral branch that is at least 1/3 the diameter of the stem to be removed.
If it is necessary to remove more than 1/2 of the foliage from a branch, remove the entire branch.