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LATE-WINTER PRUNING BEST FOR MOST TREES AND SHRUBS
I'm asked just about every month of the year when is the best time to prune. For most trees and shrubs, February and March are ideal months to prune for a number of reasons.
When trees are pruned during dormancy, research shows that the remaining branches grow faster in spring because energy stored in roots and branches can be channeled to fewer growth points.
The absence of leaves allows the pruner to get a better look at the tree's branching pattern so crossing branches and weak crotches can be more readily seen and corrected.
Frozen ground facilitates the use of larger, heavier equipment on turf and around gardens planted with perennials and shrubs. The noise and commotion caused by pruning is also less intrusive in winter when houses are closed tight and less activity is going on outdoors.
The spread of insect-born diseases such as oak wilt is eliminated. Oaks, by the way, should never be pruned during April, May or June.
The goal of pruning is to maintain a tree's structure and form by removing dead branches and by thinning large branches to reduce weight and wind resistance. Proper pruning also enables more light to penetrate inner branches.
Before you reach for the saw, give some thought to how much wood you plan to remove. Over-pruning can be just as damaging as no pruning at all, explains Dr. Patrick Weicherding, University of Minnesota extension horticulturist. Over-pruning removes too many leaves and, without enough leaves, a tree cannot gather and process sufficient sunlight to survive. Weicherding advises to never remove more than one-third of a tree's total mass.
According to Weicherding, there are six general forms of pruning.
Crown cleaning refers to removal of dead, dying, diseased and weakly attached branches and water sprouts (fast-growing vertical branches).
Crown thinning is the selective removal of live branches to increase light penetration and air movement.
Crown raising is the removal of lower branches to provide clearance for buildings, vehicles and pedestrians.
Crown reduction involves using thinning cuts to make the crown smaller when it has grown too large for the space. The remaining lateral branch to which a leader branch is cut should be at least one-third the diameter of the branch being removed.
Crown restoration refers to improving the structure of trees that have sprouted vigorously following breakage or from being pruned with heading cuts (cuts made at 90-degree angle).
Utility pruning is done to restore clearance under electrical power lines or other utility wires.
Proper pruning cuts do not injure the branch bark ridge (the raised line of bark that forms between the branch and the trunk) or the branch collar (the living, swollen tissue that surrounds the base of a branch). The size and prominence of the branch collar and branch bark ridge vary between individual trees and tree species.
I've heard speakers describe the branch collar and bark ridge as a tree's "first-aid kit" because the cells and tissue in these zones are specially adapted to help the tree heal following pruning.
The two most-common cutting errors made by non-professional pruners, according to Weicherding, are cutting too close ("flush cut") and not cutting close enough ("stub cut"). Both errors can lead to decay, cracks, insect and disease problems and hazard trees. Flush cuts destroy these tissues that defend against decay. Stub cuts leave an energy source for insect and disease organisms because the cells remaining in stub cuts are alive but lack defense mechanisms.
"Proper pruning is one of the best things a person can do for trees, and improper pruning is one of the worst," Weicherding says. "When people decide to do their own pruning, rather than hire a professional pruner, they need to arm themselves with the knowledge about what proper pruning is and what it is not."
The two best ways to help trees recover following pruning is to water and mulch them during the growing season.
There is no need to apply any type of sealant or wound dressing on trees pruned in late winter or early spring. Trees are capable of gradually sealing their own wounds when pruned properly. In fact, wound dressings may actually interfere with this process. The only time sealant is recommended is on oak trees pruned during April, May and June, and the trick here is to apply the sealant within minutes of pruning. Sealants applied to oaks hours or days later will not protect oaks because the insects that transmit oak wilt will have already infected the wound.
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