Pruning can't help but wound a plant.
But the injury doesn't have to compromise its health.
Your job, as a pruner, is to pinch, snip, lop or saw in such a way as to facilitate your plants' natural healing.
Plants have an uncanny ability to deal with wounds. Immediately after any wound, whether from high wind or from the sharp edge of your pruning saw, cells around the wound burst into activity. Their goal: to prevent the spread of infection and seal off the wound.
Unless the weather is frigid, rapid respiration ("breathing") and cell division occur, during which natural antimicrobial chemicals are released and new cells grow to seal off the wound. With little or no microbial growth in frigid weather, the plant can wait to begin repair.
Plant heal theyself (with some good cuts)
The first thing you can do to encourage healing is to make all cuts clean. Ragged edges leave more damaged cells and more surface area to close over. Sharp pruning tools are a must.
Smaller cuts leave smaller wounds, so prune off that misplaced maple limb when you can do it with hand shears rather than when you need a chain saw. Pruning away small stems, rather than large ones, also removes less stored food or food-producing tissue of a plant, which is desirable unless you are deliberately attempting to dwarf a plant.
Pruning off diseased stems can be a way to thwart diseases, but watch out that you don't inadvertently spread infection in the process. When disease transmission is a hazard -- as it is, for example, with fire blight disease of pears -- sterilize your pruning tool between cuts by wiping the blades with alcohol.
Young, actively growing stems heal easiest and quickest, which makes pinching out a growing point between thumbnail and forefinger the least damaging method of pruning. No special instructions here, except, perhaps, to clean under your fingernails. (Just kidding.)
Pruning stems, large limbs
Shortening a stem must be done with more care. Cut it back to a bud, which is where a leaf is growing or grew the previous season. To avoid leaving a dead stub, or killing the bud, shorten the stem to a little beyond the bud, at an angle, so that the cut slopes down ever so slightly behind the bud.
Removing a large limb with a single pruning cut can tear long shreds of bark from a tree as the limb comes toppling down. Avoid this by making three separate cuts. First undercut the limb one-quarter of the way through about 12 inches further out than your eventual cut.
Next, saw through from the top, near the first cut but a couple of inches further out on the limb. After the limb falls (without tearing any bark), saw off the easily held, foot-long stub that remains. But do not cut it back flush to the trunk; cut back to just beyond the ring of bark at the base of the limb.
After you have cut off a branch or limb, do nothing to the bare wound that stares you in the face. Marketing or an innate desire for nurturing has induced humans for centuries to cover wounds with dressings ranging from clay to manure to tar. Such dressings, for the most part, keep the wound moist, maintaining a hospitable environment for disease-causing microorganisms.
A good pruning cut -- not a poultice -- allows a woody plant to seal off the wound and prevent spread of infection. Take care how you cut, and appreciate a plant's natural ability to heal itself.
* Lee Reich's latest book is The Pruning Book (Taunton Press, 2010).