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Gardening: Proper pruning protects tree health

gardening by Mary Robson, Area Horticulture Agent, Washington State University
Storms for the past few weeks indicate, without doubt, that winter has come to the maritime Northwest. The canopy of deciduous trees above our heads is leafless now, and it's time to look at the shape and health of the trees to see what's needed.
   Large, mature trees contribute beauty, shade and value to landscapes, and they do need care and attention occasionally.
   When is pruning needed? Whenever trees have damaged or broken limbs, or diseased areas, these must be pruned out.
   Because the maritime Northwest has a mild climate, pruning can be done almost anytime, but for many people the best time is between late winter and early spring, after leaf fall and before trees beging to bud out with new leaves (December through early March).
   Windy winter storms rip limbs off large trees, and these resulting wounds must be attended to. Broken limbs endanger animals (including people) and buildings. Untended wounds can contribute to disease problems developing in the tree.
   If a large tree is extensively damaged or inaccessible, hiring professional service makes good sense. Tree care professionals with the most training and up-to-date methods are certified as members of the International Society of Arboriculture (ISA). Some may also note membership in the National Arborist Association (NAA).
   When contracting for tree services, especially for a prized heirloom tree, look for a certified arborist. Climbing a tall tree with a chain saw running requires a combination of mountaineering and juggling skills and is best left to trained people.
   However, lots of pruning is possible for any of us from an eight- or 12-foot ladder using hand tools: Invest in good loppers and hand saws.
   In general, pruning removes dead or diseased branches (look for depressed concave spots on branches that could be caused by cankers).
   Pruning can also cut out branches that have grown into or across other limbs, spoiling the tree's shape. And pruning encourages bloom and crops for all tree fruit.
   Cut thoughtfully. Pruning carries some of the admonition attached to carpentry, when people remind themselves to "measure twice, cut once." Observe twice (or more times) before cutting.
   Learn to understand a bit more about how trees grow. Modern tree research tells us to cut a large branch off at the "branch collar," a raised curved area encircling the branch where it meets the trunk.
   Don't cut flat against the trunk. The tree will heal faster, covering the wound with a callus, when the branch collar is left.
   Very heavy branches need to be cut in two stages to prevent further wounding the tree by having the branch rip off. Cut under the branch 12 to 14 inches out from the branch collar, about halfway through the branch. Move out one inch and saw through the branch from above. Then saw off the remaining stub, outside the branch collar.
   A common misconception about pruning is that cuts must be treated with "wound paint," a gummy substance that's brushed onto the open surface after cutting. Current research indicates that trees heal faster and more completely if the cut is left unpainted. The old-fashioned wound paint tends to allow disease organisms to thrive under its shelter.
   Shaping and opening a tree is usually done by making "thinning" cuts, which means removing whole branches back to a trunk or to a natural growth point. Proper thinning cuts allow the tree to continue growing in an open, attractive shape. Thinning cuts help to remove tangled or crossing branches and sculpt the tree effectively.
   If a branch is nipped at the tip, that's called a "heading cut." Heading cuts are useful in tree fruit pruning and will contribute bushiness where needed. Hedge trimming is a good example of over-all heading cuts, where each branch is cut back but not cut to a growth point.
   When a tree branch is whacked off at the end in a "heading cut," it often re-sprouts with a lot of little, weak twiggy branches. The worst examples of this occur when tall trees are "topped," given a sort of crew cut to lower them below wires or get them out of views. Topped trees develop weaker growth and are more prone to disease and blow-downs than those pruned carefully. Careful pruning protects the asset that is a mature tree and is worth the cost when done right; slapdash "topping" ruins trees.
   Trees are often topped because they have grown too large for the space. If a tree doesn't fit, and can't be thinned for views, remove it. Replace trees that will always be too big for the space with smaller ones. It's often difficult to take out a big tree, but in many situations, it's the only possible choice.
   Several books show good pictures of proper pruning cuts, such as Ortho's All About Pruning. W.S.U. Cooperative Extension has a publication on pruning, EB 1619 ($3.50): 296-3900 in King County, 338-2400 in Snohomish County.