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Pruning improves a tree's structure and helps keep it healthy. A bit of judicious pruning benefits a tree at many stages of life. Properly pruned young trees will grow into handsome, structurally strong trees; they require little corrective pruning when older, and will pose less risk from weak branches. Mature trees need periodic pruning to remove dead wood, crossed or rubbing branches, and to thin the crown to reduce the "sail" effect.
Get out the whetstone! Pruning shears and saws should be sharp, for your own safety and to make clean cuts in bark and wood.
Pruning should always be done at branch junctions. Where two branches, or a branch and the trunk meet, there is an area of swollen tissue called the branch collar. The ideal pruning cut leaves the branch collar intact while removing the rest of the branch. Never perform flush cuts or leave stubs. On larger branches, the three-cut pruning method is necessary to prevent bark tearing as the branch falls away.
Pruning paint or wound dressing is unnecessary. Such sealers can actually hamper a tree's natural defense system, causing problems instead of preventing them. Wound dressing is recommended only on oaks pruned or wounded between April 15 and July 1 to prevent oak wilt infection.
Many tree defects can be corrected with timely pruning. Pruning young trees greatly influences their future structure and form, and substantially reduces or avoids the need for heavy pruning as they age.
It's best to prune a young tree over a period of years, beginning two to three years after planting. Because no more than one-quarter of the tree's live crown should be removed at one time, not all defects can necessarily be corrected with the first pruning.
Each cut has the potential to alter tree growth, so remove no branch without a reason.
Stand back from the tree and look for branches that are broken, dead or diseased. Remove these problem branches first. Branches that are growing back into the center of the tree and those that are rubbing against other branches also should be removed.
Correcting double stems or forks is another important pruning objective. Bark is sometimes "included" or squeezed downward between both sides of a forked stem. This bark-included crotch is weakened and with time, is likely to split. Retain the stem that is largest, more fully crowned, more vigorous and/or in a more direct vertical line from the top of the roots.
Take another look at the tree's overall form and structure before you prune any healthy branches. Depending on the species, the main lateral or scaffold branches should ideally be spaced eight to 24 inches apart vertically and be evenly distributed around the trunk. The branches that remain, should be no more than one-half the diameter of the trunk. Leave some branches lower on the trunk – they'll help develop good trunk strength. Low branches may be removed during subsequent pruning. Never remove more than one-third of the total live crown at one time.
The second pruning takes place about five to seven years after planting. Again, remove defective branches first. Lift the crown and provide clearance for pedestrians, vehicles and structures by pruning the lower, temporary branches. Stand back and look at the form of the crown. Branches that extend outside the natural outline may be cut back to a lateral branch or branch junction.
Mature trees need occasional pruning to remove dead or hazardous branches, to restore damaged crowns, and to keep roadways and sidewalks clear. Only experienced and knowledgeable arborists should prune mature trees. Good arborists use a technique called "drop-crotch pruning" to reduce the length of branches or reduce the size of the canopy and will never "top" a tree.
The best time to prune is when the tree is dormant – after the leaves fall off in autumn, and before the buds begin to swell in the spring. The next best time is in the summer, after leaves are fully formed.
Spring flowering trees are best pruned after the flowers have dropped. Try to avoid pruning any tree during leaf formation in the spring, or during leaf drop in the fall. Storm-damaged limbs, dead branches, or branches that may be high risk should be removed as soon as possible.