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.Pruning a young tree
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.Pruning the big trees
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.