Planting a tree that will mature into a towering specimen in a small space can result in a misshapen, unhealthy tree at the cost of a lot of effort. Some trees need annual trimming to keep them compact. Others need occasional shaping to correct storm damage, and some need little help during their lifetimes. Know which trees to trim -- and why -- before reaching for the shears.
Your local master gardener or nurseryman has the expertise to help chose a tree that will fit your space when it grows to maturity. Your tree will grow to a natural shape as it ages -- round, columnar or oval, upright or vase-shaped, spreading, and conical are the more common shapes. For example, the umbrella-shaped holly oak (Quercus ilex) that grows to 70 feet tall and oval Japanese evergreen oak (Quercus acuta), a 25-foot tree, fit very different spaces, even though both are oaks. Trimming a holly oak to fit a space where a Japanese evergreen oak should be would expend a great deal of effort to end up with a misshapen tree.
Some trees, especially fruit trees, benefit from shaping in their first years. Leaders, or main branches are headed, or cut, back so the tree concentrates its growth on the lateral shoot nearest the cut. Young trees also produce numerous side shoots, and all but the few that grow at a healthy angle -- close to 45 degrees -- will eventually break of their own weight, tear away in storms or grow across other branches. Removing these minor branches decreases crowding and improves air circulation in young trees. When thinning branches, cut back to the “collar” -- the thick ridge of tissue at the base of each branch.
Even trees that seldom need pruning, such as many oaks, need periodic trimming to keep their growth compact and strong. Depending on the tree, this regular maintenance is done to trim out dead or damaged wood. Branches that cross others and rub against them need removal to avoid stripping bark that would allow insects and disease into the live wood that carries nutrients, called the phylum. Depending on how brittle the tree’s wood is, maintenance trimming takes place annually to as long as seven or eight years apart. Late winter -- when trees are dormant -- is the optimum time for trimming and shaping trees.
Mature trees that have been shaped properly in their youth seldom need trimming to shape, but when trees have lost branches in storms or grown faster on one side due to root damage, the remaining branches can be reduced to re-shape the tree. Trimming up to one-third of each overgrown or extended branch each year will eventually restore the tree’s original shape. Shaping will also help keep trees that grow in shady areas compact. Tools should be kept sharp and cleaned with rubbing alcohol.