Pruning a tree when it is newly planted and during its first few years in the landscape will not only ensure its development into a desirable shape, but will also prevent more drastic corrective pruning as the tree ages. Early pruning is easier for the gardener and healthier for the tree. Removing poorly positioned branches leaves a much smaller wound on a young tree than on a mature tree. Removing the same branches after they have been allowed to grow large means more expense, greater disfigurement, a longer time for the wound to close over, and a greater chance of infection by wood decay fungi.
In purchasing a young tree, choose one with a strong trunk. Nurseries often grow trees close together, almost as if they were in a forest. Because they are protected from the wind, these trees do not develop sturdy trunks. If transplanted into an open area, they are prone to breaking in strong winds. On the other hand, trees that are allowed to sway in the wind actually develop stronger trunks that last a lifetime. Look for a tree that was grown with enough space; identify it by a trunk that is wider at the base, tapering upward, and with a full set of lower branches. A sturdy tree like this will bend without breaking.
Try to choose a tree that will remain upright without staking. Those without sufficient trunk taper may need to be staked. In that case, do not rigidly stake or guy-wire the tree; allow it to sway in the wind so it can develop a strong stem taper. Tie the tree to the stake with a single tie, at the lowest spot that will hold the tree upright. Prune the crown to reduce the weight of branches and foliage so they can better resist the wind. If the tree is tall and the location very windy, a second tie and a taller stake may be needed to keep the tree from breaking off above the tie. The stake is a temporary crutch to be used only until the trunk is strong enough to support the tree. Remove the stake as soon as possible.
Nurseries often head young trees to 5 or 6 feet high to force the development of lateral side branches so the tree looks bushier and more appealing to the customer. Frequently, the new shoots grow from directly below the cut, the result of the sudden loss of apical dominance. As the tree grows, the cluster of terminal shoots often develops into more than one leader. Such a tree needs some corrective pruning to maintain a central leader (see below).
Developing a Leader
The first task is to determine which branch is or should be the main leader. If several are growing upright in the center, select the most vigorous or most central as the leader. Remove or shorten the competing branches to prevent the formation of a double leader, which is inherently weak and susceptible to breaking during storms. To shorten and subdue the growth of a potential competitor without removing it entirely, cut the branch back by one third to one half of its length to a lateral twig, thin to a small side branch, or head it to a leaf or bud. This removes the terminal bud and arrests the upright growth pattern.
The greatest mistake in pruning a young tree is to prune the leader — unless you thin it to a lateral, which would then assume apical dominance. The loss of apical control also means the loss of the main leader upon which the primary scaffold branches arise. Seldom is there any justification for cutting back or removing the leader of a young tree. Even in trees that form multiple leaders with age, encouraging a strong central leader when the tree is young results in a sturdier branch structure later.
Select the lowest branches to be kept permanently. The permanent lateral scaffolds, the principal branches of the tree, should arise from the central leader. Depending upon the height of the tree, none of the present branches may actually remain when the tree is mature; keep in mind that a tree adds height to its top as it grows and branches remain at the same level although they increase in diameter.
Branches on most trees should not be so low that it is impossible to sit or walk beneath them. The lowest limbs on trees overhanging a driveway or street must allow clearance for a delivery truck.
The best branches to select for main scaffolds emerge from the trunk at an angle greater than 45 degrees. These spreading lateral branches are strongly attached to the trunk with connective wood and have desirably small branch bark ridges, which are naturally weak areas on a tree.
As branches with angles narrower than 45 degrees grow larger, the bark of the trunk and branch becomes sandwiched into the crotch, forming a large branch bark ridge and a weak attachment. As these branches grow heavy with age, the angle begins to spread and the crotch can split apart at the branch bark ridge in windstorms. With the exception of trees that have distinctly narrow and upright habits like the Lombardy poplar or columnar cultivars of maples, remove or subdue any potential scaffolds with narrow angles of attachment.
Strive to keep the vertical distance between scaffolds 10 to 24 inches apart in young trees. Large trees naturally may assume several feet between scaffolds. Select branches that radiate in alternating positions to avoid shading those positioned below and to eliminate competition for food and water. An ideal pattern is five to seven branches emerging from the trunk in an ascending circle that rotates once or twice. Nature is rarely this orderly so plan ahead and mark branches to keep and branches to subdue or cut out as the tree grows.
As the tree matures and begins to express its characteristic branching pattern, it assumes one of two basic forms: central leader with a pyramidal shape and one main trunk or multiple leader with trunks equaling or exceeding the main leader.
On newly planted or young trees, do not remove all the branches below any permanent scaffolds. Leave these temporarily to protect and shade the trunk and contribute to food production. Maintaining these low branches for several years adds to the width of the base of the trunk and encourages the well-defined upward taper of a strong and flexible tree.
Although these branches aren’t removed, their growth should be subdued. Continue to do this every year until the branches can be removed entirely. A good time to eliminate these temporary branches is when their diameter reaches about 1/2 inch. A cut this size closes in about one year.
When pruning small branches with a pruning saw, hold the branch in one hand and use the other to cut along the slanting outside edge of the collar. Make saw cuts with an upstroke to avoid injuring bark in the crotch between the trunk and branch.
Corrective Pruning of Lateral Branches
Whenever the trunk or a branch forks, the main branch should have a larger diameter than the other or the crotch may split. If the diameter of a side branch is equal to or greater than the diameter of the main trunk or parent branch, subdue its growth by pruning it back with a thinning cut by one third of its total length.
Training young and recently transplanted trees will take several years of light pruning. The tree benefits when a branch is removed gradually over several seasons rather than being cut off abruptly. Most branching problems in older trees are due to lack of proper pruning when the tree was young.