Young or newly-planted trees require special care to ensure their establishment and rapid growth. Young trees must be protected from the careless operation of lawn mowers and weed- trimmers, from vandals and from harmful construction activities. They also must be given appropriate amounts of water and essential mineral elements (fertilizer) and may benefit from staking, trunk wrapping and mulching. But pruning may be the most important post-plant maintenance task to perform on young trees if they are to live up to our expectations. The time and expense invested in training a young tree will always be much less than costly and time-consuming corrective pruning of neglected mature trees.
Tree pruning must begin at time of planting. But avoid the temptation to "thin" a young tree's crown excessively. Research has shown that post-plant growth is more rapid and trees will establish sooner if pruning at planting time is limited to removing only weak, dead, diseased, rubbing or injured branches. By removing these obvious defects, young trees are able to use all available foliar resources needed to develop a strong root system and to overcome planting stress.
Also avoid removing the many small side branches that occur along the trunk. These lateral branches help the trunk increase in base diameter or caliper making for a sturdier tree. Laterals also shade the trunk, reducing the likelihood of sunscald injury and act as guards warding off equipment operators, animals and vandals.
After the young tree has become established in the landscape (usually one year after planting), pruning really becomes a job of "training". Two general concepts that will help guide the pruner are (1) training or pruning can take place progressively over the next three to five years and (2) no more pruning should take place in a single year than is needed to enhance the shape or structural strength of the tree.
When training a young tree, first identify those primary limbs (scaffold limbs) that will eventually make up the tree's framework. The height to the lowest scaffold limb will be determined partly by the anticipated activities that will occur under or near the tree. But remember, during this early development stage it is important to leave temporary scaffold limbs on the tree at lower heights than eventually will be desired. In general, two-thirds of the tree height should be left as crown (branches and leaves).
Select scaffold branches that have wide angles of attachment with the trunk, are spaced evenly and are distributed radially around the trunk. Optimally, major limbs should be spaced 24 to 36 inches apart on alternating sides of the stem. Trees such as maple and ash frequently have major branches occurring in pairs across the main stem. Pruning these alternately up to a height of 12 to 18 feet will create a structurally sound tree that is attractive and balanced. Never let one limb grow directly over a lower one.
Unless the tree has a natural multi-stemmed habit, it should be trained to a single central dominant leader. The central leader is the topmost vertical stem extending from the trunk. Prune back laterals that threaten to grow taller than the leader. Double or codominant leaders, if left unattended, can pose problems for trees as they age. One of the two stems (usually the weaker stem) should be removed to establish dominance.
As training continues in subsequent years, use other practices to promote a healthy and long-lasting tree. Laterals that have grown higher than the terminal leader or beyond the perimeter of the crown should be pruned back within the bounds of the tree. Remove any laterals that have grown inward toward the center of the crown back to their point of origin. And, remove structurally weak water sprouts and basal sprouts (suckers) to preserve the beauty and natural growth habit of the tree.
Finally, whenever removing branches back to the trunk, always cut just outside of the branch bark ridge thereby avoiding injury to the branch collar. Careful pruning will promote rapid wound closure and inhibit spread of decay in the trunk.
Every tree planted should be given a level of care that will enable it to initially survive and then prosper for many years to come. Pruning and training trees when they are young will help ensure good growth and long-term structural stability.
This article originally appeared in the March 24, 1995 issue, p. 28.