Take good care of young trees

By Ed Perry

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I recently attended a tree care conference where the main topic was pruning. The general consensus among the arborists and other tree experts in attendance was that pruning may be the most important 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 always will be much less than the 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 growth is more rapid and trees become established sooner if pruning at planting time is limited to removing only weak, dead, diseased or injured branches, and to maintain a single central leader. 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. However, you should avoid removing small side branches that may occur along the trunk. These lateral branches help the trunk increase in diameter, making for a sturdier tree. They also shade the trunk, reducing the likelihood of sunburn injury, a serious problem for young trees in our hot Central Valley summers.

Young tree pruning really should be thought of as 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 (in general, no more that 25 percent of a young tree's canopy).

Unless your goal is to develop a multistemmed tree, such as for relatively small-growing species like crape myrtle, it should be trained to a single central dominant leader. The central leader is the topmost vertical stem extending from the trunk. Prune back upright lateral branches that threaten to grow taller than the leader. Double or co-dominant leaders, if left unattended, can pose serious structural problems for trees as they age. One of the two stems (usually the weaker stem) should be removed.

When training a young tree, first identify those primary branches (scaffold branches) that will eventually make up the tree's framework. The height to the lowest scaffold branch will be determined partly by the activities that will occur under or near the tree. But remember, during this early development stage it is important to leave temporary branches on the tree at lower heights than eventually will be desired.

Two-thirds of the tree height should be left as crown (branches and leaves). Select scaffold branches that are spaced evenly and are distributed radially around the trunk, like the spokes of a bicycle wheel. Large trees should have branches spaced 18 inches apart, while smaller trees' branches should be spaced 12 inches apart.

As training continues in subsequent years, use other practices to promote a healthy and long-lasting tree. Lateral branches that have outgrown the central leader should be reduced in size or pruned out entirely. Remove any structurally weak branches, water sprouts and basal sprouts (suckers). Finally, whenever removing branches back to the trunk, make the cuts just outside the branch collar (a "shoulder" or bulge at the base of the branch) The branch collar contains trunk tissue that should not be damaged or removed.

Careful pruning will promote rapid wound closure and inhibit spread of decay in the trunk.

Ed Perry, a farm adviser with the UC Cooperative Extension, can be contacted at 525-6800 or ejperry@ucdavis.edu.