Corrective pruning can help young trees thrive

With planting season just ahead, this week’s column offers pruning advice to the gardener planting a new tree. Once you have it in the ground, immediately correct the problem created in the nursery by pre-pruning.

Pre-pruning is an industrywide practice in which small trees with widely spaced branches, the kind of tree you should be able to buy, are headed back during production in order to encourage an unnatural proliferation of lateral branches. Problems occur as the resulting crowded branches grow larger in diameter, exerting pressure on one another while becoming weak and susceptible to breakage when loaded with ice or snow.

Most people, unaware of the future problems associated with such closely spaced branches, will select a young tree with crowded branches over one with fewer widely spaced branches. But while scaffold branches two inches apart may look nice in miniature, they are going to be overcrowded and poorly anchored after they become a foot thick.

Your only recourse with a nursery-butchered tree is to do the necessary corrective pruning, thinning crowded branches to allow the remaining scaffold limbs sufficient room to develop along the trunk. The scaffold branches on large trees should be between 12 and 18 inches apart in a spiral arrangement up the trunk with about three feet between branches on the same side of the trunk. For smaller tree species, such as crab apples and Japanese tree lilacs, the space between branches on the same side of the trunk should be between 12 and 18 inches.

When should this formative pruning be done? I normally recommend little pruning during the establishment period of a tree, defined as one year after planting for every inch of trunk diameter. This delay in pruning allows for retention of maximum foliage to nourish the developing root system. But when I see the crowded canopies of nursery-pruned trees, I reach for the loppers immediately and do the formative pruning while the branches to be removed are still small rather than making larger pruning wounds down the road.

I spent last Saturday morning helping a group of adult students learn more about the formative pruning of young trees. We worked on a row of Japanese tree lilacs planted along a city street two years ago. Each 3-inch-diameter trunk rose from its tiny planting hole for about 3 feet before the first branch. Within the next 3 feet, there were a dozen or more branches spaced 2 or 3 inches apart, some already crossing and rubbing over others, others making a narrow angle with the trunk, growing up rather than out.

Tree after tree looked the same, too many closely spaced scaffold branches, the result of pre-pruning in the nursery. I asked the students to think down the road a few years, to imagine each branch increasing in diameter, reminding them that the space between branches remains the same as the tree grows. Surely something would have to give!

In two hours, 20 pruners improved the future health of 10 trees, carefully selecting each branch to be removed with a thinning cut just forward of the branch bark ridge, the point where the branch exits the main trunk. In the end, half of the branches from each tree lay on the ground, an indication of the excessive branching caused by nursery pruning.

It is a sad fact that the life expectancy for the average street tree is only seven to 10 years. There are a lot of reasons for this, chief among them the planting of nursery-pruned trees without the necessary corrective pruning.

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