Every year in early spring I spend a couple of Saturday mornings helping small groups of gardeners learn more about pruning shrubs and trees. If the snow is not too deep, we might focus on proper pruning of early flowering shrubs such as forsythia and spiraea. If these plants are still buried in snow, we concentrate on the formative pruning of young trees.
A few years back, we spent an hour working on a collection of Japanese tree lilacs in the parking lot of a local business, then left the pruning tools behind and took a walk down the main corridor of town, closely examining a line of street trees that had been planted the previous summer.
Our tour began with a ginkgo tree, its 3-inch-diameter trunk rising from its tiny planting hole for about three feet before the first branch. Within the next two feet, there were 12 branches spaced between 1 and 2 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, ginkgos and tree lilacs, all looked the same — too many closely spaced scaffold branches, a result of pre-pruning done in the nursery. In this industrywide practice, small, unbranched trees are headed back to encourage a proliferation of lateral branches. The problems associated with nursery pruning occur as the crowded branches grow larger in diameter, exerting pressure on each other while becoming weak and susceptible to breakage when loaded with ice or snow.
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.
Pre-pruning is done to sell trees. 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.
Pre-pruning is the standard in production; you are not likely to find trees that are otherwise. Your only recourse with a nursery-butchered tree is to do the necessary formative pruning to thin crowded branches, allowing the remaining scaffold limbs sufficient room to develop along the trunk.
Branches on large trees should be at least 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 crabapples, the space between branches should be at least six inches with a foot between branches on the same side of the trunk.
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 trees such as these street trees, I want to reach for the loppers immediately, do the formative pruning while the branches to be removed are still small, rather than making larger wounds down the road. Of course, this is the strongest argument for buying small trees that will establish in a single year, trees no more than one inch in diameter.
So, if you planted a nursery-pruned tree last year, now is a good time to perform the essential task of formative pruning. There is nothing gained, and much that could be lost, in waiting.
It is a sad fact that the average urban tree lives for only 10 years. There are a lot of reasons for this, chief among them putting large pre-pruned trees in small planting holes and never looking back.
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