Too often, pruning is urgent damage control, or catch-up work on long-neglected trees. Ideally, for transplanted landscape trees, it is a long-term and low-intensity process beginning when the tree has begun to form branches.
All too commonly landscape trees have problems making them susceptible to structural failure that could have been prevented with a bit if thoughtful pruning early on.
Here are a few examples:
— Scaffold branches crowded too close together. This concentrates physical strain at one point, making the trunk and branches subject to breaking.
— Branches with a narrow “V” angle of attachment and “included bark,” i.e., branch bark pinched against trunk bark. It results in the tree wounding itself over time and branches failing at the weak attachment.
— High “branch aspect ratio”: Branches nearly equal to the trunk diameter have inherently weak attachment. The trunk cannot get a good grip on a branch that is nearly equal to it in diameter.
After viewing an International Society of Arboriculture video on “Young Tree Pruning,” I devised a mnemonic for the process — a fairly simple device, if slightly silly, for remembering the main objectives: the “CLPST” method.
It goes like this:
C is for “Clean.” Prune for cleaning to remove dead, broken, dying and diseased branches.
L is for “Leader.” Retain a central leader, or select a branch that will serve as the main central axis.
P is for the first “Permanent” branch, the lowest limb. Its height on the trunk, diameter in relation to trunk diameter, and orientation in the landscape are critical to the physical strength and function of the tree.
S is for “Scaffold” branches, the architecture of the tree. These emerge over a period of years as the tree grows from sapling to youth, and early maturity.
T is for “Temporary” branches: These might be too low on the trunk to serve as the first permanent branch, or too crowded to serve as scaffold branches, but retaining and controlling some of them can protect the trunk from sun injury or physical damage, and “feed” the trunk, helping to build strength.
To elaborate a bit: Cleaning is rarely necessary for a new tree from the nursery, but the tree should be examined for problems. This step should generally be the only pruning in the first year after planting.
While a straight central leader is classic for redwoods, many other conifers, and some of the broadleaf species like Liquidambar, trees having naturally rounded overall structure or multiple trunks will benefit in strength when pruning encourages strong, tapering main stems.
The first permanent branch elevation is critical for clearance where trees are adjacent to streets and walkways. Its compass orientation can be critical for the same reason and for aesthetics. The angle from which the tree is most often viewed should show off this crucial element of structure. Look at classic bonsai trees. They typically have a “front” with the first branch growing to one side, not the front or back.
Scaffold branches on small trees, like dogwoods, might already exist on a newly transplanted tree. But on large species, like oaks, scaffolds should be spaced farther apart and might not emerge from the leader for many years after planting. Their diameter should be no larger than half the diameter of the leader/trunk at their point of attachment.
Temporary branches should also be less than half the diameter of the leader and pruned to keep them short. They can be removed gradually as the tree matures.
The CLPST system also applies to maturing branches, just as it does to the young tree. It can help pruners get our heads around an otherwise complicated, bewildering task.