Pruning at the branch collar, and when to make the cut

We see more trees ruined by pruning than by neglect: a cut made flush because that looks tidy, a stub left because that looks safe, the work done in May because that is when the ladder came out. A tree cannot heal a cut the way skin heals. It can only wall the wound off and grow over it, and whether it succeeds depends on where the cut lands and when it is made.

Find the collar before you cut

On most trees there is a slight swelling at the base of a branch, the branch collar, where trunk wood and branch wood overlap each year. A raised line of rough bark runs down at an angle from the crotch: the branch bark ridge. Together they mark the boundary: inside is trunk tissue with a chemical protection zone that resists decay, outside is branch.

The correct cut starts just outside the branch bark ridge and angles down and slightly away from the trunk, ending just outside the collar. It removes nothing that belongs to the trunk, and callus grows from the collar’s rim inward until the wound is closed.

A flush cut removes the collar and its protection zone: the wound is larger, it exposes trunk wood, and decay moves into the stem. A stub, the opposite error, cannot close at all; the tree must wait for it to rot back to the collar, and rot often reaches the trunk first. This is the compartmentalization that lets a tree live for centuries with hollows and old wounds, and a collar cut works with it rather than against it.

Three cuts for anything the hand cannot hold

A branch heavier than your free hand can hold will tear as it falls, stripping bark down the trunk. The fix is three cuts:

1. Undercut. About a foot out from the collar, cut upward from the underside, a quarter to a third of the way through.
2. Drop cut. An inch or two farther out, cut down from the top until the branch falls. The tear stops at the undercut.
3. Finish cut. Remove the stub with one cut just outside the ridge and collar, supporting it as it comes free.

When to prune

For most trees the best time is the dormant season, from late winter until just before bud break. The structure is visible, wound closure is about to be at its fastest, and the insects and fungi that colonize fresh wounds are mostly inactive. Dead, broken or hazardous limbs can come off at any time.

The exceptions are worth knowing:

• Oaks where oak wilt occurs (much of the Midwest, Texas and scattered parts of the East). Sap beetles carry the fungus to fresh wounds within hours. Do not prune oaks from bud break through early summer; the usual advice is no cuts from April through July, and Texas starts the ban in February. If storm damage forces a cut in that window, it is the one case where a wound dressing is recommended, applied at once.
• Maples, birches, walnuts, elms and yellowwood. Cut in late winter, they bleed sap for weeks; harmless, but messy and alarming. Prune them in summer once the leaves have expanded, or in early winter before the sap runs.
• Spring-flowering trees. Dogwood, redbud, serviceberry, fringetree and magnolias set next year’s flower buds in summer, so a winter cut removes the bloom. If the flowers matter, prune right after they fade.

No paint, and not too much

Wound dressings, tar and paint were standard for a century, but they do not stop decay; they seal moisture against the wood and can slow closure. Leave the cut open to the air, except in the oak wilt case.

Take little. A young tree can lose up to about a quarter of its live crown in a year, and a mature tree far less, more like a tenth. Every live branch removed is food the tree no longer makes and a wound it must close, and old trees close wounds slowly; if a job needs more, spread it over several years.

Raising and thinning, never topping

Good pruning does three things: removes dead, damaged, crossing and rubbing wood; raises the crown by taking off the lowest limbs for clearance; and thins it by removing whole branches back to their origin so light and wind move through. Thinning does not mean stripping the interior and leaving tufts at the tips, which is the fastest way to break a limb.

Topping is cutting a trunk or major limb off at an arbitrary point, leaving a stub with no collar. The tree responds with a mob of weakly attached sprouts from dormant buds below the cut while the stub decays, so within a decade it is taller than before and far more likely to drop a limb. Where a tree is genuinely too big for its spot, the honest options are reduction cuts back to a lateral at least a third the diameter of the piece removed, or replacement with a species that fits.

Tools, and the point where you stop

Bypass pruners handle branches to about three quarters of an inch, loppers to an inch and a half, and a sharp pruning saw takes the rest. Anvil pruners crush live wood; keep them for dead stems. Sharp blades leave a clean surface; wipe them with rubbing alcohol between trees when fire blight or another disease is around.

Call an arborist for anything that needs a ladder and a saw together, any limb over a roof, a road or a place where people sit, anything near a power line (utility work, not homeowner work), and any big limb on an old tree, where the cut itself is a wound the tree may never close. Ask for certification from the International Society of Arboriculture, and walk away from anyone who offers to top the tree.