When done properly, pruning improves the safety, health and beauty of a street tree. Friends of the Urban Forest prunes young street trees for three years after they have been planted. After that, the tree is considered “established,” and the city of San Francisco takes over responsibility for pruning it. You can find more information about StreetTreeSF, the city’s street tree maintenance program, here.
If you want to prune an established street tree yourself, or hire an arborist to prune it, you must obtain permission from the Bureau of Urban Forestry (part of San Francisco Public Works). You can call them at (415) 554-6700 or email them at email@example.com.
If you prune a tree, there are three main types of cuts you can make:
A thinning cut is the removal of a smaller branch from a larger one. The idea is to cut close to the main limb without making a flush cut or leaving a stub. Look for the branch collar, the swollen area where the smaller branch meets the larger one, and cut just to the outside of it.
Reduction or re-leadering cuts
A reduction cut shortens a limb to a lateral branch that’s at least 1/3 the diameter of the shortened limb; this causes the lateral branch to grow more vigorously. Don’t cut too close to the lateral branch, but also don’t leave a stub (an excessive amount of the shortened limb).
A heading cut shortens a branch back to a bud. This should be a cut of last resort! A correct pruning cut has its lower point even with the top of a growth bud and slants upward at about a 45° angle.
To make a proper close pruning cut, hold the pruning shears with the blade closest to the growth that will remain on the plant. A stub results when you reverse the position and place the hook closest to the plant.
For larger limbs, use three cuts. The first is an “undercut” to keep the bark from peeling down the trunk; cut beneath the branch, a third of the way to half-way through it. Make the second cut beyond the first cut, all the way through, to remove the weight of the limb. The third is your finish cut. Here you must guide your saw just to the outside of the branch collar (ridges at the limb’s base) with clean strokes so the final result is smooth.
“Topping” is removing more than 30% of your tree’s foliage at any one time, cutting or removing the tallest branch, or cutting or removing the ends of all or most of the branches. Topping will severely weaken your tree, and may even kill it. It will result in a high-maintenance, expensive, dangerous and potentially ugly tree. It’s illegal in San Francisco, and is punishable by fine. Do not top your tree!
Here’s more information about tree topping, and about how to report it.
It’s OK for tree branches to be in and around telephone wires and low voltage wires as long as there is no stress or undue abrasion. Begin training the young branches to go out and around these wires. Avoid large pruning cuts.
Want to learn more? FUF leads free, hands-on pruning workshops.