Xericoasis: Clearance Pruning Mature Trees

Back in the prehistory of the last millennium, Stacy and I headed down to Florida for our honeymoon. I snapped a shot of this southern live oak (Quercus virginiana) in St. Augustine. The ancient tree looks as if it's about scoop up and run off with my new bride.

Low, lateral branches like this are a kid's best friend. Which is why, a Boise forester explained to me, city trees are limbed up long before they can develop these low branches. A pity.

A pity too that upon occasion, mature trees need to be trimmed because of clearance issues. Better to take out just low limbs than an entire tree though, and better to perform the trimming in such a way as to minimize the stress on the tree.

A new driveway is scheduled to be installed right under the large, low limb of this ash. Rather than make the Airstream slated to be parked there do the limbo, the owners asked me to trim up the tree (arborists call this "raising the canopy").

Removing large limbs on a tree creates open wounds that make the tree more susceptible to disease and pests which can then lead to decay. It's critical that pruning cuts are performed correctly to expedite the healing process. This healing process is accomplished by a process that the late Dr. Alex Shigo called the "Compartmentalization Of Disease In Trees" (CODIT). Cells form walls around the wound, effectively quarantining the area to prevent the spread of decay to the rest of the tree.

So, where and how do you make the cut? Let's take a look.

Preparing for the first cut. (Yes. I will get off the branch first, smarty-pants.)

It's very easy to see the demarcation between the branch and the trunk of the tree in this photo. That line is called the branch bark ridge, and it's where the bark from the trunk joins the bark of the branch.

Nestled in the branch bark ridge is another area called the branch defense zone. Cells in this area are responsible for the growth of woundwood that will ultimately grow over the cut.

Woundwood callous is close to sealing off pruning cut

If the branch collar is damaged (often by flush cuts), the woundwood will not grow from the damaged area. This prolongs the tree's susceptibility to decay.

These branches weigh hundreds of pounds so the standard three cut process ain't gonna work. I trim the branch in small sections, working my way in from the tip until I have a piece small enough to hold. An undercut a quarter of the way up, and a final cut from the top (carefully trimming along the branch bark ridge), and viola! A clean cut.