Branch Collar
The branch collar is the shoulder or bulge at the base of a branch, formed by overlapping layers of branch and stem tissue that grow each year. The branch bark ridge is the ridge of bark in the branch crotch, formed as stem and branch tissues grow against one another. Together they mark where branch tissue ends and trunk tissue begins.
Source: How to Prune Trees (NA-FR-01-95, revised 2012), USDA Forest Service, Northeastern Area State and Private Forestry
Where the final cut goes
Pruning cuts should remove only branch tissue and leave stem tissue undamaged. Clemson Extension describes the final cut as beginning outside the branch bark ridge and ending just outside the branch collar swelling on the underside, angled down and away from the trunk.
For a branch too heavy to hold, the Forest Service guide uses three cuts to avoid bark ripping: a shallow notch on the underside outside the collar, a second cut farther out that takes the branch off and leaves a short stub, then the final cut just outside the branch bark ridge and collar.
Sources: How to Prune Trees (NA-FR-01-95, revised 2012) ; Pruning Trees
When there is no collar
A collar forms only when the side branch is clearly smaller than the trunk. Colorado State University Extension says that when a side branch is over half as thick as the trunk beside it, no collar or branch defense zone forms, so the final cut should be angled slightly further out to keep the wound smaller.
Codominant stems
Two or more upright leaders of near-equal size are codominant stems. Colorado State University Extension calls this a major structural defect, because no branch collar forms where codominant leaders join. Included bark often forms in them too; see included bark.
Source: Structural Pruning of Young Shade Trees (CMG GardenNotes #613)
How the ISA arborist exam tests it
The exam returns to the collar again and again: where the final cut starts and ends, what to do when no collar has formed, and why codominant unions are weak. Try 3 reviewed practice questions:
An arborist is making the final cut to remove a live branch that has a visible branch bark ridge and branch collar. Where should the cut begin and end?
Show answer and explanation
Correct answer: B. Begin outside the branch bark ridge and angle down and out to just past the collar
Clemson Extension describes the final cut as starting outside the branch bark ridge and ending just outside the branch collar swelling on the underside, angled down and away from the trunk. This removes only branch tissue and leaves the collar, which helps wall off decay, intact. A flush cut or a cut into the collar injures trunk tissue, while cutting farther out leaves a stub that delays wound closure.
- A: A flush cut removes the collar and injures trunk tissue, inviting decay.
- C: Cutting well out on the branch leaves a stub that delays closure.
- D: Cutting into the collar removes the tissue that helps resist decay.
Source: Principles & Practices for Pruning Trees (HGIC 1003), Clemson Cooperative Extension, Home & Garden Information Center
Report a problem with this question · ID prun-012
A side branch to be removed is more than half the diameter of the trunk, so no branch collar has formed at the union. According to Colorado State University Extension, how should the final removal cut be adjusted?
Show answer and explanation
Correct answer: D. Tilt the cut slightly farther out to keep the wound smaller
Colorado State University Extension says that when a union has no branch collar because the side branch has a diameter more than half that of the trunk beside it, the final cut should be angled slightly farther outward to keep the wound as small as possible. It cautions that a union without a collar also has no branch defense zone to trigger rapid woundwood growth and a strong reaction against decay, so keeping the wound small matters. A flush cut or a cut started inside the branch bark ridge enlarges the wound into the trunk, and a stub leaves tissue that must be closed over.
- A: Without a collar there is no branch defense zone, so a flush cut only makes a larger wound in the trunk.
- B: A stub does not substitute for a collar; the advice is a final cut angled slightly outward.
- C: Starting inside the branch bark ridge wounds trunk tissue; CSU advises angling the cut outward instead.
Sources: Pruning Cuts (CMG GardenNotes #612), Colorado State University Extension ; Pruning Cuts (CMG GardenNotes #612), Colorado State University Extension ; Pruning Cuts (CMG GardenNotes #612), Colorado State University Extension
Report a problem with this question · ID prun-048
Why is the union between two codominant stems of similar diameter structurally weaker than a union where the side branch is much smaller than the trunk?
Show answer and explanation
Correct answer: C. No branch collar forms between them, and included bark often develops
A branch collar, where the trunk's growth rings overlap the side branch's rings, forms only when the side branch is clearly smaller than the trunk. Colorado State University Extension notes that where codominant leaders join, no branch collar develops, and the union is prone to included bark, which keeps the wood of the two stems from knitting together. Purdue Extension pictures a codominant union as a V, like a slingshot, so its angle is narrow rather than wide.
- A: Growth rate and wood density are not what weakens codominant unions.
- B: Codominant stems typically form narrow, V-shaped unions, not wide ones.
- D: Shading is not the reason codominant unions are weak.
Sources: Structural Pruning of Young Shade Trees (CMG GardenNotes #613), Colorado State University Extension ; Corrective Pruning for Deciduous Trees (FNR-563-W), Purdue Extension
Report a problem with this question · ID prun-018
0 of 3 correct
Common mistakes in the field
Flush cuts
Cuts that start inside the branch bark ridge or the collar injure stem tissue and can lead to decay.
Leaving stubs
Stub cuts delay wound closure and can let in canker fungi that kill the cambium.
Bark ripping
Cutting a heavy limb in one pass lets it tear the bark below the union. Use the three-cut sequence.
Sources
- How to Prune Trees (NA-FR-01-95, revised 2012), USDA Forest Service, Northeastern Area State and Private Forestry
- Pruning Trees, Clemson Cooperative Extension, Home & Garden Information Center
- Pruning Cuts (CMG GardenNotes #612), Colorado State University Extension
- Structural Pruning of Young Shade Trees (CMG GardenNotes #613), Colorado State University Extension
Written in our own words from the public sources above, checked October 19, 2026. Arborist Practice Test is not affiliated with ISA.