Crown Reduction and Thinning Procedures

Why this matters

Crown reduction and crown thinning are two of the most common arboricultural procedures, and they're frequently confused (or done poorly). Crown reduction reduces the overall size of the tree by shortening the longest branches; crown thinning selectively removes branches to reduce density without changing the overall shape. Each has specific applications, technical requirements per ANSI A300, and consequences for tree health when done improperly.

When to use each

Crown reduction:

• Tree has outgrown its location
• Storm damage or risk of breakage
• Distance from structures, utilities, or property lines
• Customer aesthetic preference (subordinate to tree health)

Crown thinning:

• Wind resistance reduction (dense crown catches more wind)
• Light penetration for understory plants
• Reducing weight on long limbs
• Improving aesthetic without size reduction

ANSI A300 standards

The ANSI A300 (Part 1) Pruning standard establishes:

• Pruning cuts must be made at the branch collar (not flush, not inside the collar)
• No more than 25% of foliage removed in a single operation
• No topping (the indiscriminate cutting of branch tips)
• Specific cut techniques: drop crotch reduction, thinning cut, etc.

These are not suggestions; they're the professional standard. Customer's tree care should follow them.

Crown reduction procedure

The goal: reduce overall height/spread while maintaining tree health.

Step 1: Identify the target reduction

1. How much height / spread reduction does the customer want?
2. Is the requested reduction achievable without harming the tree?
3. ANSI A300 limit: no more than 25% reduction in a single year

Step 2: Identify target branches

The longest branches typically:

1. Reach to (or beyond) the desired size
2. Are candidates for reduction cuts

Step 3: Make drop-crotch reduction cuts

For each target branch:

1. Identify the largest lateral branch growing from the target branch
2. Cut the target branch BACK to the lateral branch
3. The cut is just above the lateral (similar to a thinning cut)
4. The lateral becomes the new "tip" of that branch

This is the "drop crotch" technique. Each major branch is shortened by cutting back to a lateral.

Step 4: Verify the cut

The cut should:

• Be just outside the branch collar of the parent branch
• Not damage the lateral branch
• Be smooth (clean cut, no tearing)
• Properly sized relative to the tree

Step 5: Repeat for additional branches

Walk the tree, identify branches needing reduction, make drop-crotch cuts.

Step 6: Step back and assess

Verify the tree's overall shape after reduction:

• Should look natural (not square, not stub)
• The reduced branches should follow the lateral's growth pattern
• The tree should look smaller but well-formed

If the result looks unnatural or "lollipop" shaped, the cuts were excessive or in the wrong locations.

Crown thinning procedure

The goal: reduce density while maintaining size.

Step 1: Identify the level of thinning

1. Light thinning: remove 10-15% of foliage
2. Moderate thinning: remove 20-25% of foliage (ANSI A300 limit per year)
3. Heavy thinning: NOT recommended (exceeds A300)

Step 2: Identify target branches

Look for:

• Branches growing inward toward the trunk
• Branches crossing or rubbing against others
• Dead, diseased, or damaged branches
• Suckers / water sprouts (vigorous vertical shoots)
• Lower branches that aren't producing leaves

Step 3: Make thinning cuts

For each target branch:

1. Cut at the branch collar (the swelling at the base of the branch)
2. Three-cut method for larger branches:

• First cut: undercut a foot from the trunk
• Second cut: top cut beyond the first cut
• Third cut: clean cut at the branch collar

1. The three-cut method prevents bark tearing

Step 4: Verify the cuts

The cuts should:

• Be flush with the branch collar (NOT flush with the trunk)
• Leave the branch collar intact
• Be clean (no tearing)
• Be sealed by the tree's natural compartmentalization (don't apply sealant)

Step 5: Step back and assess

After thinning:

• The tree should look less dense but still complete
• Should still have a natural shape
• Should NOT look "stripped" or denuded
• Customer should approve the result before more cuts are made

What NOT to do

Don't top trees

Topping (cutting back the tree's main trunk OR major branches without regard for laterals):

• Violates ANSI A300
• Creates stubs that decay
• Promotes vigorous regrowth that becomes weakly attached
• Results in long-term tree failure
• Considered unprofessional in the trade

Don't lions-tail (over-thin lower branches)

Removing all lower branches leaves the foliage only at the top (like a lion's tail):

• The remaining limbs are heavier (whip in wind)
• Increases breakage risk
• Stunts the tree's growth in the lower trunk

Don't violate 25% removal limit

ANSI A300 limits removal to 25% in any 12-month period:

• Mature, healthy trees: 25% maximum
• Stressed or declining trees: less
• Newly planted trees: minimal (only structural correction)

Removing more than 25% stresses the tree significantly; multiple years may be needed for substantial reduction.

Branch collar identification

The branch collar:

• Slight swelling at the base of a branch where it meets the parent
• Contains specialized tissue that compartmentalizes (seals) the cut
• Critical to cut just outside the collar, NOT through it
• Cutting through the collar prevents the tree from sealing the wound

Field test: feel the base of the branch where it meets the trunk. There should be a visible swelling or different tissue. Cut just outside this swelling.

Cut technique

For larger branches (over 1.5 inches in diameter):

Three-cut method:

1. Undercut: 1/4 inch deep cut, 12 inches from the trunk, on the underside
2. Top cut: cut from the top, 1 inch beyond the undercut, removing the branch weight
3. Final cut: at the branch collar, clean cut

This prevents bark tearing that would occur if the branch fell during cutting.

For smaller branches:

• Single clean cut at the branch collar
• Sharp pruning saw or chainsaw appropriate for the size

When to use each tool

Each tool has limits; use the right tool for the size.

Tree species considerations

Different trees respond differently to pruning:

References

• ANSI A300 (Part 1) - Pruning
• ANSI Z133 (Safety Requirements)
• ISA (International Society of Arboriculture) Best Management Practices for Tree Pruning
• USDA Forest Service tree pruning publications
• Manuall internal: Tree Pruning Service SOP, Chainsaw Safety, Tree Species Guide