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Fir Tree Root Cutting for Retaining Wall: Damage Assessment

WA State: Building wall near 40–50 ft firs. Cut 12" circumference root (thigh-sized) for trench. More roots likely. Kill trees?

Root Cutting Threshold

≤25% roots = usually safe. Root zone = 1.5× canopy radius. Cutting "several feet" from trunk on large firs = high percentage. Foliage dieback will reflect loss.

Compounding Factor

Retaining wall changes drainage → alters water availability.

Action

Water remaining root zone during dry weather. Monitor. Outcome uncertain.

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