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root zone

Determining Critical Root Zone


When doing any type of construction work around existing trees, an area protecting the root area of the tree from damage should be determined and fenced off for protection. Determine the amount of space needed for the tree protection area. This is done by calculating the critical root zone of individual and groups of trees.

A measurement of the trunk should be taken at diameter breast height (DBH) or 4.5 feet above the ground. Then measuring outwards from the trunk 1.25 feet for every inch of trunk diameter, the resulting area is the critical root zone. For example, a 20-inch diameter tree would require a 30-foot area of protection around the tree for the critical root zone. This is a minimum area of protection, but a larger area is preferred. A minimum of 6 feet must be protected around trees regardless of diameter.