What is the radius of the critical root zone around a tree for every inch of its diameter at breast height?

• A. 1 ft
• B. 1.5 ft
• C. 2 ft
• D. 2.5 ft

Answer: (B)

The critical root zone is defined as a radius that grows with the tree’s trunk size: you use 1.5 feet of radius for every inch of diameter at breast height (DBH). DBH is measured 4.5 feet above the ground. So the radius equals 1.5 times the DBH in inches. For example, a tree with 12 inches DBH has a CRZ radius of 18 feet, and an 8-inch DBH tree has a 12-foot CRZ radius. This rule—1.5 feet of radius per inch of DBH—explains why the best answer is 1.5 feet per inch of DBH.