For an 8-inch DBH tree, the critical root zone radius is how many feet?

• A. 10 ft
• B. 12 ft
• C. 14 ft
• D. 16 ft

Answer: (B)

The concept here is estimating the critical root zone radius from the tree’s diameter at breast height (DBH). In Maryland practice, the standard rule is that the critical root zone radius equals 1.5 feet for every inch of DBH, so the radius in feet is 1.5 × DBH (in inches). For an 8-inch DBH, that’s 8 × 1.5 = 12 feet. This zone identifies the area around the trunk that should be protected from heavy disturbance because it contains key roots that help anchor the tree and supply water and nutrients. The other numbers would come from using different multipliers, but 1.5 per inch is the accepted guideline for this calculation.