Critical Root Zone (CRZ)

The CRZ of a tree, also called the “tree protection zone”, is often defined as an imaginary circle on the ground that corresponds with the “dripline” of the tree.  However, the dripline is very irregular and misleading, so you refer to the trunk diameter.

To determine a CRZ:
– Measure tree diameter 4.5 feet above grade
– Multiply this diameter by 12 inches

Keep in mind that while this is a generally accepted method for measuring CRZ, root systems will vary in depth and spread based on size of tree, soil quality, water table, species, and other related factors.

The above CRZ drawing also defines the “Perimeter CRZ” (PCRZ) and “Interior CRZ” (ICRZ).  Generally, the full PCRZ is considered the optimum amount of root protection for a tree.  (The ICRZ is identified as the inner half of the CRZ radius.)  As root impact occurs within the PCRZ, greater post care will be required for the tree to remain alive and stable.  The absolute maximum disturbance allowed must still leave the ICRZ undisturbed if the tree is to have any chance of survival.  If the root impact were this severe, the tree would then need to be treated and maintained as a transplanted tree.  In this case, the tree would require extensive post care, including but not limited to; regular irrigation, misting, root treatment with special root hormones, mulching, guying, and monitoring for several years.