The size of the critical root zone depends upon the size of the tree- larger trees have larger critical root zones. To calculate the size of the critical root zone (measured in feet), measure the diameter of the tree in inches at 4.5 feet from the ground and multiply it by two. A trees root system ranges well beyond the dripline. The critical root zone (CRZ) has been established to set a practical limit beyond which any loss of roots would not have a significant impact on a trees survival. Certain conditions may require larger critical root zones to expect tree survival.
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