What is the Critical Root Zone of a Tree?

Knowing where your tree’s roots are located and how far they extend from the trunk is vitally important to their proper care and maintenance. The roots not only absorb water and nutrients from the soil, but they also store carbohydrates. In short, healthy trees have healthy, undisturbed root systems.

The Root System of a Tree

The root system of a tree can be divided up into zones: the Total Root Zone and the Critical Root Zone.

The Total Root Zone area is the total soil area where a tree’s root system extends out from the trunk. Care and concern for the roots throughout this zone should be considered. While damage or disturbance to the roots outside of the Critical Root Zone might not cause critical disease to the tree, the health of the tree will be impacted.

The Critical Root Zone (CRZ) is located within the Total Root Zone. It includes a significant amount of the tree’s moisture and nutrient gathering roots grow. Depending on the soil conditions and the species of the tree, the roots may grow just a foot below the surface.

How To Calculate Your Tree’s Total Root Zone and CRZ

Total Root Zone

1. Measure the circumference of the tree at about chest height, using inches.
2. Divide the circumference of the tree by π (3.14) to get the “diameter at breast height” (DBH). DBH = C ÷ π
3. Multiply the DBH by 18 inches = Total Tree Root Zone.
4. From the trunk of the walk out, in any direction, by the number calculated in Step 3.

Example calculation:

1. Trunk circumference = 32 inches
2. Trunk diameter (DBH) = 32 ÷ π = 10.19
3. 10.19 x 18 inches = 183.42 inches (15.29 feet)

Critical Root Zone

1. Circumference of the tree at chest height, using inches.
2. Calculate the DBH. DBH = C ÷ π (3.14)
3. DBH x 15.6 inches = Critical Root Zone
4. From the trunk of the walk out, in any direction, by the number calculated in Step 3.

Critical Root Zone Care

To ensure a tree’s growth and vitality, it is critical that great care be taken with the area known as the Critical Root Zone. Avoiding cutting, nicking, overwatering, underwatering, as well as soil compaction to the roots. (Driving vehicles or heavy equipment over a tree’s Critical Root Zone can be very harmful to a tree’s health.)

Keeping the Critical Root Zone covered with an adequate amount of mulch can help ensure the soil remains moist and free of disease and pests. The slow breakdown of locally sourced mulch will also help supply the tree with needed nutrients.

Spreading compost over the Critical Root Zone is highly recommended. This adds nutrients back into the soil which have been removed by construction and development in the suburban environment. Compost can give your tree an extra vitality boost!