Mulch a Tree's Critical Root Zone

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Mulch Your Tree's Critical Root Zone

Critical Root Zone
Critical Root Zone. Athens-Clarke County Community Tree Program, Georgia

The Critical Root Zone of a Tree

Your tree critical root zone (CRZ) should be protected and mulch should be considered just as important for tree health as fertilization, protection from pests and trunk damage. Vertical mulch along the edge of the CRZ is one way to insure tree health and well worth the investment in time and effort. I will explain vertical mulching in detail after defining the critical root zone of your tree.

The CRZ of a tree, which is also called the “tree protection zone”, is often defined as an imaginary circle on the ground that corresponds with the “dripline” of the tree. The dripline of a tree is where the greatest extent of a tree’s branches end. For narrow-crowned trees the dripline distance should be expanded to insure that the critical tree roots will be protected.

A more effective way to define your tree's critical root zone is to calculate a circle on the ground below the tree that has a radius equivalent to 1.5 feet for every inch in trunk diameter. A tree with a trunk diameter of 12 inches has a critical root zone radius of 18 feet.

The trunk diameter of a tree should be measured at 4.5 feet above the ground. You can estimate this diameter, or calculate it by first measuring the circumference of the tree with a tape, then dividing by 3.14 (a constant known as pi). For example, a circumference of 36 inches is roughly equivalent to a diameter of 12 inches.

How to Vertically Mulch a Tree

Vertical mulching is an excellent technique used to partially alleviate soil compaction within the critical root zones of trees. Soil compaction is harmful as it reduces the amount of pore space in the soil normally filled by oxygen (micro-pores) and water (macro-pores).

Vertical mulching will also lessen damage due to excessive water, preserve necessary aeration during wet periods, allow sub-soil water penetration during dry periods, and promote the formation of fine feeder roots.

To vertically mulch your tree, power auger or drill two inch wide, 18" deep holes in the soil on 12-20" centers under the affected trees starting at about 8 feet from the trunk and out to the drip line of the branches (where fine feeder roots are located). This area is also called the edge of the critical root zone. If you hit a large root, just move over and complete the hole. You should have a grid of holes toward the outer edge of the CRZ.

Fill holes with a mixture of pea gravel, or sand, or a mixture of compost with pea gravel or sand. A 3-4 inch layer of good quality organic mulch, such as aged wood chips, leaves, compost, or pine straw spread evenly over the entire critical root zone will also help trees substantially.