Many trees thrive well without detailed attention to their health. But there are economic and other specific tree types that need special attention. Therefore, mulching is critical to their health and wellness, sometimes as important as fertilizing and watering them. However, you cannot mulch a tree without knowing its critical root zone.

To find the critical root zone (CRZ) radius, measure its circumference. The radius increases by one foot, six inches for each tree trunk. When you get the circumference, divide it by pie 3.14, which gives you the trunk’s diameter. Next, multiply the diameter by one foot, six inches, or 18 inches, and you get the critical root zone.

This article provides details on measuring and getting a tree’s correct critical root zone.

The critical root zone of a tree, also called the root protection zone, is an imaginary circle around the tree that aligns with its drip line. The branches and leaves typically spread wide, and the wider the branches, the bigger the tree. This fact means the drip line should be broader but also means the tree’s circumference should be more extensive.

However, the drip line of a tree is hardly ever regular. You are more likely to find an irregular drip line, making its measurement hard to determine. Otherwise, it would be easier to check the drip line to determine the critical root zone without measuring the tree. Nevertheless, there is a method to get an accurate measurement.

Getting the correct tree circumference is critical if you want to check where the zone starts and ends for a tree. Start your measurement at four feet off the ground to get the correct measurement.

Measuring the critical root zone

Here is how you can measure the critical root zone of a tree.

Suppose the tree measures 60 inches in circumference, and you need the CRZ. Divide 60 by 3.14, which gives you 19.1. The diameter of that tree is 19.1. Since the radius of the tree increases by 1.5 feet for each inch of the tree trunk, multiply the diameter by the radius. You can convert the radius to inches, which means 1.5 feet equals 18 inches.

For this example, the result of multiplying 19.1 by 18 inches should be 343.8 inches. Converted to feet, it should be 28.65 feet. So the critical root zone for this tree is 28 feet 8 inches.

The next step is to measure 28 feet and 8 inches in four directions – north, south, east, and west – going out from the tree trunk. It does not matter if the measurement does not cover the end of the roots; some trees spread their roots wider than the critical root zone.

Once you get the correct measurement in the four directions, stop measuring and connect the points to make a circle. You can make a mental picture or opt to draw it for a better reference. The circle acts as a boundary for the CRZ so that you can accurately and adequately mulch the area.

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Factors to consider

What is the importance of a critical root zone? When an area is getting developed, there may be a need to cut tree roots. Since the roots provide the life source for the tree, you may be killing the tree if you cut the vital parts of the roots.

Even though some roots spread wide, there is a point where you may tamper with them without killing the tree. This point is where the best practices of dealing with critical root zone comes into play.

Root system

Every tree has a differing root system. Some spread wider than others. For example, the roots of an oak tree can spread three times wider than the critical root zone. Some arborists tell you that some trees can spread up to seven times wider than their crown.

Tree size and age

The size of a tree does not always determine the spread of its root. The same is true for its age. A mature tree may have a drip line closer to the critical root zone than a younger tree. It is always recommended to check the spread of the roots of a tree, regardless of the size of the tree.


When the soil is rich and accommodating, the roots of a tree tend to spread wider than you may anticipate. They can be opportunistic, so you may find a tree with roots becoming fibrous and numerous if the soil is moist, uncompacted, and fertile.

However, the roots may not be numerous if the soil is compacted and dry, with little or no organic matter. But that usually means they have more space to grow larger and extend far from the drip line.


Trees you plant on soil with bedrock or a high water table may spread their roots wide without developing deeper in the soil. The restriction pushes the roots to find other ways to develop, causing them to extend several feet in different directions.

Additionally, growing trees too close together leads to a narrow root system. Such trees may graft with the roots of other trees of their kind. While it may look like a disadvantage, root-grafting encourages sharing nutrients, reducing stress on the trees, and stabilizing each other.

However, if you plant them in the open with adequate spaces around them, the roots spread wider.

Protecting and treating a tree’s critical root zone

If you have a lot of trees vital to maintaining the environment, you may be interested in finding the critical root zone. And when you find it, the next step is treating and protecting the trees. Each tree will have a different critical root zone, but they are easy to find using the simple method in this article.

Mulching is an excellent way to ensure the trees receive the nourishment they need, and vertical mulching is one method. Vertical mulching is ideal for the soil within the critical root zone because of soil hydration and aeration.

If soil is too compacted, it cannot get the adequate amount of water, air, and nutrients it needs to thrive. As a result, any plant or tree on that soil may not perform well because the soil is not equipped to handle it.

How to mulch

Vertical mulching keeps the tree from getting damaged due to too much water, promotes deep soil hydration in drought, and encourages feeder roots formation. To mulch the critical root zone around the tree, appropriately drill several holes.

Each hole should be about 18 inches deep and 2 inches wide. Go about eight feet from the tree trunk and slowly make the holes out toward the edge of the critical root zone.

When you have these holes, the next step is to fill them with a mixture of sand and compost or pea gravel. Finally, get quality mulch and spread some of it over the CRZ area.

Author: Ben McInerney is a qualified arborist with over 15 years of industry experience. He uses his in-depth knowledge of the tree service industry to give readers to most accurate information on tree service costs and helps to educate them about the benefits of using a certified arborist for tree trimming and removal work.