Protecting Your Trees Roots

There is a common belief that a trees root system is reflected by its crown above. Unfortunately this is not true. So if you are looking to dig around your tree, whether it is to plant other plants or even to build something such as a house or garage, its best to understand how a trees root systems work so that you can protect your tree's health.

There are many ways a tree can be damaged, the less obvious is the damage that occurs underground. Roots are vital to a tree's health as they are responsible for its nutrients and water uptake, they help the tree store energy, and they anchor the tree. Trees are never the same shape below ground as they are above, so it is difficult to predict the length or location of a tree's roots. The rule of thumb is that approximately 90-95 percent of a tree's root system is in the top three feet of soil, and more than half is in the top one foot. If you are planning to disturb the ground around a tree, the part of this root system in which construction damage should be avoided is called the Protected Root Zone (PRZ).

One common method used to identify the tree's PRZ is to define it as the "dripline"--the area directly below the branches of the tree. However, many roots extend beyond the longest branches a distance equal to two or more times the height of the tree. For this reason you should protect as much of the area beyond the dripline as possible.

While the above mentioned is ideal, unfortunately, in most places, space around a tree, especially on construction sites, is limited. In these situations, the rule of thumb sometimes has to be bent. When this occurs, one then has to evaluate how close to the roots one can dig without threatening the tree. The good news is that healthy trees can survive after losing 50 percent of their roots. However, other species are extremely sensitive to root cutting, even outside the dripline.

When digging, if possible, try not to disturb no more than 25 percent of the roots within the dripline for any tree. When dealing with trees that have been growing in the forest or that naturally have a narrow growth habit, an approach called the "critical root radius" is more accurate than the dripline method for determining the PRZ. This is particularly true for columnar trees and for those where competition has reduced the canopy spread.

To calculate critical root radius, begin by measuring the diameter at breast height (dbh). This is done by measuring the tree's trunk diameter (thickness) at a point 4.5 feet above the ground. The measurement should be done in inches. For each inch of dbh, allow for 1.5 feet of critical root radius for sensitive trees, or 1.0 feet for tolerant trees. For example, if a tree's dbh is 10 inches, then its critical root radius is 15 feet (10 x 1.5 = 15). The PRZ is an area around the tree with a diameter of 30 feet (2 x radius), and is the area in which a critical amount of the tree's roots may be found.

If construction is to occur in an area with numerous trees, it may be necessary to remove certain trees and decide which ones will be protected. In this case, develop a landscape protection plan. Obtain a set of site development plans that include where proposed buildings will be placed or even sidewalks and utility lines. Work with the builder on which trees can be saved and protected. Keep in mind that the builder can possibly shift locations of buildings slightly if there are certain trees you are set on protecting.

There are many ways to protect your tree's root systems when working around them, the first requires evaluating and understanding how a tree grows and its root sensitivity before putting a shovel into the ground.

Andrew Johnson is the owner of Central Texas Tree Care, a leading tree service provider in Central Texas (Travis County and surrounding areas) offering services such as pruning and removals, cabling and bracing as well as arborist reports, diagnostics, pest management, fertilization and Austin tree service trusts. For more information please visit http://www.centraltexastreecare.com.