Blog 2: Trees, Roots, Water, Air

This is an excerpt of CSU Extension Fact Sheet No. 2.926, written by J.M. Sillick and W.R. Jacobi. We thank them for this information.

Quick Facts

Most tree roots are located in the top 6 to 24 inches of the soil and occupy an area two to four times the diameter of the tree crown.

Roots obtain water, oxygen and minerals from soil. They do not grow toward anything or in any particular direction.

Soil compaction, change in soil depth and improper watering can injure roots, increasing stress and susceptibility to disease and insects.

To avoid root disease, maintain a healthy, vigorous environment around a tree. Once a root system is severely affected, the tree usually must be removed.

The root system of a tree performs many vital functions. In winter, it is a store-house for essential food reserves needed by the tree to produce spring foliage. Roots absorb and transport water and minerals from the soil to the rest of the tree. Roots also anchor the portion of the tree above ground. It is important to keep the portion above ground healthy to ensure an adequate food supply for the roots to continue their important functions.

Where Roots Grow

Tree root systems consist of large perennial roots and smaller, short-lived feeder roots. The large, woody tree roots and their primary branches increase in size and grow horizontally. They are predominantly located in the top 6 to 24 inches of the soil and occasionally can grow deeper 3 to 7 feet if soil conditions allow. Root functions include water and mineral conduction, food and water storage, and anchorage.

In contrast, feeder roots, although averaging only 1/16 inch in diameter, constitute the major portion of the root system’s surface area. These smaller roots grow outward and predominantly upward from the large roots near the soil surface, where minerals, water and oxygen are relatively abundant. The major function of feeder roots is the absorption of water and minerals. Under normal conditions, feeder roots die and are replaced on a regular basis.

Large roots and small feeder roots occupy a large area under ground. Typically, the root system of a tree extends outward past the dripline, two to four times the diameter of the average tree’s crown.

Why Roots Grow Where They Do

Roots grow where water, minerals and oxygen are found in the soil and allow root growth. Roots need some water and oxygen but if soils are saturated with water, most roots will die. Because oxygen is usually located in the upper surface layer of soil, the largest concentration of feeder roots exists in this zone.

Other factors that determine root growth include soil compaction (reduction in air pockets resulting from soil particles being packed together) and soil temperature. In general, as the depth increases, soil compaction increases, while the availability of minerals, oxygen and soil temperature all decrease. In some instances, hard, compacted soil (hardpans) can occur near the surface, which restricts root growth.

Causes of Root Injury and Disease

There are many ways to injure tree roots and stress trees. Some injuries are unintentional and cannot be avoided. However, most root damage can be avoided with some care.

One of the biggest killers of urban trees is use of heavy clay subsoils instead of topsoil, and soil compaction. Heavy clays and soil compaction restricts water and oxygen uptake by roots, and is associated with use of deep sub soils as fill for landscaping and compaction from construction of roads, parking lots, and from foot traffic, construction machinery, livestock, poor soil preparation, and a host of other factors.

Changes in soil depth around trees can also cause injury to root systems. The addition of only 4 to 6 inches of soil over an existing root system drastically reduces the amount of oxygen and water available to the roots (Figure 2). The opposite issue of removal of soil around an existing tree can expose and injure roots, change the soil conditions where roots grow, and reduce water availability (Figure 3).

Other causes of root problems include over- and underwatering, improper fertilization, and competition between roots. Overwatering causes the soil pore (air) spaces to fill with water and restrict oxygen uptake. Underwatering does not provide sufficient water for proper root development. Overfertilization can injure or kill the roots, while underfertilization results in a lack of the minerals essential to maintain a healthy tree. Competition for water and minerals between tree roots, bushes, grass and flowers can also stress trees. Trees can be stressed from root damage by routine soil preparation in the tree’s root zone for flower planting.

Other practices that increase root injury and disease susceptibility are: improper use of herbicides, deicing salts and other chemicals; wounding through digging and trenching; and adding deep mulch (over 6 inches), plastic or pavement that restricts or suffocates roots. After a tree is established, anything that changes the soil condition or the oxygen and water supply can be extremely detrimental.