Healthy Roots and Healthy Trees

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 small, 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. 

Feeder roots, although averaging only 1/16 inches 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. 

Typically, the root system of a tree extends outward past the drip line, two to five times the diameter of the average tree’s crown (Figure 1). 

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 affect root growth include soil compaction (which reduces the amount of large air-containing pores in soil and makes soil harder for roots to penetrate) and soil temperature. In general, as the depth increases, the amount of plant-available minerals and oxygen decrease, restricting root growth. 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 the use of heavy clay subsoils (instead of topsoil) and soil compaction. Heavy clays and soil compaction restrict water and oxygen uptake by roots and are 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).