Tree and shrub roots require oxygen and water to grow. Their growth rate is variable and is affected by the soil depth, water supply, aeration, mineral supply and temperature.
Root systems are made up of large, permanent roots for support and stabilization, and many small, temporary feeder root and root hairs. These small roots are the primary water and nutrient absorbers. Most roots can be found in the top 6 to 18 inches of soil, where water, nutrients and oxygen are found.
Roots generally extend up to two or three times the height of the tree, but can extend as far as seven times the height of the tree. Large, mature trees may have thousands of feet of root system searching for nutrients. Roots will be less extensive in clay soils than in sandy or well-drained soils.