Healthy Roots Make Healthy Trees
The root system of a tree is a store-house for essential food reserves needed to produce spring foliage. Roots absorb and transport water and minerals from the soil to the rest of the tree. They also anchor the portion of the tree above the ground.
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 annually increase in size and grow horizontally. They are located in the top 6” of the soil and do not grow down more than 3-7 feet.
Feeder roots average a size of 1/16” in diameter and are the major portion of the root system’s surface area. These smaller roots grow outward and upward from the large roots near the soil surface where minerals, water and oxygen are abundant. Their major function is water and mineral absorption. Under normal conditions feeder roots die and are replaced on a continual basis.
Typically, the root system of a tree extends outward past the dripline, 2-4 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. The greatest supply of these are located in the surface layer of the soil so the plant has the largest concentration of feeders in this zone.
Causes of Root Injury and Disease
- One of the biggest killers of urban trees is soil compaction. It restricts water and oxygen uptake by roots, and is associated with roads, parking lots, foot traffic, construction machinery, livestock, poor soil preparation, and other causes.
- Changes in soil depth around trees, by adding only 4-6” of soil over a root zone, drastically reduces the amount of oxygen and water available to the roots.
- Removal of soil around a tree can expose and injure roots, change the soil conditions where roots grow, and reduce water availability.
- Over-watering causes the soil pore (air) spaces to fill with water, restricting oxygen uptake.
- Under-watering does not supply sufficient water for proper root development.
- Over-fertilization can injure or kill roots.
- Under-fertilization results in a lack of the necessary minerals essential to maintain a healthy tree.
- Competition for water and minerals between tree roots, bushes, grass and flowers can seriously stress trees. Also beware of routine soil preparation for flowers if it might damage tree roots.
- Other causes are improper chemical use, wounding through digging and trenching, and addition of deep mulches/plastics/pavement that restrict or suffocate roots.
- After a tree is established, any activity that changes the soil condition and/or the oxygen and water supply, can be extremely detrimental.
Types of Root Diseases
Fungi that occur on small feeder roots are numerous (Phytophthora, Pythium, Fusarium). These break down the feeder roots and reduce the trees mineral and water absorbing capability.
Fungi that attack large, woody roots suppress growth, decay food transporting cells, reduce food storage and reduce structural support for the tree.
Root Disease Signs & Symptoms
Typical symptoms associated with root diseases are often confused with mineral deficiencies because high numbers of dead roots reduce water and mineral uptake.
Symptoms of root disease include small, yellow chlorotic foliage, reduced growth, scorch, tufted leaves at the end of branches and branch die-back, mushrooms or conkers at the base of a tree, white fungal growth under the bark. Construction damage may appear several years after the damage was done.
To verify a disease carefully excavate roots removing a small patch of the bark. A brown coloration beneath the bark indicates a dead root, while a living, healthy root usually appears white or light colored.
Control & Prevention of Root Injury & Disease
The most effective way to prevent any problem is to keep your plant healthy and vigorous.
Maintain a healthy root environment with adequate growing space for root development, well-conditioned soil 16-24” deep, and sufficient water and oxygen.
Check the water and soil condition of the root environment by digging a hole outside of the dripline to determine if the soil is wet, dry, or compacted. Soil moisture is fine if the soil can be formed into a ball with a little bit of pressure.
Water long and deep over the entire root system and allow some time for the soil to dry before the next watering. Frequent, shallow waterings will harm your plant.
Always water at least once a month during the winter months.
Avoid injurious practices such as soil compaction, soil depth changes, mechanical injury, improper watering and fertilization techniques.
To minimize soil compaction, it should be removed and replaced with new soil after adequate drainage is achieved. An organic amendment such as peat moss, wood chips, tree bark can be used around the base of a tree to improve aeration and water availability.
Fertilization damage can be avoided by applying nitrogen fertilizer to established trees immediately after spring leaf expansion, not in the late summer or fall.
Once a tree is infected it is difficult to control root disease. Since the disease probably entered the tree due to some form of stress or injury, the first step is to eliminate the stress and restore the tree’s vigor. Adequate watering and fertilization are musts.
If the root system is severely damaged, tree removal is usually recommended, for the tree may become a dangerous liability. Planting in the same area following death should be preceded by removal of as much of the dead stump and roots as possible. Fumigation of the soil to remove certain disease organisms is also recommended.