Biggest Myth About Tree Roots
Many people believe that tree roots grow mostly downward and mirror the top of the tree. Not true.
Most Important Things to Know About Trees
Fact: In our area, the Piedmont region of Georgia, tree roots spread out just under the surface of the soil surrounding the tree.
Fact: Roots survive and grow only where there is adequate water and oxygen. Our soils have high clay content, and deeper soils do not contain sufficient water and oxygen for tree root growth.
Fact: The vast majority of tree roots are in the top 18 inches of soil (some sources say as much as 98% of tree roots).
Fact: Providing adequate room for a tree's roots to grow without being disturbed is one of the most important aspects of keeping a tree healthy.
Protect the Root Zone and Structural Root Plate
Structural Root Plate: The woody roots close to the trunk, known as the structural root plate, help anchor the tree and provide physical stability. Destroying or disturbing these roots may leave the tree unable to hold itself up. This type of damage can result in disaster. The City of Atlanta Tree Ordinance prohibits damage to the structural root plate of trees.
Critical Root Zone: The majority of roots reach out just beneath the soil surrounding the tree. The roots usually extend one to two times the reach of the branches. For some types of trees, this critical root zone can be two or three times as wide as the extent of the branches. The City of Atlanta Tree Ordinance defines the critical root zone as a circle having a radius of one foot for each one inch of diameter at breast height of the tree for a free-standing tree with no apparent root restrictions. For trees to be considered "saved", no more than 20% of the critical root zone may be damaged.
When it is done correctly, mulching a tree can often be the single most beneficial aspect of tree care. Mulch contributes to soil nutrients levels, helps retain soil moisture during drought, reduces compaction, and can even help control some common root diseases. It also reduces the tree's need to compete with other plants and turf for water and nutrients.
Ideally, mulch should be applied in a circle from near the tree trunk to the edge of the tree canopy, and covers about the same area as the critical root zone. On a practical note, however, sometimes it is not possible to mulch the entire root zone, so mulch should extend as far from the tree as possible within the scope of the site and landscape area.
Mulch should be spread only two to four inches deep, and should not touch the tree trunk or buttress roots, so leave some breathing room at the base of the tree. Never start mulching next to the trunk of the tree. Instead, the mulch should start a foot away from the base of the trunk to prevent damage to the base of the tree. Piling a heap of mulch against the tree trunk is sometimes called 'volcano mulching' and can cause insect and disease problems, often killing the tree within a few years. Deeper mulch is NOT better and can harm the tree. Covering a wider area is better.
Wood chips, pine straw, shredded cypress, pine bark, and bagged mulch all work quite well. Artificial mulches and stone ground covers (like lava rock, pebbles or crushed granite) may cause changes in soil pH and do not have the same beneficial effects as natural materials that biodegrade over time.
Root damage can sometimes result in an immediate effect on a tree. You might see leaves dry or drop during the wrong season or limbs die back. More often, however, the symptoms of root damage appear years later. It can take as many as three to seven years for symptoms of root damage to become evident in the crown of the tree. This damage is often irreversible and results in the death of the tree.
Tree "murder" by root suffocation can be the "perfect crime" since the injured tree can take several years to die after the offending act. New homeowners often report having moved into a newly constructed home with "beautiful, perfectly healthy mature trees" that mysteriously enter a spiral of decline several years later. This is because trees typically store as much as 66% of the energy they make. After tree roots are damaged and the tree is unable to make new energy, it pulls from its "bank" of energy for up to seven years before ultimately dying.
Soil Compaction Irreversible
Soil compaction should be considered permanent. University of Georgia studies show that under normal forest conditions, soils do not readily "come back" from compaction. Recovery time is at least two human generations. Damage to the trees occurs because the roots cannot function properly in compacted soil. Arborists sometimes recommend mechanical means of aerating the compacted soil around trees, but this can be difficult to accomplish without damaging roots.
Reducing the Negative Effect of Soil Compaction
Sometimes root damage cannot be avoided, but it can be minimized by restricting access to the areas close to trees during construction. If the area near the tree must be disturbed by, for example, heavy foot traffic or delivery of construction materials, the roots can be protected by softening and distributing the impact. This can be accomplished by placing heavy mulch, plywood, or metal driving pads under the trees. Physical properties of soil can be improved with the addition of organic matter and aeration to increase pore space. When tree roots must be cut, they should be cut cleanly with a sharp pruning shear or saw. When root damage is severe or occurs to woody roots, the whole tree should be evaluated to determine whether it should be removed. If a damaged tree is to remain, it should be monitored and maintained to promote tree health. Maintenance may include mulching, aeration, and irrigation.
REFERENCES City of Covington Tree Preservation Ordinance and information, prepared by Connie Head 01/04/2001. International Society of Arboriculture Arborist Certification Study Guide 1010. Soil Compaction Impacts On Tree Roots, Dr. Kim Coder, University of Georgia, July 2000; www.extension.iastate.edu/forestry/publications/for00-008.pdf
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