Planting gardens under trees
By Ray Ridlen
Raising the grade or adding soil over the roots of an established tree is not recommended.
Changes in soil grade can seriously injure trees. About 90 percent of the tree's root system lies within the upper 12 inches of the soil and can extend double the distance beyond the drip line in older established trees. Covering tree roots with as little as 3 inches of soil can cause damage by suffocating roots.
The degree of suffocation is directly related to the depth of soil added. Toxins are often produced by bacteria, which thrive under the anaerobic conditions created by the added soil. Some tree species may tolerate fill over the roots better than others, but all can be injured by raising the grade. Species that are most sensitive to changes in grade are oak, maple, and pine. However, most tree species are sensitive, depending on the degree of grade change, the environment, and other stress factors.
Caution should be exercised when constructing a raised bed to serve as a planter around the trunk of an established tree. Tree trunks do not tolerate prolonged exposure to soil and soil microorganisms.
Soil in contact with the trunk restricts the movement of oxygen and carbon dioxide into and out of the portion of the trunk covered by soil. Over a period of years, tissues in the lower trunk die which leads to root dieback and root collar disorders.
The only time installing raised planters can be done safely is when the tree and planter are established at the same time.
Usually it is three to five years before a site disturbance produces obvious symptoms.
Trees affected by site disturbance often lapse into a progressive deterioration and decline in health. Early symptoms include leaves becoming smaller in size, early fall coloration, premature defoliation, and dieback of twigs and progressively larger branches beginning in the upper crown and moving downward. Sucker growth may become more prolific on the trunk and larger branches. Decline usually progresses from the top to the bottom of the tree. Leaf or needle drop may begin within the canopy near the trunk and progress outward. Despite therapeutic intervention, most trees affected by decline eventually die.
Extreme care should be taken when digging up or tilling the soil under a tree. Many large and small roots will be cut by such digging, especially if it occurs close to the trunk.
Minor disturbances, such as suddenly exposing trees to direct sunlight by removing neighboring trees, mowing, fertilizing, mulching, and shallow cultivation, have also been observed to cause trees to decline and die.
The best way to cover the area under a tree is to use mulches or ground covers. Organic mulch such as wood chips or bark should be applied around the tree to a depth of two to four inches and renewed as necessary to keep a constant layer around the tree. Avoid using rocks that cause soil compaction, or plastic sheeting that suffocates the root system.