Building Around Trees

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Published on: May. 2, 1998

Last revision: Oct. 28, 2010

As aspiring homeowners contemplate a wooded lot, they usually imagine their dream home surrounded by healthy, vigorous trees that shade the summer sun and whisper with every breeze. After the home is built, however, the trees that made the location desirable often are either gone or dying, the victims of damaging construction work.

No matter how good our intentions, any construction work near trees will have some impact on them, because of the close relationship between a tree and the soil surrounding it. Trees are much more than the visible trunk, branches and leaves.

Their root systems, close to the surface and wide spreading, are easy to damage, even far from the trunk. Trees cannot be repaired or restored to their original condition after a construction project is finished. Therefore, it is better to prevent construction injuries to trees, rather than attempt to treat them after the fact.

How Trees Are Harmed by Construction

Trees can be harmed by construction work in several ways. Any break or tear in a tree's bark disrupts the flow of vital fluids and exposes wood to invasion by disease and decay microorganisms, which the tree must then expend energy to deal with. A trunk wound does not always cause corresponding loss of branches or foliage, so the consequences may not be fully apparent. But a large wound in the trunk of a tree is serious-it cannot be repaired and will almost certainly result in future decay and loss of stem strength.

Just as serious, although not as visible, is damage to the root system. Roots can be severed by excavation or smothered by earth fill or compacted soil.

Compaction, the loss of tiny air spaces within the soil from foot or vehicle traffic, is especially insidious. Not only will existing tree roots be affected, but future root growth also will be impaired. Symptoms of root damage from compaction include slow growth and branch dieback in the top of the tree. Soil compaction may kill trees, although no other damage occurs. New trees, shrubs or ground covers planted in the dense soil also will suffer.

Injuries are cumulative. Construction work will compound problems trees may have received from earlier drought, insects or other natural causes. This means that trees in poor condition before construction work are not as likely to tolerate further damage. It also means that trees that do not succumb to construction disturbance may be left weakened after the work is finished.

How to Protect Trees During Construction

Are we just lucky if trees survive construction? Not at all. We can greatly increase the chances of keeping trees healthy by using these strategies:

Survey the entire construction site well before work begins to determine where trees are and what condition they are in. It is best to plot all trees on a scaled drawing, but this also can be done on the ground, especially for small construction projects. Healthy, vigorous trees with solid stems and a full complement of live branches are the best candidates for saving. Large, old trees may not tolerate much disturbance. Don't overlook smaller, understory trees, such as dogwood, redbud, serviceberry and ironwood. If you are working with builders, inform them of your desire to save trees.

Develop a construction plan that blends the buildings with the trees on the site. This is the time to decide which trees will be removed and which will remain. Consider alternate locations for footings, walks, drives and changes in the ground line to save the best trees.

Keep in mind that it may be necessary to remove some good trees simply because there is not sufficient space for them. It is much easier to make these choices before the construction work begins. The plan can be drafted on a layer that overlays the survey plan, or mark the ground with stakes or flags. Remember that you need to keep a relatively large undisturbed area around each tree to help protect it. The larger the tree, the larger the area needed surrounding it.

Establish tree protection zones around individual trees or groups of trees to be saved. Exclude any type of construction disturbance, including grade changes, vehicle parking or storage of materials around protected trees. Set steel fence posts with flexible, snowfence-type fabric around the perimeter of each protection zone.

Route trenches as far away from trees as possible. Utilities that may require trenching include sanitary sewer, water, gas, electricity and telephone or television cable. Some utilities may be advantageously placed in the same trench. Placement of some utilities is flexible, while others are not. If a trench cannot be placed by the builder to avoid coming close to a valuable tree, consider going under it. Dig the trench directly up to a tree trunk on both sides. Then bore or force a tube or line through the soil below the tree. Rerouting or tunneling for utilities may add to the cost of the project, but also will increase the chances of saving trees.

If branches or roots must be severed, cut them with care. Generally, remove entire limbs or branches at their origin. Use the 3-cut method to avoid stripping bark below the limb and to promote proper wound closure. Roots should be cleanly cut with a saw to maximize root regeneration and minimize chances for decay. Do not leave ragged ends. Dig carefully around large roots and allow them to pass through a trench. Place utility pipes or lines below the roots. Backfill trenches with loose soil placed on top.

Use wood chips as a protective blanket over the ground. A layer 4 inches or more deep will help prevent soil compaction, especially where construction work near trees cannot be avoided. Chips help protect soils anywhere on the site where new trees, shrubs or turf will be planted. Replenish chips as they deteriorate or wear thin.

Avoid post-construction activities which could further stress weakened trees. Refrain from adding topsoil around trees, installing underground irrigation pipes or using herbicides within tree rooting areas. Do not prune trees heavily, until normal growth rate returns.

Plan for new trees, shrubs, and ground covers, which are compatible with a wooded environment. Plant shade tolerant shrubs and small trees around saved trees to maintain a wooded appearance and help preserve the original root environment. Retain and expand the natural forest floor with bark mulch. Plant turfgrasses in more open, sunny areas where they will grow better and compete less with tree roots.

Building homes, offices or other buildings on wooded sites requires taking precautions to preserve the trees. Consider tree needs before construction begins. Find the best trees and concentrate on saving them. You may wish to protect small trees that have the potential to grow into shade trees.

During construction, protect as much undisturbed area around each tree as possible, remembering to take into account both the visible and the fragile underground parts of the trees. Finally, continue to care for your trees after construction is finished. Your efforts on behalf of the trees will make your dream home in a woodsy setting a reality.

How Much? How Close?

No two trees will react the same to disturbance because of differences in soil type, species, age and condition. Healthy trees generally can tolerate limited injury if they have a good growing environment for recuperation. The more severe the damage and adverse the growing conditions, the higher the risk.

How close to the tree can a tree's roots be cut?

An easily recognizable limit for root disturbances is the ground outside the branch spread, or dripline. Soil excavation inside this point may result in some root loss. But if damage is not done on all sides, a healthy tree can likely tolerate it. If roots are exposed, cut them off cleanly with a saw to promote better regrowth.

How much soil can be added over the roots?

Preferably, none. Added soil can suffocate roots from lack of oxygen. If soil must be added, use the thinnest possible layer of loose soil over the smallest possible area. Think in terms of inches rather than feet. Willows or cottonwoods can tolerate more fill, ashes less and white oaks little, if any, added soil.

How much soil can be removed around a tree?

Because many fine roots are at or near the surface, lowering the grade around trees should be avoided. A healthy tree may tolerate removal of a few inches of soil inside the dripline on one side.

How many limbs can be removed?

Because leaves manufacture food for a tree, removal of more than one fourth of the live branches threatens a tree. Weak trees, or trees with root damage, for example, may tolerate less. Removing dead limbs will not hurt a tree.

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