Blending new homes, existing trees requires care

Friday, March 8, 2002

As aspiring homebuilders contemplate a wooded lot, they may imagine their dream home surrounded by healthy, vigorous trees. After the home is built, however, the reality may be far different. Trees that made the home site unique or desirable are often eventually lost, victims of damage during construction work.

No matter how good our intentions, any construction work near trees will have some impact on them. Any break or tear in a tree's bark will disrupt the flow of fluids and will expose wood to disease and decay.

Less obvious is damage to the root system. Roots can be severed by excavation or smothered by added soil.

Soil damage, especially the loss of small air spaces in the soil from vehicles and foot traffic, will harm existing roots and impair new root growth. Symptoms include slow growth and branch dieback in the top of the tree. Soil compaction can disfigure or kill trees, even though no other damage occurs, and is very hard to correct. The health of newly planted trees will also be affected by compacted soil.

Cumulative damage

Injuries are cumulative. Construction work will only compound any existing problems trees may have. Trees in poor condition should be removed prior to construction, saving only the best trees.

Survey the entire construction site before work begins to determine tree location and condition. Healthy, vigorous trees with solid trunks and a full complement of live branches are the best candidates to save. Large, old trees may not tolerate disturbance. Don't overlook the value of smaller, understory trees, such as dogwood and redbud. You may wish to protect small trees that will become future shade trees. Be sure to inform your builders of your specific desires.

Develop a construction plan that blends the proposed building with trees on the site. This may be done on paper or on the ground. Decide which trees will be removed and which will remain. Not every tree can stay. Consider alternate locations for footings, walks, drives and changes in the ground line to save the best trees. It is much easier to make these choices before construction work begins.

Establish tree protection zones around individual trees or groups of trees to be saved.

Saved trees will need an undisturbed protection area around them. The larger the tree, the larger the protection area needed. Work with your builder to set steel fence posts and snow fence-type fabric around the perimeter of each protection zone. Any type of construction disturbance, including grade changes, vehicle parking, or storage of materials should be excluded.

Route utility trenches as far away from trees as possible. If trenching near a valuable tree cannot be avoided, consider boring or forcing a line through the soil below the tree. Rerouting of utilities or boring may add cost to the project, but it will also increase the chances of saving valuable trees.If branches or roots must be severed, carefully cut them using proper pruning techniques.

Roots should be cleanly cut with a saw to maximize root regeneration and minimize chances for decay. Do not leave ragged ends.

Use wood chips as a protective blanket to help prevent soil compaction, especially where construction work near trees cannot be avoided. A layer 4-6 inches deep is desirable.

Avoid post-construction activities which could further stress weakened trees, such as adding topsoil, 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 that are compatible with the site.

Different reactions

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 roots be cut? At a bare minimum, protect the ground under the branches out to the branch tips or dripline. Soil excavation under the branches will result in root damage.

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.

Trees can't be "fixed" after construction damage is done. Take steps to save trees before work begins to make your home in a woodsy setting a reality. A conservative approach is best; consult a tree or landscape professional if you have any doubts.

Joe Garvey is a district forester with the Missouri Department of Conservation.

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