Trees and Construction

— Written By Sarah Roberson

Every year I get questions about dieback and death of trees that occurs following construction. The cause is often related to soil disturbance and root damage during construction. One of the biggest misconceptions about trees is that since they are big, they are indestructible. What often happens is a wooded lot is purchased and a house is built among the trees left standing. In many cases no steps are taken to protect the trees. Over a period of three to ten years, the homeowner has to pay to have trees removed. Larger trees and those close to the house are removed at great expense since they often overhang the house and have to be cabled down. This is a hard lesson to learn.

Why do trees on so many construction sites have to be removed in three to ten years? The environment on a construction site is altered greatly. Let’s take a look at some of the individual changes that occur on construction sites. I hope this will help develop an understanding of how trees can be damaged during the construction period.

Altered Drainage Patterns–Sewers, streets, and gutters at a site will alter the drainage patterns. Water that once soaked in is now channeled into the storm sewer, bypassing the ground water system. The result is soil that dries out faster since less water is stored. Under these situations, drought conditions occur more quickly. On the other hand, altered drainage patterns can result in excess water moved to areas and held where it didn’t go before. Major problems can result for trees not tolerant of wet soils.

Debris in the soil–In some cases, materials get buried at the construction site. Brick, concrete, sheetrock, paint and plywood are examples of construction debris that cause disturbed soil profiles and alter soil moisture distribution patterns. I have heard of an example where a foundation planting had failed over several years. When the area was probed, a 4 X 8 sheet of plywood was found buried about 6 inches below the soil surface

Cut and Fill—Removing soil or trenching around existing trees is as serious as adding soil around them. The majority of absorbing roots are in the top 9 inches of soil. When soil is removed, a large number of these roots can be lost. When fill is placed on top of the normal grade, root damage can occur. As little as 3 to 4 inches of soil may kill a mature forest tree. You should also be concerned where soil is stockpiled. Large piles of soil near the base of a tree can kill it. A pile of soil sitting on the tree roots for four to six months will cause problems.

Disturbed soil profile–Builders are faced with soil disposal problems. One of the problems is what to do with soil removed from digging a basement or footings. The lowest cost solution, but not the best, is to spread the soil over the existing soil. The result is a disturbed soil profile. Usually the material removed from basements or footings is clay. Placing clay over a coarser textured soil causes problems. The finer clay will hold on to water and not let it move down into the sand or loam soil. In the process, sensitive roots are killed.

Environmental changes–Wooded lots are generally cooler than lots without trees. The first step in the building process is to remove enough trees so construction can begin and there is a place for lawn. As trees are removed, shade is reduced. The result is an increase in temperature and light intensity. Forested situations also have lower soil temperature due to shading and an organic mulch layer. When construction starts, most of the organic litter is removed so soil temperatures warm. Plants are very sensitive to these changes in environment and will quickly let you know they are not happy.

Taking measures to protect trees up front is cheaper than removal once a house is on site. If you purchase a wooded lot and plan to build and save trees, seek the services of a Certified Arborist during the planning process before any equipment goes on the site. The best protection is to place organic mulch 2 to 4 inches deep over a large portion of the root zone. A good rule of thumb is to mulch 1 foot away from the tree for every 1-inch in trunk diameter. Put temporary fencing around the area with signs that indicate it is a tree protection zone. Protecting trees in groups is often better than trying to protect individual trees. If you have questions about trees in your home landscape contact the Pitt County Master Gardener Volunteers at 902-1705 or Yours treely, Danny Lauderdale.