Spruce Trees and Soil Compaction

The following question was sent to the P&PDL diagnosticians here at Purdue University:

Question: We are building a new home and in the side yard are two, huge 16-year-old blue spruce. We are trying to save them so we designed a retaining wall to include these trees, but huge concrete trucks have had to go rather close to them. I'm worried they have damaged the roots. The trees have dead tips hanging from some of the branches. What can I do to help save these beautiful old trees? I have another spruce farther away, and I don't see any damage on it. Please help!

Answer: I'm sorry my answer probably comes too late to be of service. In fact, it was probably already too late when you wrote to this column. By the time damage is apparent on an evergreen, it's usually irreversible.

In order to protect trees during construction, temporary fencing should be put up to keep equipment off the root zone. This is an important part of reducing soil compaction during construction.

Heavy equipment on the site compacts the soil and reduces the pore space for air and water. Typically, the top four to eight inches of soil are compressed into a dense mass. This damages existing plant roots, increases surface runoff and erosion, decreases water moving into the soil, decreases soil aeration and makes it more difficult for plant roots to grow. It's difficult and expensive to correct soil compaction. It's much easier to prevent it.

If you do any more remodeling, keep vehicular traffic routed away from desirable trees. Also, make sure construction materials are not stored near trees. Fence off an area equal to 1.5 times the drip line.

If vehicles MUST go over the root zone of the trees, mulch with a six-inch thick layer of wood chips to reduce the compaction. This layer of mulch can be removed after construction.

Do not increase or decrease the amount of soil covering the roots, unless it is absolutely necessary. The majority of tree feeder roots are located in the top four to eight inches of soil. When the soil grade is decreased, not only is the nutrient rich topsoil removed, most of the tree's feeder roots are removed also. If the soil grade must be increased, aeration will be reduced. Few species will tolerate fill of even a few inches placed against the main trunk.

Now that the construction is over, if your trees have survived, make sure you give them regular watering and fertilizing for the next several years.

-Beverly Shaw, Purdue Advanced Master Gardener