Expert: Jim Hyland - 9/4/2006

Question
We want to level off the ground around or pine trees. How much soil can we add around the trees? I've heard their roots cant be buried very deep. (Mostly White Pine in MN)

Answer
Several large roots grow downward to support the tree, but 90 percent of a tree's roots occupy the top 12 to 18 inches of soil. These fine feeder roots spread out, pancake-like, away from the trunk. Roots can extend from the trunk to a distance of 1 to 3 times the height of the tree. Roots take up water, nutrients and oxygen, and release carbon dioxide. Because roots need oxygen, any activity that compacts the soil or raises/lowers the existing soil level can damage tree roots and diminish tree health.

The working part of the root system of all trees (and shrubs) has nothing to do with tap roots (which are largely mythical) and everything to do with an expansive network of very shallow and fine roots. Changing the grade drastically is not something that trees can tolerate very well. At the most, a 2 inch layer of coarse soil at a time is the most one should apply.
You see, those very shallow roots are responsible for absorbing all of the water for the plant, for the uptake of dissolved minerals, and for holding the plant in place. Those top few inches of top soil provide all that the roots will seek, including plenty of oxygen without which they (roots) will die. By adding that much more soil on top, you will be smothering those roots. 2 or so inches during a season and the roots can slowly grow UP, before adding another 2 inches. Use good top soil as the fill.

Also keep the soil away from the tree trunk about a foot. the tree needs the swell of the trunk as it enters the ground-covering it will effect the health of the tree.