Tree RootsTrees suffer damage or are killed when grading is done around their trunks or their roots. In one class of cases it is due to cutting the main roots, as when pavements are laid. When the roots are in direct line of the pavement damage can seldom be avoided, but wherever possible all roots should be left intact.
In another class are the trees that have earth filled in deeply over their roots and the bases of their trunks. Misinformed people argue that because roots grow, often deeply, in the ground the trees will not suffer by an increased depth of earth to a foot or even a yard. But countless sickly, dying and dead trees prove this to be incorrect.
The fact is that roots and tree trunks need air as well as moisture in the soil. This may be assured by building wells around the tree trunks. The wall of the well need not go any deeper than the original soil surface, but it should be wide enough to allow the trunk ample room in which to expand as it grows. Such wells, when built of large stones, laid without mortar or cement are better than solid walls because they allow a much larger and freer passage of air into the soil.
Adapted from: Gardening Short Cuts