Ask Mr. Smarty Plants

Ashe junipers (Juniperus ashei) are tough evergreen trees that grow in rocky, well-drained soils making them prone to problems if their roots are not allowed to get enough air exchange by being covered too suddenly by an additional layer of soil and mulch. The mulch layer, if it is shredded cedar for example, usually has enough integrity to not compact easily and usually has enough air pockets that it won't cause too much damage. It is the soil layer that is denser and will smother plant roots by not allowing for enough air and gas exchange that is the issue here. A safer way to add mulch and soil is to do a slow increase in soil depth over several years won't harm the tree as the roots will have a chance to grow into the new soil layer each year.

The Morton Arboretum on their webpage has the following information about grade changes and tree roots..."Roots grow much closer to the soil surface than is often believed. Since roots are near the surface and depend on oxygen, raising the soil level around an established tree can have serious impact. This new soil will drastically reduce the oxygen supply to roots. When grade changes are necessary, avoid changing the grade within the dripline of the tree. The fewer roots that are impacted, the better the chances that the tree will survive. Another alternative would be to construct a retaining wall outside the dripline to accomplish the grade change."