I am a volunteer gardener with the St. Mary’s Cathedral in the archdiocese of Kingston. We have 8 large and very old trees on the site we maintain and have been adding soil around the trees in order to plant hostas and annuals. We are concerned about how much soil we can add. We talked to the National Capital Commission and they advised that soil could be added to a depth of 8 inches around the tree but we have been getting other conflicting opinions and would like more information.
There are a number of things to consider when adding soil under a tree in order to plant a garden. Tree roots require adequate oxygen and moisture to survive. Most active roots are in the top 3 feet of soil and the majority are in the top 12 inches. Adding a layer of soil that is more than 2 inches deep , can reduce moisture and oxygen availabilities and hinder absorption causing trees to suffer and even die. However, up to 3-4 inches of soil can be added as long as the soil texture is coarser than the native soil. New soil should be graded away from the base of the trunk. Be sure to maintain the original soil level at the base of the trunk. Feel free to add a 2-3 inch layer of coarse, organic mulch to conserve moisture and help keep weeds down. Ensure the mulch is kept at least 12 inches away from the base of the tree to avoid trapping moisture against the bark which will promote rot.