One method of rejuvenating roots on a tree that has had soil compaction and/or fill soil added is Radial trenching which involves excavating narrow trench spokes outward from the trunk. We like to start at a distance from the trunk equal to three times the trunk diameter. So if the tree diameter is 20 inches, then we would start the radial trench 60 inches (5 ft.) from the trunk. The radial trenching procedure should always be followed with a 2 to 3-inch application of organic mulch placed directly over the soil. Do NOT use a weed mat… ever.
The radial trenches should extend out to the dripline of the tree. The depth of the trench depends upon soil conditions, tree health and desired effect. Some researchers recommend cutting small roots that will quickly regenerate and grow into the compost-filled trench. Other experts only recommend making the trench but leaving all the roots intact. I prefer to cut small roots when the tree is reasonably healthy. I won’t cut roots on a stressed tree.
The most commonly used radial trenching technique utilizes the Air Spade. The Air Spade will not damage utilities such as electrical or water lines that are often found in an urban landscape. If desired, the small pencil-thick roots are cut with a chain saw or hand saw. Then a compost mixture is added to the trench and raked and mixed with the existing soil.
Dan LeBlanc of Taylor Tree Services is assisting us with a more extreme method of radial trenching which uses a trenching machine that makes a 3-inch wide trench that is about 18 inches deep. The trench is refilled with a mixture of soil and compost. If the soil quality is good, we use the same soil to mix with the compost. If the soil quality is poor as is sometimes found on construction sites, we add new better-quality soil and combine radial trenching with a compost stirring technique described below. The trenching machine can only be used when there are no underground utilities that can be damaged.The trenching machine will sever smaller roots that cross the path of the trench. When the treated tree has reasonably good health, the severed roots should quickly sprout and grow into the compost-soil mixture in the trench where nutrients, organic matter and general conditions are better than in the surrounding soil. Cutting larger roots over 1-inch in diameter should be avoided.\