The decay of this part of the tree can cause the tree to become unstable. Keep mulch 12 to 18 inches away from the trunk for any size tree. Mulch should not be stacked against the trunk (top) because roots can grow there (below) and surround the trunk. It's essential for proper tree maintenance, but when the mulch is too high and touches the tree trunk, it can cause problems.
Trees, especially younger trees, tend to benefit more from a 2- to 3-inch layer of mulch around their base. However, for the first 12 inches that immediately surround the trunk, dilute the layer of mulch so that it covers the soil. This will avoid common problems associated with excessive use of mulch. Mulch should never touch the tree trunk.
As mentioned, applying too much mulch can be very problematic for the overall health of trees. We recommend that mulch be stacked no more than 3 or 4 inches off the ground. It's also imperative to keep the mulch away from the base of the tree, creating the appearance of a doughnut-shaped hole around the tree with the surrounding mulch. We recommend keeping mulch approximately 12 to 18 inches away from the trunk of any tree.
Ideally, the mulch around the tree would spread along the branches or over an area of at least 4-5 feet in diameter around the tree trunk. The larger the area covered with mulch, the more beneficial. In general, the depth of the mulch should not exceed a total of 3 inches, including the remaining mulch from previous years and the current season's application. Mulching mimics the natural environment found in forests, where leaves and branches cover the soil surface, replenishing nutrients as they decompose and creating an ideal environment for root growth.
A slight raking of existing mulch may be all that is needed to refresh old mulch and break up compacted or crusty layers that may develop. A mulch volcano is often the result of people building circular raised beds around their trees and then filling raised beds with mulch. Volcanic mulching, also known as overpadding, is the improper mulching technique that stacks mulch around the tree against the tree trunk. If you have smaller trees or trees that have only had volcanic mulch for a few years, you can probably dig up the mulch yourself with some garden tools.
The mulch will dry out more quickly than the soil and much of the root system that has spread all over the mulch will die, affecting the overall health and survival of the trees.