Mulch should never be piled up in a mound around the base of the trunk. Piling the mulch deeply around the base of the trunk exposes the trunk to dark, moist conditions. Decayed organisms can take advantage and invade the trunk.
When mulching trees, the mulch should be spread out in a flat disk about 2 to 4 inches deep and pulled back slightly from the trunk. As the mulch thins out and decays, add more mulch as necessary.
Not only does this protect trees from string trimmers, but keeping the area mulched and free from grass encourages faster growth on young trees. Research indicates that in some cases, trees that were mulched grew twice as fast as trees that were not.
Source: LSU AgCenter
