From: https://extension.unh.edu/blog/2020/04/volcanoes-kill-trees
Volcano mulching is an improper tree care technique where, year after year, mulch is piled against the trunk of a tree. Properly planted trees (or naturally grown trees) will develop a characteristic root flare near ground level. Volcano mulch buries the root flare or, in young trees, prevents a proper flare from ever developing. Trees without proper root flare lack stability and are more prone to windthrow. Mulch piled against a tree’s stem also encourages disease and decay because the bark is almost always wet. To make matters worse, huge mulch piles interfere with good root development. For example, if the top few inches of soil are always moist, roots may not spread deeply into the ground. During a summer drought, your tree may be left high and dry!
So, what is the proper mulching technique? Spread a layer of mulch 2-4 inches thick under your trees, ensuring no mulch touches the trunk. For younger or newly planted trees, spread the mulch to the dripline (the outermost part of the tree’s canopy where water drips onto the ground). For older, well-established trees, try the “3-3-3 Rule.” Spread a 3-inch layer of mulch in a 3-foot donut around the tree, leaving at least 3 inches between the mulch and the tree stem. With a bit of know-how, properly mulching trees is a simple task that will enhance the health of your trees.