Look around your neighborhood or your local shopping center and you will see that mulch volcanoes have erupted everywhere in recent years. What is a mulch volcano you ask? They are those large, sometimes comically enormous, piles of mulch that are commonly placed around the trunks of our neighborhood trees.
What started out as a well-intentioned practice to protect tree trunks from lawnmower damage and reduce competition from weeds has become a tree killer in itself. When done properly a mulch circle provides many benefits to tree health. In addition to providing a barrier against lawnmower and weed trimmer injury, mulch helps to retain soil moisture, adds organic material to the soil as it biodegrades, and reduces root competition from turf for water, nutrients, and space.
Done improperly, too much mulch predisposes the trunk to decay, disease, and insect attack. It can also hide problems such as improper planting depth or the presence of stem girdling roots. In short, mulch volcanoes kill trees!
A proper mulch circle should be no more than 2-4" deep and never actually come into contact with the trunk itself. The shape should be more like a flattened donut than a tall volcano. Extend the circle out as far as is feasible, more is better. Leave a few inches of air space around the trunk.