What is volcano mulch you might ask? As I drive around every spring, in addition to beautiful flowers I see excess mulch piled high around the base of trees in the shape of a volcano. Excess mulch cuts off oxygen to the root system, invites pests and slowly kills the tree. Mulch should never be placed over the bark of a tree. Two to three inches of properly applied organic mulch in the tree well will help retain moisture, moderate soil temperature and reduce weeds. As this organic mulch decomposes it provides the soil with beneficial bacteria and nutrients.