A: Over mulching and incorrectly mulching trees and shrubs is occurring in epidemic proportions. The mulch “volcanoes” seen around trees are detrimental to the life of the tree. Mulch material should never be more than 3 inches thick and never placed close to the trunk of the tree or shrub. To find out more about over mulching and to see how to correctly mulch trees and shrubs visit RCE fact sheet. FS099, Problems With Over-Mulching Trees and Shrubs.