Mulch does three things — holds water, suppresses weed seed, and moderates soil temperature — and it does all three at about three inches. Under two it stops suppressing weeds; over four it starves roots of air and holds damp against stems. The mulch piled into a cone around a tree trunk, which you will see on most streets, rots the bark underneath and eventually girdles the tree. We pull it back to the flare every time, whether or not we put it there.