Mulching can be one of the best things you do for your trees and shrubs. It also can beone of the worst things you do for your trees and shrubs.
First, let's get our definitions in order.
"Mulch" is anything with which you blanket theground, be it straw, black plastic, pebbles, even the marbles I once saw blanketing aportion of ground in a contemporary California landscape.
Any of these materials, when spreadbeneath a tree or shrub, will keep lawn and, hence, lawn mowers at bay.