Do's and Don'ts...for effectively mulching your trees
put down approximately 2" of mulch
extend your mulch ring as far out to the drip line as is feasible in your situation (to the edge of the canopy at least)
use organic mulch that is slightly composted
use chemically treated mulch
push mulch to the trunk of the tree (keep it 3"- 6" away!)
build "berms" for new trees - it keeps water near the trunk and prevents outward root growth.
Well, there you have it. Mulch is one of the best things you can do for your trees as it not only insulates the soil, keeps weeds and grasses from competing, but it also feeds the soil as it breaks down, improving its condition and thereby keeping your trees healthy and thriving.