Reality:  Mulch is too thick when a tree root starts growing into it.

The right amount of mulch is very beneficial for trees. Start by applying 3 to 4 inches, and inspect the mulch several times a year for root growth. Keep mulch at least 6 inches away from the trunk to reduce chances o
f rodent injury and infection by pathogens. Excessive amounts of mulch may disrupt soil moisture and aeration.