We see these wondrous landmarks everywhere, especially in shopping centers or along roadsides, but these mulch volcanoes are bad and are killing trees. They can cause trunk rot, increase insects and they decrease the amount of water and oxygen getting into the soil leading to a stressed and nutrient deficient plant.
When it comes to mulch, less is more. 3-4″ thickness is all you need, with a topdressing of 1-2″ to refresh every year or two as the old mulch breaks down into organic matter. What many fail to do, is rake away the old and they tend to just pile it up creating the volcano effect. A nice cut bed line will contain the mulch and create a barrier for water to catch and seep down into the drip line of the tree.
Save your trees and keep your mulch low!