Mulch piled to stratospheric heights around tree trunks has been called many things (some not printable) such as pyramid mulch and mountain mulch. My personal favorites are mulch volcanoes (or volcano mulch) for stratovolcano-like creations and mulch mounds for those that resemble shield volcanoes.
These mulch monstrosities have been a source of continual frustration for anyone who cares for trees or cares about trees. Yet, despite years of educational efforts, we just can't seem to stamp out the abominable practice.
Adding insult to injury (or the coup de grâce), mulch volcanoes are often accompanied by "tree moats" created by excavating a moat-like ring around trees or shrubs at about the dripline, or slightly beyond. This is done with an edger or a shovel. In either case, there always appears to be a concerted effort to sever roots growing beyond the excavation zone.
Why do these horticultural horrors appeal to people? Tree moats take extra time. Mulch volcanoes cost more money. Both can cause slow tree death. Perhaps that's the problem: they don't kill trees right away!
Leave it to the Bard to provide the perfect metaphor in "King Henry VI Part III", Act 2 scene 1:
"And many strokes, though with a little axe,
Hew down and fell the hardest-timbered oak."
Volcano mulch and tree moats are little axes.
Mulch Done Wrong
Volcano mulch does not kill trees outright; if it did, people wouldn't do it. Instead, it produces subtle, long-term, ill-effects that are mostly hidden from our view.
1. Bark Damage: Tree bark is dead, dry tissue that protects trees from a wide range of challenges such as dehydration, oxidation, and direct access by plant pests and pathogens to the living tissue beneath. Mulch piled against tree trunks can retain water elevating the moisture content of the bark making it susceptible to decay. The result is analogous to what happens if we suffer severe skin damage.
2. Root Dehydration: Although bark mulch may at first appear light and airy, it ultimately becomes compacted as it degrades to interfere with oxygen reaching tree root cells. Trees respond by growing a secondary root system into the mulch; it's the same response seen in trees planted too deep in the soil. However, the roots growing into the mulch can become exposed and dehydrate as old mulch eventually degrades and disappears.
3. Stem Girdling Roots: Secondary roots growing into mulch piled high on the trunk will encounter the slopes of mulch volcanoes causing the roots to turn; they can't grow into thin air! Eventually, these roots encircle the tree trunk and merge with the stem tissue. As these errant roots increase girth, they gradually girdle the trunk and restrict vascular flow. Thus, they are known as "stem girdling roots."
4. No Water Infiltration: As the organic mulch decomposes and dries out, it will eventually start to repel water; it becomes hydrophobic. You can observe hydrophobicity of dry organic matter when you try to moisten a bag of dry peat moss. Of course, water repellency ultimately causes infiltrating roots to dehydrate.
5. Tree Stress: The deleterious nature of volcano mulch is not immediately apparent. While moisture starvation and vascular strangulation can ultimately kill a tree, along the way they produce tree stress. This can induce trees to drop their defenses against infestations by opportunistic insect pests such as native borers or infections by plant pathogens. Of course, the pests and diseases get blamed if a tree succumbs, not the volcano mulch that set into motion the tree's demise in the first place.
The Flatheaded Appletree Borer (Chrysobothris femorata, family Buprestidae) is a good example of a native borer that attacks stressed native and non-native trees. Despite its common name, this borer attacks a wide range of trees including maples and other hardwoods.
Mulch Done Right
Organic mulches such as aged bark mulch are a wonderful thing. The mulch moderates soil temperature, preserves soil moisture, suppresses weeds, and as the organic mulch slowly decays, it contributes to the organic content of the underlying soil. If used properly, the dark colors enhance landscape aesthetics. Of course, the availability of organic mulch dyes make other colors available; shopping for mulch can be like using an artist's palette.
In 2017, I highlighted an outstanding example of tree mulching in Glenwood Gardens, Great Parks of Hamilton County (GPHC) ("Glenwood Gardens: A "Volcano" Mulch-Free Zone"). I'm including a few of the images from that Alert or you can read the entire report by clicking this hotlink: https://bygl.osu.edu/node/721
Jerry Frankenhoff (Urban Forester, GPHC) told me that the mulch job was performed by a group of volunteers from General Electric. They worked with staff members from the GPHC landscape department to apply 60 cubic yards of mulch in Glenwood Gardens and Winton Woods on Earth Day.
Their application of mulch followed all of the general recommendations for using hardwood mulch around trees. The mulch rings were as large in diameter as practical and mulch depths were no more than 2 - 3 inches. Mulch that found its way onto the tree trunks was pulled away from the trunk flare. What a fantastic Earth Day statement!