Dr. Jekyll’s Good Mulch Matters
Organic mulches such as aged hardwood bark mulch are a wonderful thing. It moderates soil temperatures, preserves soil moisture, suppresses weeds, and as the 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 makes other colors available; shopping for mulch can be like using an artist's palette.
The proper application of hardwood mulch around trees starts with producing mulch rings as large as is practical. Keep in mind that organic mulch is an effective stand-in for leaf litter found beneath trees in forests. It’s also a great way to reduce direct competition between turf and tree roots. However, the mulch should be applied to a depth of no more than 2 - 3 inches. Mulch that finds its way onto the tree trunks should be pulled away from the trunk flare.
Mr. Hyde’s Bad Mulch Method
Organic mulch has an evil side locked within that may be unleashed if the mulch is piled high around tree trunks. This mulching method has been called many things (some not printable) such as pyramid mulch, mountain mulch, mulch mounds, and my personal favorite volcano mulch, or mulch volcanoes, for stratovolcano-like creations.
These mulch monstrosities have been a source of continual frustration for anyone who 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), volcano mulch is 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. Volcano mulch piled high on tree trunks cost more money. Both can cause slow tree death. Perhaps that's the problem: the effects are insidious; trees don’t die right away!
Mr. Hyde’s Mulch Method 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 until the evil is unleashed.
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 to the living tissue beneath by plant pests and pathogens. Mulch piled against tree trunks can retain water elevating the moisture content of the bark making it susceptible to decay.
2. Root Dehydration: Although bark mulch may at first appear light and airy, it will ultimately compact 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 the mulch degrades.
3. Stem Girdling Roots: Secondary roots growing into mulch piled high on the trunk will eventually 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 the 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 the tree's demise into motion 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.