The horticultural horrors commonly called “mulch volcanoes” and “tree moats” defy explanation. These mulch monstrosities are not the work of anyone who cares about the long-term health of trees and shrubs.
They produce a range of detrimental effects from vascular strangulation to moisture starvation that wreaks havoc on plant health. Add in other stress-inducing issues and tree and shrub health can be tipped over the edge. Of course, that’s the Grim Mulcher’s plan.
When Mulching is Done Right
Organic mulch, such as aged hardwood bark mulch, serves as a stand-in for leaf litter found beneath trees in forests. Arguably, organic mulch may be the single most important component in healthy, sustainable Ohio landscape ecosystems.
Organic mulch moderates soil temperatures; preserves soil moisture; suppresses weeds and of course; enhances landscape aesthetics. The mulch also contributes to the organic content of the underlying soil which ultimately improves soil structure.
Some types of organic mulch degrade faster than others. For example, large “chunk” mulch like pine bark nuggets last longer than aged hardwood bark mulch. However, the fact that hardwood bark mulch does not last forever is its true value.
Hardwood bark mulch supports an ever-changing community of macro- and microorganisms as it degrades on its way to becoming small particles of humus. In turn, the microorganisms improve soil structure as they exude sticky substances known as “microbial glue” that sticks together soil particles to form soil aggregates.
As the graphic below shows, macropores between the aggregates provide a pathway for water to infiltrate and move through the soil. Indeed, research has shown that aggregated clay soils can drain as fast as sandy soils.
The large air spaces between the aggregates allow air to penetrate the soil. This places oxygen, the #1 element required for root growth and development, in direct contact with roots.
In contrast, bare soil beneath trees and shrubs allows soil temperatures to fluctuate widely. Water easily evaporates from the surface producing a boom-and-bust condition with soil moisture. Cracks in clay soils exacerbate the problem. Of course, bare soil also provides an open range for opportunistic weeds.
Worse, organic matter rapidly oxidizes from bare soil. The loss of support for beneficial microbes means soil aggregation suffers lessening the development of good soil structure.
The proper application of organic mulch such as hardwood bark mulch starts with producing mulch rings as large as is practical. 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.
When Mulching is Done Wrong
Mulch piled high around the stems of trees and shrubs has been called many names; some unsuitable for BYGL Alerts. Descriptive names include pyramid mulch, mountain mulch, mulch mounds, and my personal favorites volcano mulch, or mulch volcanoes, for the stratovolcano-like creations.
Digging out turfgrass to add definition to a mulch ring can add to aesthetics if done with extreme care to avoid damaging tree roots. Unfortunately, overly enthusiastic excavations to produce “tree moats” can have negative short-term and long-term impacts on a tree’s root system.
Likewise, volcano mulch-induced changes to a tree’s root system can remain throughout the life of a tree. The damage is irreversible if not recognized and corrected early.
The Six-Step Grim Mulcher Assisted Herbicide Program
The impacts of tree moats and mulch volcanoes are insidious and perhaps that’s the problem. The mulch madness would be stopped if trees died right away.
Here is a six-step assisted herbicide program approved and practiced by the Grim Mulcher:
1. Damage the Bark: 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. Cut the Roots: Studies have revealed that the majority of a tree’s roots grow well outside of the dripline. Overly enthusiastic excavations around mulch rings that morph into deep tree moats may cause injury to the roots making them more susceptible to infections from root-destroying plant pathogens. Completely severing the roots creates a concentrated root mass within the mulch ring where resources such as water and nutrients are quickly depleted.
3. Let's Make Secondary Roots!: Tree root cells acquire oxygen directly from the soil rather than from an internal source; plants don’t have a cardiovascular system! Numerous studies have revealed that most of a tree’s feeder roots are found in the upper soil strata where the concentration of oxygen is highest.
Planting trees too deep causes dormant buds within the bark on the buried trunk to produce so-called adventitious roots that grow from the trunk. Indeed, the same mechanism is exploited to root cuttings.
The production of adventitious secondary roots on trees that are planted too deep is a survival strategy to grow roots near the soil surface. The result can be the production of a new, elevated secondary root system.
Although bark mulch may at first appear light and airy, it can ultimately compact as it degrades to behave much like soil. As a result, adventitious secondary roots are produced that grow into the mulch volcano. As shown in the image below, the production of secondary roots can start very early once the mulch is piled high on the main stem.
However, unlike secondary root systems on trees that are planted too deep where the roots remain in the soil, the secondary roots infiltrating the mulch are elevated high above the surrounding soil. They acquire the same confined shape as the volcano mulch which limits the area available to the tree for the uptake of water and nutrients.
This is illustrated in the series of images below. I took pictures of volcano-mulched trees around a motel in 2020. Even then, it was apparent the mulch was having a negative effect: note the stem-girdling root. The motel closed and the trees have not been mulched. Consequently, the bark mulch decayed to reveal a heavily mulch-modified tree root system that I photographed a few weeks ago.
The adverse effects that mulch volcanoes can have on the tree’s root structure can be irreversible as shown in the images below. These trees were volcano-mulched for many years. Unfortunately, there is no way to correct the obvious problems with these trees' root systems without causing substantial, possibly lethal, injury.
4. Dehydrate Roots: As the organic mulch degrades, the secondary roots growing into the mulch can become exposed. The decomposing mulch can also become hydrophobic if allowed to dry out; it repels water. 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 roots that infiltrate the mulch to dehydrate.
5. Girdle Those Stems: Secondary roots growing into mulch piled high on the trunk will eventually encounter the slopes of mulch volcanoes causing the roots to turn. Although root cells require an external oxygen source, 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."
Symptoms of stem girdling roots include flat sides directly above the compression zone. This is a non-reversible condition and can eventually lead to a loss in structural integrity provided by the cylinder-within-cylinder morphological structure of tree stems. In other words, trees break and fall over. Stem girdling roots can also lead to bark splitting as the choked-off phloem dies. Ultimately, the disruption in vascular flow produces a thinning canopy.
6. The Coup de Grâce; Say Hello to My Little Friends: 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.
Sap-sucking insects such as soft scales or the newly arriving Spotted Lanternfly (Lycorma delicatula) present another opportunity for the Grim Mulcher to shift the blame. While not considered outright tree killers, both the scales and lanternfly feed by withdrawing copious quantities of sap. The sublethal effects of the sap-sucking insects combined with the chronic stress effects of erupting volcano mulch may push trees over the lethal edge. Of course, the Grim Mulcher returns for the harvest.
