The spring malpractice of piling mulch high on tree stems to approximate the look of a volcano is well underway in Ohio. Commonly known as “volcano mulch,” this harmful use of mulch has been the subject of eight BYGL Alerts since 2017. Perhaps these horticultural horrors now deserve a scientific name: Horticulturus horribilis var. volcanicus.
Adding insult to injury, volcano mulch is sometimes accompanied by the bizarre practice of excavating a moat-like ring around the mulch boundary. 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. The excavations are sometimes called “mulch moats,” or H. horribilis var. volcanicus ‘Mulch Moat.’
Why do these horticultural horrors appeal to people? Tree moats take extra time. Mulch volcanoes cost more money. Both can cause slow tree death, and perhaps that's the problem: They don't kill trees right away!
Leave it to the Bard to provide the perfect metaphor through the Messenger in "King Henry VI Part III", Act 2, scene 1:
"But Hercules himself must yield to odds;
And many strokes, though with a little axe,
Hews down and fells the hardest-timbered oak."
Volcano mulch and mulch moats are little axes. If they immediately caused trees to die, people wouldn't do it.
Who keeps producing mulch moats and mulch volcanoes despite all horticultural reasoning? We contend it’s the work of the horticultural villain known as The Grim Mucher (G.M.). Perhaps shame will work where reasoning has failed.
The Good, The Bad, and the Ugly
The Good
Organic mulch, such as 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, and suppresses weeds. Of course, mulch also enhances landscape aesthetics.
As the organic mulch decays, it contributes to the organic content of the underlying soil, which in turn supports soil biota and ultimately improves soil structure. Soil organisms exude sticky compounds that “glue” soil particles together to create soil aggregates (peds). Macropores in the aggregated soil bolster drainage and improve oxygen infiltration.
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.
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.
The Bad
Although I can find no studies detailing the deleterious effects of mulch volcanoes, I believe an accurate picture can be drawn from a range of scientific studies, along with observations. The observations are presented in photographs in this Alert, and the studies are listed under “Selected References.”
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.
If trees are too deep in the soil, dormant buds within the bark on the buried trunk are induced to produce so-called adventitious roots that grow from the trunk. Indeed, the same mechanism is exploited to root tree cuttings.
The production of adventitious secondary roots on trees that are too deep in the soil is a survival strategy to grow roots near the soil surface to acquire oxygen. The result can be the production of a new, elevated secondary root system.
Although bark mulch may initially appear light and airy, it can ultimately compact as it degrades, behaving much like soil. As a result, adventitious secondary roots are produced that grow into the mulch volcano. It mirrors what happens when trees are too deep in the soil.
As shown in the images below, the production of secondary roots that infiltrate the volcano mulch starts quickly once the mulch is piled high on the main stem. The first image below shows maples planted and volcano-mulched in 2016. The second image was taken before the trees were re-mulched in 2022. You can see that elevated adventitious roots had invaded the volcano mulch in less than five years.
The secondary roots of trees positioned too deeply in the soil are found just beneath and parallel to the soil surface. However, the secondary roots infiltrating the mulch volcanoes are elevated high above the surrounding soil. The secondary root system ultimately acquires the same confined shape as the volcano mulch, which limits the area available to the tree for water and nutrient uptake.
The graphic below shows the stepwise progression of the development of an elevated root system in volcano mulch. Secondary roots growing into mulch piled high on the trunk will eventually encounter the slopes of mulch volcanoes. Although root cells require an external oxygen source, they can't grow into thin air! Some roots grow downward to create an elevated root system. Others will encircle the tree trunk. If these errant roots merge with the stem tissue, they become stem girdling roots by gradually girdling the trunk to restrict vascular flow.
Symptoms of stem girdling roots include flat sides directly above the compression zone. This is an irreversible 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.
The image below was taken in 2020 and shows a tree in the landscape of a hotel. It was one of several maples planted in a row that had been subjected to consecutive years of volcano mulching. The stem girdling root peeking through the mulch on the right side of the stem makes it apparent that the volcano mulch was having a negative effect. On a side note, this tree was not planted too high! The hotel is not far from my home, and I’ve observed the landscape since the trees were installed.
The image below, taken in 2025, shows the same tree, although from a slightly different angle. The hotel closed around the time I took the first image, and the trees in the landscape were never re-mulched. Consequently, the bark mulch eventually decayed, revealing a heavily mulch-modified tree root system.
Changes to a tree’s root system induced by mulch volcanoes can remain throughout the life of a tree. The image below shows the row of trees in the hotel that have had their root systems irreversibly reshaped by stratospheric mulch volcanoes. The previous hotel perpetrator was no doubt an acolyte of the G.M.
As shown in the images below, there is no practical way to correct the malformed, elevated root system without causing substantial, possibly lethal, injury to the tree. The ghost of volcano mulch will remain for the shortened life of the tree.
Finally, organic mulch eventually disappears through decay and oxidation. This is why we periodically reapply mulch. However, as the volcano mulch structure degrades, the elevated secondary roots that have infiltrated the mulch become exposed, making trees more susceptible to drought stress.
The Ugly
The excavation of mulch moats is discretionary. However, it’s an option offered enthusiastically by the G.M. at no additional cost because the synergistic impact, along with volcano mulch, will hasten the demise of the tree.
Studies 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. Of course, roots are constantly at risk of dehydration during droughts.
Final Point: Don't Get Stoned
I’ve focused on the detrimental effects of piling organic mulch high onto the main stems of trees. However, bark mulch presents other risks that can inflame landscape managers.
It’s important to remember that, as with dry organic duff in forests, dry bark mulch is flammable. Positioning smokers in a sea of mulch is just wrong.
As a solution, the G.M. recommends simply stoning trees and shrubs. Of course, while the inorganic mulch is non-flammable, it can make unwatered trees and shrubs that die and dry out flammable.
Selected References
Arnold, M. A., McDonald, G. V., & Bryan, D. L. (2005). Planting depth and mulch thickness affect establishment of green ash (Fraxinus pennsylvanica) and bougainvillea goldenraintree (Koelreuteria bipinnata). Arboriculture & Urban Forestry, 31(4), 163-170.
https://auf.isa-arbor.com/content/isa/31/4/163.full.pdf
Chalker-Scott, L. (2007). Impact of mulches on landscape plants and the environment—A review. Journal of Environmental Horticulture, 25(4), 239-249.
https://www.conservationtreecare.com/uploads/1/1/5/9/115910725/chalker_scott_2007_mulch_review.pdf
Day, S. D., Wiseman, P. E., Dickinson, S. B., & Harris, J. R. (2010). Contemporary concepts of root system architecture of urban trees. Arboriculture & Urban Forestry (AUF), 36(4), 149-159.
https://auf.isa-arbor.com/content/isa/36/4/149.full.pdf
Gilman, E. F., Harchick, C., & Paz, M. (2010). Planting depth affects root form of three shade tree cultivars in containers. Arboriculture & Urban Forestry (AUF), 36(3), 132-139.
https://auf.isa-arbor.com/content/isa/36/3/132.full.pdf
Hauer, R. J., & Johnson, G. R. (2021). Relationship of structural root depth on the formation of stem encircling roots and stem girdling roots: Implications on tree condition. Urban Forestry & Urban Greening, 60, 127031.
Perry, T. O. (1989). Tree roots: facts and fallacies. Arnoldia, 49(4), 3-29.
https://arborcaresolutions.com.au/treerootfacts.pdf
Sun, X., Ye, Y., Liao, J., Soromotin, A. V., Smirnov, P. V., & Kuzyakov, Y. (2022). Organic mulching increases microbial activity in urban Forest soil. Forests, 13(9), 1352.
https://www.mdpi.com/1999-4907/13/9/1352
Sun, X., Zhao, J., Wang, G., Guan, Q., & Kuzyakov, Y. (2023). Fine root extension in urban forest soil depends on organic mulching. Agroforestry Systems, 97(2), 235-247.
https://link.springer.com/article/10.1007/s10457-022-00801-3
Wells, C., Townsend, K., Caldwell, J., Ham, D., Smiley, E. T., & Sherwood, M. (2006). Effects of planting depth on landscape tree survival and girdling root formation. Arboriculture & Urban Forestry (AUF), 32(6), 305-311.
https://auf.isa-arbor.com/content/isa/32/6/305.full.pdf
