Mulch the Right Way

When done right, organic mulch, particularly hardwood bark mulch, will provide a wide range of benefits to trees and other woody ornamentals. The 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 creating 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.

Mulch the Wrong Way

Mulch piled high on the main stems of trees has been called many names; some unsuitable for BYGL Alerts. The descriptive names for this horticultural horror include pyramid mulch, mountain mulch, and mulch mounds. I’ve always favored volcano mulch, or mulch volcanoes, for the stratovolcano-like creations.

The mulch monstrosities serve no purpose other than to waste mulch and provide proof the perpetrators do not care about long-term tree health. Indeed, the “Grim Mulcher” was first introduced in a BYGL Alert in 2018 as the Monarch of the Mulch Malefactors.

Mulch volcanoes produce a range of detrimental effects that wreak havoc on tree health from vascular strangulation to moisture starvation. Add in other stress-inducing issues and tree health can slip over the edge to slide towards tree removal and replacement. Of course, that’s the Grim Mulcher’s ultimate plan.

Digging Deeper: The Grim Mulcher’s Five-Step Assisted Herbicide Program

Although I can find no studies specific to detailing the deleterious effects of mulch volcanoes, I believe an accurate picture can be drawn from a range of frequently cited scientific studies along with observations. Those observations are photo-documented below. I’ve also included a selection of relevant studies under “Selected References.”

1. Use Mulch Volcanoes to Induce 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 oxygen concentration 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 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. It mirrors what happens when trees are too deep in the soil.

As shown in the images below, the production of secondary roots can start very early 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.

Unlike secondary root systems on trees that are too deep in the soil where the roots remain in the soil, the secondary roots infiltrating the mulch are elevated high above the surrounding soil. They ultimately 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 image above, taken two weeks ago, and the next two images. The picture below shows a volcano-mulched tree in the landscape of a motel in 2020. It was one of several maples that were being mistreated this way. Even then, it was apparent the mulch was having a negative effect: note the stem-girdling root. On a side note, this tree was not planted too high! The motel is not far from my home, and I’ve observed the landscape since the trees were installed.

The image below was taken two weeks ago and shows the same tree although from a slightly different angle. The motel closed around the time I took the first image and this and other trees in the landscape were never re-mulched. Consequently, the bark mulch eventually decayed to reveal a heavily mulch-modified tree root system.

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 adverse long-term effects are shown in the images below. As with the maples in the motel landscape, these trees were not planted too high. They were just subjected to years of mulch volcanoes! The perpetrator was no doubt an acolyte of the Grim Mulcher. However, the damaging mulch practice was stopped when the HOA changed landscape management companies. Unfortunately, the damage was already done, and the problem has been inherited by the new company.

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 the mulch volcano will remain for the shortened life of the tree.

2. Cut the Roots

This step is optional. However, if applied with great enthusiasm, the synergistic effect in conjunction with volcano mulch will hasten the opportunity to select and plant a new tree.

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. Dehydrate the Roots

Although organic mulch is highly beneficial to the overall health of trees, the mulch will eventually disappear through decay and oxidation to expose bare soil. If the mulch is not periodically replaced, the loss of support for beneficial microbes means soil aggregation suffers lessening the development of good soil structure that helps to moderate wide swings in soil moisture.

Of course, as the organic mulch degrades, the secondary roots growing into the mulch 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.

4. 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.

5. 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 commonly get blamed if a tree succumbs, not the volcano mulch that served as an accomplice to the tree's demise. It may require a deeper diagnostic investigation to recognize that volcano mulch was a criminal partner.

Sap-sucking insects such as soft scales or the new to the scene Spotted Lanternfly (Lycorma delicatula) present another opportunity for the Grim Mulcher to shift the blame. While not considered outright tree killers, both feed by withdrawing copious quantities of sap. The sublethal effects of the sap-sucking insects combined with the chronic stress induced by the erupting volcano mulch may push trees over the lethal edge. Of course, the Grim Mulcher returns for the harvest.

Is there a Sixth Step?

Some BYGL readers may assert that the Grim Mulcher missed a possible sixth step: 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 by plant pests and pathogens to the living tissue beneath the bark.

There is a common belief that mulch piled against tree trunks can retain water elevating the moisture content of the bark to make it susceptible to decay and opportunistic borers. I also held this belief and set out to photo-document the damage. However, despite excavating volcano mulch around dozens of landscape trees, I’ve yet to observe and photograph such damage.

Of course, I've commonly observed substantial bark cracking and sloughing on volcano mulched trees; however, my excavations consistently revealed stem girdling roots. The connection between errant stem-encircling roots and the death of stem phloem has been documented in numerous studies. I’m not claiming bark rot never occurs, I’m simply speculating that much of the damage stems from an indirect impact of volcano mulch.

Final Point: Don’t Get Stoned

I’ve focused on the detrimental effects of piling organic mulch high on the main stems of trees. However, as you see in the image below, the Grim Mulcher is also testing the effects of stoning trees and shrubs.

Selected References

Perry, T.O., 1989. Tree roots: facts and fallacies. Arnoldia, 49(4), pp.3-29.

Arnold, M.A., McDonald, G.V. and Bryan, D.L., 2005. Planting depth and mulch thickness affect establishment of green ash (Fraxinus pennsylvanica) and bougainvillea goldenraintree (Koelreuteria bipinnata). Journal of Arboriculture, 31(4), p.163.

Wells, C., Townsend, K., Caldwell, J., Ham, D., Smiley, E.T. and Sherwood, M., 2006. Effects of planting depth on landscape tree survival and girdling root formation. Arboriculture and Urban Forestry, 32(6), p.305.

Chalker-Scott, L., 2007. Impact of mulches on landscape plants and the environment—A review. Journal of Environmental Horticulture, 25(4), pp.239-249.

Day, S.D., Wiseman, P.E., Dickinson, S.B. and Harris, J.R., 2010. Contemporary concepts of root system architecture of urban trees. Arboriculture & Urban Forestry (AUF), 36(4), pp.149-159.

Gilman, E.F., Harchick, C. and Paz, M., 2010. Planting depth affects root form of three shade tree cultivars in containers. Arboriculture and Urban Forestry, 36, pp.132-139.

Hauer, R.J. and 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, p.127031.

Ron Wilson