Horticultural Horrors: Mulch Volcanology

There are few more terrifying garden situations than the dreaded mulch volcano: an ill-performed attempt to achieve… who the heck knows. They can be seen in many places, but are most common in public or commercial spaces, where uneducated landscapers and gardeners are hired to complete tree installation and regular maintenance duties. Despite their widespread use, these piles are very harmful to tree health, resulting in disease, decay, root rot, pest issues, and the eventual death of the tree. Also called 'Pyramid Mulch' and 'Mountain Mulch', these landscape atrocities are the bane of many arborists and horticulturalists who are called in after the fact to remedy the, in most cases, resulting hopeless situation.

The Down-Low on Mulch Volcanoes

It is difficult to truly pinpoint the exact time this landscaping trend emerged and why it has become so prevalent considering the catastrophic damage it does to trees. Some believe it began when HOAs and similar organizations (for example, builders of cookie-cutter housing developments) began simply dumping overly generous amounts of mulch on top of the previous years' layer, over time causing the level to rise. Others believe it is due to lazy landscapers who purchase mulch in bulk and are reluctant to carry the extra away, instead piling it up at the base of trees. It is, after all, organic material that will eventually be absorbed into the earth. In this specific situation however, more is definitely not better, and in many cases, it is much worse.

On the whole, mulch is a positive force in the garden, suppressing weed growth, maintaining consistent soil temperatures, and increasing the amount of moisture soil holds on to against the action of evaporation. It is used around the bases of many different plants, and also provides a clean, aesthetically-pleasing look to garden beds. Mulch is meant to mimic the natural leaf litter on the ground in native landscapes, and is an indispensable tool that most home gardeners use responsibly to increase plant health and overall soil quality and texture.

Most plants, including trees, can benefit from a layer of mulch, though typically this layer should only be about 1-2 inches in depth. When mulch is piled higher than this, it causes a cascade of problems. Naturally, trees will develop a root system that is visible above ground level and extends down into the earth. A natural layer of mulch around a tree is kept away from the trunk, and is a few inches deep. This allows for roots to access oxygen and water, for the bark to breathe, to allow moisture to evaporate from where it should not be kept, and to slowly add nutrients and organic matter to soil over time.

When trees are subjected to this type of volcanic environment, they undergo many issues. Having mulch touch any stem of any plant can invite fungal and pest diseases to take hold, but with trees, it also causes damage to an important organ: the bark. Tree bark is dry tissue which protects the inner layers of the trunk from pests, diseases, dehydration, and oxidation. It needs to be open to the air so that the lenticels (small openings in the bark) can freely exchange gases and so that any moisture can evaporate quickly. Mulch piled against bark can increase water retention, which elevates the moisture content in bark and causes decay.

Additionally, although mulch appears light and airy at first glance, it actually has a lot of mass and weight. When it is piled in large quantities, it creates a somewhat impenetrable layer that air and moisture have trouble getting through to reach the root system. Trees, however, can compensate for this issue, typically by extending their root system above the soil level to grow in the mass of mulch on top. This secondary root system can allow the tree to survive for a while.

It is easy to see this secondary root system when the mulch is finally removed, as pictured. The issue with having roots at this height on the tree trunk is that they can girdle the tree, causing it to cut off its own circulatory system and reducing vascular flow. 

Although trees can produce this stop-gap measure, over time as the mulch decomposes it makes the surface hydrophobic, which means it will not take in water. This causes even these above-ground roots to dehydrate. This causes the tree a lot of stress, and can ultimately cause it to fade and die. When it is vulnerable, it is more apt to develop issues with disease and pests, which can cause further and faster decline and even open other nearby plants to developing pest and disease issues. These pests and diseases often are unfairly blamed for the ultimate failing of the tree in the landscape.

After the Volcano

Fortunately, cleaning up the aftermath of a mulch volcano is much easier than cleaning up actual lava flow. The first step is relatively straightforward: remove the mulch. Using gloved hands and working from the trunk outwards, begin brushing away the mulch from the above-ground roots, similar to an archeologist uncovering a dinosaur. An air compressor can also be used for this purpose, though it can kick up a lot of dirt and dust into the air. Once most of the mulch is removed, take a few minutes to assess whether an above-ground root system has formed and how developed it is. Keep an eye out for any roots that do not grow straight out from the trunk, and instead wrap around the mass of roots:

These are called girdling roots, and as the tree trunk grows in diameter over time, it will eventually reach the girdling root and start to compress. This will result in an unstable tree base around the root flare and also restrict the flow of nutrients, photosynthates and water from leaves to roots and vice versa. Even if the mulch volcano is removed, these girdling roots will cause the tree to perish prematurely and be much less stable, increasing the odds of it being taken down during a storm. The bottom line is, these roots will need to be removed for the tree to have a chance at a healthy life. Using cleaned and sharpened pruners, loppers, and/or saws (depending on how large the roots have gotten) remove all roots above the main root structure. If girdling roots have merged with others, saw through the girdling root and avoid damaging the ones growing correctly. Take off any smaller roots at this time as well. The photographs below illustrate a before and after for a tree rescued from a mulch volcano:

After the chopping is complete, the root system should look clean of dirt and debris (any remaining mulch), and for the most part only have roots that flare out from the trunk, except for any portions of girdling roots that had to be left on because they fused to others. The next step in the process is to reapply a healthy layer of mulch at the base of the tree that does not cover this above-ground root flare.

Mulching Correctly for Tree HEalth

As stated before, mulch can improve the health of trees in many ways, and adding mulch around the base of trees has become an automatic step in installation. Mulch should not cover the main roots or the root flare (where the trunk begins to spread out and increases in diameter). It should be kept several inches away from the roots themselves, be between 2 and 3 inches deep, and have a level top surface. This depth allows both air and water to penetrate the mulch, but still allows for increased protection against temperature changes, evaporation of soil moisture and weed suppression. The somewhat naked-looking portions of the above-ground roots that were exposed by the removal of the mulch volcano will form bark similar to the bark on the trunk over time.

Paying attention to how trees and other plants are mulched and how healthy the mulch layer in a garden bed is is an important step in monitoring the health of a landscape. It is an integral part of good garden culture, and your plants will thank you.

Happy gardening!

Resources and References

Tree Care: Helping Trees Recover From Mulch Volcanoes

Treemergency: Mulch Volcanoes are Erupting in Landscapes