By Enrique Arayata
October 24, 2024
The kwanzan cherry tree in question.
Recently, I was talking to one of my neighbors about typical home and outdoor activities, which sparked the conversation about the kwanzan cherry tree in my front yard. He let me know that it was mulched incorrectly, which caught me off-guard seeing as I took mulching advice from my coworkers Lindsey, an ISA Certified Arborist®, and TJ, an ISA Board Certified Master Arborist®. I kept an open mind and asked him what was wrong and he informed me that it’s best to form a volcano-like shape up towards the trunk of the tree so that way the tree will absorb more water and to increase curb appeal. For him, since it looks more intentional and less natural, it shows that someone is caring for the tree. I communicated to him that multiple coworkers as well as the Arborists’ Certification Study Guide have conflicting information. He said he’s been doing it this way for decades and everything has been fine, completely ignoring the fact that his 50-60 foot tall sugar maple tree was removed due to heavy decay at the base of the tree. We agreed to disagree and went on with our days. Now, I’m not saying mulch volcanoes will cause your tree to decay and be removed, but I am saying that it is detrimental to a tree’s health and that this method of mulching needs to be addressed, stopped, and changed.
A mulch volcano. Note the lack of an exposed trunk flare and the mulch exceeding the height of the surrounding grade.
So what is a mulch volcano? A mulch volcano, also sometimes known as a mulch mound, mulch pyramid, or a mulch shield, is a circular volcano-shaped pile of mulch above ground level formed up to the trunk of a tree. Mulch volcanoes are created with good intention as tree mulching adds nutrients to the soil, aids in water retention, helps the tree outcompete other plants for resources, protects the tree from trunk damage by lawn mowers and string trimmers, minimizes soil compaction, and increases curb appeal. However, all of the aforementioned benefits apply only to properly mulched trees and mulch volcanoes are an improper form of mulching that eliminates most of these benefits. Most of the time, the person applying mulch to the tree does not even know about this incorrect practice. So if you have ever created a mulch volcano, know that I am not upset or mad. My goal is to educate and discuss the harms of mulch volcanoes and the proper ways to mulch a tree.
An example of stem girdling roots.
Limited Oxygen and Stem Girdling Roots
Mulch volcanoes bury the roots of a tree and simulate it being planted too deeply. This limits the availability of oxygen and water which can cause stem girdling roots, roots that wrap around the trunk of the tree in search of oxygen and nutrients and inevitably “choke” the tree. Stem girdling roots can lead to dieback, stunted growth, and delayed or shrunken leaves popping out in the spring. This can be treated via root collar excavation.
Limited Water
For a mulch volcano with compacted mulch, less water is actually retained and runs off to the bottom perimeter of the mulch. This results in less water availability for the tree as it may run off elsewhere on the property or be absorbed by grass and other plants instead of by the tree.
Excess Water and Bacteria
For better or for worse, mulch is great at retaining water. In the case of mulch volcanoes with porous mulch, this is bad because water can sit and dampen the trunk. Sitting water is bad for a tree because it can carry bacteria that can lead to disease.
Soil pH
Overmulching in this fashion may also limit nitrogen availability and detrimentally alter the soil’s pH. A change in pH outside the ideal range for the tree can lead to nutrient deficiency, chlorosis, stunted growth, defoliation, dieback, or even worse, the death of a tree.
Another mulch volcano. Note the tip dieback on the right side that could be a result of the mulch volcano. An in-depth inspection would be needed for confirmation.
Rodents
When mulch is piled high, it may be enticing to rodents as a form of shelter and concealment. They have also been observed to feed on the lower stems and bark of trees, leaving open wounds that could invite decay, disease, and pests.
Aesthetics
Mulch volcanoes can make a trained eye who knows proper mulching practices cringe similar to nails on a chalkboard. They convey that the tree manager or mulch installer may be unaware of the best care for the tree. Depending on who you ask, mulch volcanoes are subjectively an eyesore and look goofy or out of place.
A tree mulch ring 12 feet in diameter.
Proper Tree Mulching
So how do you properly mulch a tree? Apply a level 2 to 4 inch deep layer of mulch at least 6 feet in diameter or up to the canopy drip line of a medium-to-larged sized tree. The exact size will depend on your preference or landscape limitations. Keep the root flare exposed and not in direct contact with any mulch. If water runoff is a concern, a small 1 inch tall, wall-like berm can be formed by hand around the perimeter of the mulch to encourage water to remain inside of the mulch. Linked below is my previously written Arbor Ed article on how to properly mulch a tree. It contains a video and even additional tips on how to create a crisp edge for enhanced visual appeal! I strongly encourage you to check it out. You can even follow along with the video when mulching your own tree! For those in central Ohio looking for other tree care needs, one of our 20+ ISA Certified Arborists® are happy to provide a free tree inspection and service quote!
Additional Arbor Ed Articles
Sincerely,
Enrique Arayata I Media Production Coordinator, Russell Tree Experts
In his free time, Enrique enjoys working out, hanging out with his girlfriend, video production and photography, cars, technology, cooking, and watching new movies and YouTube videos. He has a BA in moving image production with a double minor in film studies and studio art specializing in photography from The Ohio State University, an AA in web graphic design from DeVry University, and is an FAA Certified Remote Pilot.