Over mulching Trees | Mulch Volcanoes Kill Trees

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Robert Pavlis

As Penn State puts it: “Mulch Volcanoes are Erupting Everywhere“, and they are killing trees. It is a slow death, but one that is almost inevitable. Gardeners and landscape professionals see over mulched trees everywhere and they copy the method, not knowing that they are mulching incorrectly.

The main cause of death has been attributed to increased trunk rot, but that is a myth. This post will debunk this myth, explain the real cause of death, show you how to mulch trees and shrubs properly and how to fix the problems caused by mulch volcanoes.

huge cone shaped pile of mulch around a thin tree
An extreme mulch volcano. Notice that all of the trees in the background are also over mulched, source: Paul Hetzler, Adirondack Almanack

What is a Tree Volcano?

A tree volcano is created when mulch, usually wood chips, is piled high around the trunk of a tree, forming a high cone as shown in the above picture. I’ve never heard a logical reason for making them, but one homeowner told me that they like the look of it! I suspect, landscape contractors order too much mulch and have no better place to put it.

Notice that the wood mulch used in the above picture is very fine textured – that is not a good option for mulching trees. Coarse material is better.

Growing Great Tomaotes, by Robert Pavlis

How Does Over Mulching Kill Trees?

There are four different explanations for tree deaths:

  • Tree bark rots under the mulch.
  • Fungal pathogen growing in the mulch infects trees
  • Dry conditions under the mulch prevent root establishment.
  • Girdling roots.

Rotting tree bark is the most popular explanation, but it’s wrong. The other three are possible issues.

Excavating a Mulch Volcano

Ever wonder what is under a mulch volcano? In this video I exhume a tree that has been over mulched for several years.

YouTube video

Mulch Volcanoes Cause Trunks to Rot

The common belief is that mulch volcanoes stay too wet and keep excess moisture next to the trunk of the tree. This moisture causes the trunk to rot which seems like a very plausible explanation. The interior of the mulch pile does stay wet and it keeps the trunk wet. Rotting pathogens do like a moist environment. But just because something seems to make sense, does not mean it’s true.

I looked for a study that compared rot in mulch volcanoes to unmulched or properly mulched trees. I did not find any.

I then reached out to Dr. Arthur James Downer, University of California. He researches the pathology of landscape ornamentals, root rots and mulches. He confirmed that no one has studied the effect of excess moisture to see if it caused rot under an over mulched tree. I then connected with Dr. Joe Boggs, Ohio State University Extension. He is also an expert in ornamental trees and mulching, and has written numerous articles about mulch volcanoes. He confirmed that there is no study that shows trees are harmed by the excess moisture. He had just given a presentation at the Ohio Tree Care Conference (OTCC) where they discussed mulch volcanos and tree rot. The consensus was that they do not cause rot in trees. Dr. Boggs has examined many such trees and has never seen one with rot.

There seems to be no study on this topic and field experience by professionals indicates that mulch volcanoes do not cause rot on the trunk.

Read about more mulch myths in: Wood Chip Mulch Myths.

Mulch Volcanoes Grow Fungal Pathogens

Moist wood is a perfect place for fungi to prosper and it has been proposed that pathogenic fungi such as Phytophthora cinnamomi would prosper in the mulch and thereby infect the trees. There is little evidence that this happens. The current thinking is that saprophytic fungi living in the mulch grow even better and they produce enzymes that are toxic to root rot pathogens. That white growth you see in mulch is actually fighting pathogens and keeping your tree safe.

Microbe Science for Gardeners Book, by Robert Pavlis

Can properly applied wood mulch transmit disease to trees?

Infected needles and shoot tips collected from diseased Austrian pines and then used as mulch transmitted Sphaeropsis tip blight to healthy saplings in the field. Heat treating the mulch did kill the pathogens. Botryosphaeria canker and Armillaria root rot diseases were not transmitted to redbuds or oaks when heavily infected wood chips were used.

In an effort to understand how well pathogens survive in wood mulch, one study burred infected pieces of wood in wood mulch. At various intervals the pieces were retrieved and tested. Thyronectria austroamericana, the causal agent of Thyronectria canker in honey locust, survived and was viable after 10 weeks. Survival was not affected by moisture levels but pieces of wood laid on top of the pile were less infectious than those buried in the pile.

Pathogens can and do persist in wood chip mulch. Aging the mulch does reduce the number of organisms in the mulch, which may lower the chance of infection. I found no clear evidence that a mulch volcano would increase the chance of infection, but it does expose more of the tree to pathogens.

Soil Under Mulch Volcanoes is Dry

One of the benefits of mulch is that it keeps moisture in the ground and that is quite true. However, once the soil under the mulch is dry, that same mulch prevents water from light showers and light irrigation from reaching the soil. This problem is worse in deeper mulch piles.

New plantings have all of their roots under the mulch volcanoes for a couple of years. A dry condition can prevent proper establishment and in extreme cases, kill the newly planted tree.

Girdling Roots Grow in Mulch Volcanoes

A girdling root is one that goes around the trunk of the tree, or at least passes close by the trunk. Woody roots are different than herbaceous roots in that they get thicker and woodier with time. The trunk does the same thing. That means that both the girdling root and trunk continue to get thicker and soon the two are squeezing each other, literally to death. You may know that all of the water and nutrients travel up the tree in the xylem which is located just under the bark. As the trunk gets strangled, this transport slows and eventually stops sending water and nutrients to the upper part of the tree. The tree can usually overcome some strangling, but in more severe cases the tree dies.

How do girdling roots form? Planting too deep so that the root flare is below ground is a very common cause. Such planted trees will make roots that grow up to the surface and then out in all directions. If some of these grow back towards the trunk, you end up with girdling roots.

Learn how to plant a tree correctly: Planting Trees the Right Way

tree with no mulch showing girdled roots around the trunk
Girdled roots caused by a mulch volcano, source: Joe Boggs

Trees that are planted at the correct depth and then buried in a mulch volcano act just like trees that are buried too deep. Roots grow into the mulch and then form girdled roots.

The biggest problem with mulch volcanoes is that they produce girdled roots that eventually kill the tree.

Is Colored Wood Mulch Safe?

Some wood mulch is colored red, brown or black and there is concern over the chemicals used to make these. The chemicals are not a problem since they are food dyes. However, it does take energy to make the dyes, ship them around the country and spray them on wood. That means dyed mulch is not as sustainable and they cost more. Why use them?

There are other concerns with these. The material is usually shredded finer than regular wood chip mulch and the source may come from reclaimed lumber; old skids, lumber from old homes, etc. These materials may be contaminated with chemicals and bits of metal. Colored mulch is just not a good option.

tree with circle of wood chips around it
Properly mulched tree, source: Eli Sagor

Proper Way to Mulch Trees

Select a good quality wood chip mulch that comes from trees that are not obviously diseased (if possible). If you are unsure of the source, let it sit for a couple of months before you use it. The mulch should consist of large chunks of wood. Stay away from finely shredded material because it repels water more than coarse material.

  • The finished mulch layer should be no more than 2-3 inches thick.
  • It should not touch the trunk of the tree.
  • Make the circle of mulch around the tree as big as possible.
  • In dry weather, water the soil heavily before adding the mulch. Mulch laid on dry soil keeps roots dry.

Fixing Mulch Volcano Problems

Remove the excess mulch as soon as possible and have a look for roots above the soil line.

If there are no roots growing above the soil line, mulch properly as described above and the tree should be fine.

If roots are already growing above the soil line but they haven’t grafted to the trunk (i.e. became stuck to it), cut all roots at the soil line and remove them. Add mulch but keep it well away from the trunk of the tree. You don’t want to encourage the cut root ends to grow into mulch.

If the problems are more advanced and some of the roots are grafted to the trunk, the removal process is much more complicated and each tree will be different. In general, you want to sever the roots, remove them if possible, while at the same time doing limited damage to the trunk. This is easier with smaller young roots and much more difficult with large older roots. The following pictures provide advice on removing roots in different stages of grafting.

tree with two smaller girdling roots
These girdling roots are still small and are just starting to graft to the trunk. They are fairly easy to remove, source: Maggie McCain

The root in the above picture is growing around the trunk and it’s starting to graft in one spot or maybe two spots, but the graft seems fairly new and not well established yet. In this case it is best to remove the whole root as far back as possible. Try to cut the root near its start and pry it off the trunk. Chainsaws, reciprocating saws and even chisels can be used for this.

larger girdling root showing two cut lines along the root.
Larger girdling root that is starting to graft to the trunk, source: Missouri Botanical Garden

The root in the above picture is larger and more developed. It is quite possible that it has grafted to the main root under it. Cut the girdling root at two spots as indicated by the purple lines. Once the left and right sections are removed you should be able to remove the center piece, but it might take some force. Try to damage the trunk as little as possible.

girdled root that is half engulfed by the trunk
This girdled root has been there a while and is well grafted to the trunk, source: U. of Maryland

This girdled root is well grafted to the trunk and is already being engulfed as the trunk grows. Removing it entirely will cause a lot of damage. It is best to make two cuts similar to the previous tree, to remove the left and right ends of the girdled root. Leave the remaining section attached. It will either rot and fall away on its own, or the trunk will engulf the whole piece.

base of the trunk is enlarged to form a bulb shaped growth
Girdling root has cut off much of the movement of liquids up and down the tree which results in the development of a bulge above the root, source: U. of Florida

The trunk of this tree has been strangled for quite some time. The girdling root is barely visible but you know it is there because the trunk has expanded right above it. The tree can no longer send sugars from the leaves to the roots. They are accumulating at the strangle point and forcing the trunk to expand. In this case the damage is too far gone and probably can’t be repaired. It might be best to remove the tree. If you want to try and save the tree all of the strangling root needs to be removed.

Answers to Your Mulch Questions

Q1. What does it mean to mulch a tree?

Mulches are materials placed over the soil surface around a tree to help it grow better and to keep equipment from damaging the trunk of the tree. Organic mulch will improve soil, keep moisture in the soil and keep down weeds.

Q2. How does mulch affect soil?

Mulch has a significant effect on soil as detailed in Mulch – How Does It Affect Soil?

Q3. What is a major downside to mulching?

Provided it is done correctly, there is no downside except that it does have to be renewed every few years because it decomposes. The upside to this is that the decomposition adds organic matter to soil, thereby improving it.

Q4. When is the best time to mulch?

You can mulch at any time of the year. It is best to add mulch when the previous mulch layer is 1″ or less.

Q5 How far out from the trunk should you mulch?

As far as possible. The wider the ring of mulch is, the better it is for the tree. Very large circles may not look as good aesthetically.

Q6. Should you put landscape fabric under the mulch?

Never. Landscape fabric does not stop weeds, but it does prevent water from reaching roots. It also shows over time and looks ugly.

Q7 Does cedar mulch repel pollinators and other insects?

Cedar mulch does not repel most pollinators but it may repel some insects like ants, but it also attracts others. Overall, cedar mulch is a good option, provided it is available locally. Don’t buy it if it was trucked a long distance to get to you. Try to use local material.

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

7 thoughts on “Over mulching Trees | Mulch Volcanoes Kill Trees”

  1. Thank you for drawing attention to this. One objective of forestry advocacy efforts in my city are correcting the volcano mulching practices of arborists contracted by the city to care for its trees.

  2. Much of the problem seems to stem from advice to apply mulch annually, which people follow without regard for whether more mulch is actually needed. I’d estimate I apply an inch of partially composted, chipped hedge cuttings & branches every 2-3 years & that’s sufficient to control weeds without any build-up.

  3. I’ve seen this issue over and over again with contractors either building new homes or flipping homes and who don’t call in professionals to do the planting and mulching. There are several homes in my area with dead shrubbery and trees because they were buried in mulch.

  4. I thought there was something about “air roots” at the base of the trunk that required air in order for the tree to survive and that is why it is not recommended to plant a tree deeper than when you purchase it from the landscape retailer or over mulch close to the trunk. Is this incorrect?


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