Recently, I’ve seen several situations where trees are dying, with rotting around the lower trunk.  Several things can cause this, including diseases and injury.  But sometimes, it’s because the homeowner has created a mulch volcano.

Now mulch, in itself, is a good idea.  Mulch keeps weeds down.  It reduces water needs by reducing evaporation.  And depending on the type of material used for mulch, it may add nutrients for your plants to grow, as it decomposes.  So mulch is a good idea.  But mulch volcanoes are not.

First of all, let me explain what a mulch volcano is not.  You may have seen a large pile of grass clippings in someone’s yard smoldering away.  The grass clippings have reached a critical mass, and the pile has heated to the point that spontaneous combustion has started.  The pile may actually smolder or burn down until all the grass clippings are gone.  While there are many reasons why it is a bad idea to let this happen, this isn’t what I mean by a mulch volcano, even though it may look like a volcano made out of mulch.

 The term “mulch volcano” is the way Dr. Chris Starbuck, the University of Missouri State Woody Ornamental Specialist, likes to describe the practice of piling up mulch against a tree or other woody ornamental plant, to the point that it is way too deep for the good of the tree.  Sometimes, you can see this done to the point that the mulch is conical in shape, hence the term “volcano.”

What’s the problem with a mulch volcano?  Anytime you pile mulch on too deep, too close to a tree, you will end up burying the tree’s bark.  This allows moisture, which the mulch does a good job of retaining, to stay in constant contact with the tree’s bark.  Few trees can handle this.  Bark is not made to be surrounded by constant moisture.  It’s meant to be exposed to air, so that it can dry out after a rainstorm.

If there is too much mulch, this interferes with gas exchange around the bark.  The conditions may actually go anaerobic.  Conditions like this may favor the development of pathogenic fungi which may attack the bark.

A tree in this situation may eventually start developing major disease problems.  The bark may become diseased.  If it gets to the point that the bark actually starts to come off the tree, you may be in danger of losing your tree.

Another problem is the potential for roots to grow upward into the mulch.  If the mulch later does dry out, then the roots will suffer, and perhaps die.  This obviously puts a stress on the tree.  Dr. Starbuck has also seen cases where roots growing upwards into a mulch volcano have actually girdled the tree, and killed it.

A final problem with mulch volcanoes is that certain kinds of mulches may develop a waxy coating with age, which sheds water.  This means that water falling on the mulch is not reaching the soil underneath the mulch, but running off.  This can put the tree under drought stress, especially if the tree is young.

 So keep mulch away from the tree.  A small amount of mulch next to the tree is fine, but place it in such a way that the tree’s bark is not covered up.  As the distance increases from the tree, you may deepen the layer of mulch, as desired.  And keep the mulch stirred up from time to time, to make sure that water is getting through the mulch layers to the soil beneath.

A similar situation that I have sometimes seen is where people actually bury the tree’s bark with soil by making significant changes to the level of their yard.  This is even worse than a mulch volcano.  Bark was designed to be in the air.  Roots were designed to be in the soil.  Bark covered by soil is asking for trouble.  It’s a recipe for disaster.   It’s best to maintain the original soil line of the tree, as much as you can.  Otherwise the tree may be placed under severe stress, and you may eventually lose the tree.