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A Merry Gardener

Horticulture for daily living
volcano mulch

Volcano Mulch


The tension is building in the hearts and minds of gardeners as the wet and cold weather continues to delay most outdoor garden activities. Before the weather breaks and releases the surge of pent up garden energy, I want to get on my soapbox for a moment to attempt to turn the tide against one of the worst practices landscaping that I continue to see everywhere I look.

Well-meaning homeowners and landscapers seem to understand that mulching around trees is a good idea, but have no concept of what proper mulching looks like.

Exhibit A: Volcano Mulch

What is wrong with this picture? This is a classic example of volcano mulch; the trunk of the tree erupts from the center of a tall, sloping mound. Mulch applied in this fashion spells death for the tree. How can a little pile of mulch kill a tree, you ask?

Mulch, as you likely know, is great at retaining moisture. Imagine now, what happens when mulch is applied directly in contact with the bark of the tree. The mulch holds moisture against the bark, which causes the bark to begin to rot. Left in this condition, the rot will eventually go through the outer layer of the bark into the cambium layer, the vascular tissue of the tree. Rotting through the cambium cuts off the flow of water from the roots and kills the tree. Beyond the potential for rot damage, mulching up against the tree trunk also invites insects and rodents like voles and mice to burrow in around your tree and start chewing on the bark.

Needless to say, volcano mulching is a harmful practice.

Now that I have elaborated on this improper mulching technique, I will give you some pointers for how to mulch around your trees properly.

1) Leave a two-inch gap between the trunk of the tree and your ring of mulch.

2) Apply the mulch 2-4 inches deep. You will likely need to make your application up to two inches deeper than your target depth to allow for settling. In other words, applying 4-6 inches of mulch will result in a stable 2-4 inch layer after the mulch has settled.

3) Make your ring as wide as possible/practical. Ideally, you would mulch all the way out to the dripline of the tree.

Proper mulching makes a tremendous difference for maintaining a healthy tree. Changing the soil environment around your tree from turf to mulch mimics the natural environment of the forest floor. Adding mulch reduces the competition from turf grass for water and nutrients, and helps to keep the soil moist in between watering or rain events. The mulch will also serve as a slow-release fertilizer. As soil microbes gradually decompose the mulch, they release nutrients into the soil. All of this adds up to great benefits for the health of the tree.


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