Volcano Mulching: Too Much of a Good Thing!

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Volcano Mulching: Too Much of a Good Thing!

Written by Linda Brandon, Extension Master Gardener I Volunteer in Guilford County

If you follow any of our writings here at Cooperative Extension, you probably know that we’re very keen on mulching landscape plants. If you’re new to our corner of the world, then a logical question might be, “What is mulch, anyway, and why are we even talking about it?”

Mulch is simply a layer of any substance—organic or inorganic—placed on the surface of the soil around landscape plants. (We’re not overly enthusiastic about inorganic mulches, but that’s a topic for another discussion.) A two to four inch layer of organic mulch–pine needles, wood chips, or bark that will decompose and actually improve the soil over time–can provide many benefits. It

  • reduces evaporation from the soil surface, cutting water use by up to 50%
  • promotes soil microorganism activity; those microorganisms, in turn, improve soil texture
  • stabilizes soil moisture
  • prevents compaction of soil
  • controls weeds, which can rob the soil of needed nutrients and moisture
  • moderates extremes of temperature
  • controls erosion
  • enhances the appearance of the planting, providing a finished look. (http://www.ext.colostate.edu/mg/gardennotes/245.html#benefits)

An additional benefit of mulching, especially around trees, is that mulch creates a “mow-free zone” around the base of the trunk. Since mulch controls the growth of weeds or turf close to the tree trunk, it eliminates the need to get close to the trunk with either a mower or a string trimmer. Both of those implements can cause substantial damage when they accidentally “ding” the tree trunk, cutting into the bark and creating ideal conditions for disease or insect problems. One little ding on a dogwood tree may cause no problems, but nick upon nick, ding upon ding, cut upon cut will add up to serious damage over time.

Wood bark or wood chip mulch is ideal for use around tree trunks. The old myths about wood chips tying up soil nutrients as they break down, thus causing deficiencies in plants, have been proven to be just those myths.  Linda Chalker-Scott, PhD, of the University of Washington, notes that “a zone of nitrogen deficiency exists at the mulch/soil interface, inhibiting weed seed germination while having no influence upon established plant roots below the soil surface.” (http://puyallup.wsu.edu/~linda%20chalker-scott/horticultural%20myths_files/Myths/magazine%20pdfs/Woodchips.pdf)

So, mulch is a good thing. But is mulch always a good thing? Emphatically not!

Take a look at the photos that accompany this article. Or do a Google Image search on “volcano mulching.” A “mulch volcano” is what results when you pile mounds of mulch right up to a tree trunk, often with a depression at the top to (theoretically) collect water. Since this approach looks like a volcano, the term has been widely adopted. While the top photo is more extreme than any mulch volcano I’ve ever seen in Guilford County, it’s certainly not beyond the realm of possibility.

         pic 1

Image from Metropolitan Forestry Services, Inc.                                   


first pic

So if mulch is good, what’s wrong with mulch volcanoes?

First off, they can cause the decline and death of young trees. It doesn’t get much worse than that. How does it cause this damage? Let’s look at some of the issues. First, mulch that actually touches the base of a tree is a problem, no matter its depth. Mulch can trap and retain moisture, and tree bark does not benefit from constant moisture. When the mulch remains moist, that moisture eventually penetrates the bark and suffocates the cells of the layer of tissue (phloem) that transfers food up and down the plant. Once that food supply from the leaves is compromised, the roots can die back, creating a vicious cycle in which insufficient water goes up the tree from the roots, too little nutrition goes down the tree to the roots, and so on. This can lead to a slow decline and eventual death of a substantial landscape investment.

Once the tree is weakened by the moisture/rot issues, secondary problems can develop. Insects, including borers, fungi, and bacteria, will take advantage of the stressed situation and start to inhabit the area.

The third main reason mulch volcanoes are dangerous to a tree may sound completely wrong, since we’ve just discussed moisture as a problem: a too-thick layer of mulch can actually keep water from reaching the soil under the tree. Imagine what it would take to get a rainfall to penetrate a foot or so of mulch, or think about how long a sprinkler would have to run to thoroughly saturate all that mulch and reach the soil below it. If insufficient water penetrates the mulch, feeder and secondary tree roots may actually migrate up into the mulch seeking water.  These roots grow above the tree’s main roots, and once they reach the drier edge of the mulch volcano, they can start to encircle the tree. This is simply not a good situation for tree growth.

So, what do you do if you realize that you (gasp!) have a mulch volcano in your yard? Don’t panic, but act as soon as you’re aware of it. It’s a simple process: using a shovel, trowel, or other tool, carefully get all of the mulch away from the trunk. Try very hard to avoid damaging the trunk in any way. You can blast the remaining bits and pieces away from the tree with water from a hose if you need to. If you see secondary roots in the mulch, cut them off. You should be able to see the trunk and the flare of the tree–the portion of the tree that widens as it goes into the soil.


pic 3         

Image from The Garden Professors


Once the tree is free of old mulch, you can start over with mulch—the right way—by allowing several inches between the trunk and the first bit of mulch. Think of surrounding the tree with a “donut” of mulch that leaves room for air and water to flow freely in the two to three inches immediately around the trunk.

Then add mulch to a depth of four inches or so out as far from the trunk as you want. If you have an area in your landscape with lots of trees and shrubs, you can save yourself the time, energy, and money you would normally waste mowing around the plants by converting the entire area to a “naturalized” planting; cover the entire soil surface under the trees and shrubs in mulch. You can create a natural-looking outline by using a garden hose to create lay out a pleasing shape for the bed. The mulch will help control weeds in the area and, since grass always loses in a competition for nutrients against trees and shrubs, you can make the most of the situation rather than fighting to grow a lush lawn in an area where it will never succeed.

Enjoy your landscape, and use mulch for all its many benefits, but remember to avoid those volcanoes!

Written By

Photo of Karen NeillKaren NeillCounty Extension Director & Extension Agent, Agriculture - Urban Horticulture (336) 641-2400 karen_neill@ncsu.eduGuilford County, North Carolina
Updated on Oct 7, 2016
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