The Dangers of Over-Mulching

When it comes to the practice of mulching, property owners should take a page from Hippocrates’ book: First, do no harm.

How Mulching Can Help

The benefits of mulching are many. The proper application of mulch will suppress weeds, help soil retain moisture, lower soil temperatures during periods of extreme heat, and slowly add nutrients to the soil as the mulch decomposes. Placing mulch around trees and shrubs also prevents mowers and weed trimmers from getting close enough to the plants’ trunks to harm them.

But bad mulching practices will harm your plants. So here are some valuable caveats…

Mulching Rule #1: Apply only 2-4” of mulch material to the bed.

Please, don’t go crazy with mulch. If you allow mulch to exceed four inches in depth, you can actually harm the plants by depriving them of water and air.

  • A too-thick mulch pile can become hydrophobic, or water repellent, which causes water to simply shed off the top layer rather than sinking in to the soil where roots can drink it up.
  • Too much mulch also reduces the amount of air in the soil below it, and roots need air to survive.
  • When too much mulch is applied over a tree’s root ball, the roots will grow up and into the mulch in search of water and air, causing a potentially deadly condition known as stem girdling.
  • Do not cover emerging perennials with mulch. Just as mulch suppresses weeds, it will also suppress any newly-emerging shoots of the plants you do

So heed the “no more than four” rule, and be sure to measure the mulch depth before you add to an existing mulch bed. A thin layer of mulch is necessary only every two or three years. If you want to freshen up a tired-looking mulch bed, a light raking will bring some of the more attractive darker mulch to the top.

Mulching Rule #2: Keep mulch away from the trunks of trees and shrubs.

Mulch Volcano

The dreaded Mulch Volcano (a.k.a., The Cone of Shame)

When mulching a tree bed, don’t let the mulch touch the trunk. The root flare must always be visible, whether the tree is newly planted or long established. Therefore, don’t subject your trees to the negligent and far-too-frequent practice known as “volcano mulching,” so called because the tree trunk looks like it is erupting out of a mountain of mulch. Since constantly moist bark loses its ability to protect the tree from pathogens, rodents, and insects, volcano mulching will stress the tree and cause the development of girdling roots, shallow root systems, and death of trunk tissue.

Example of properly mulched tree

Example of properly mulched tree. Photo credit Brock Eastlund.

To properly mulch your tree bed, start 12 to 18 inches from the trunk (regardless of the size of your tree), lay down a two-inch layer of mulch and gradually increase the depth to four inches as you move away from the trunk. In so doing, your tree will reap the benefits of the mulch, and you’ll have created a useful watering basin to nourish the root system below. Be sure not to cover the base of the tree or its root flare with mulch; otherwise, the tree could unwittingly be harmed.

Rule of Mulch #3: Work (quickly) to remedy past mulching mistakes.

Stem girdling

An arborist treats the stem girdling of an over-mulched tree.

If you have unwittingly broken the rules of good mulching, all is not lost. Pull excess mulch away from trunks of trees and shrubs and follow the mulching guidelines above. If your tree is already showing significant signs of stress, such as root stem girdling, call in your local arborist to help correct the situation and assess the root damage. By using specialized tools such as air spades, the arborist can deftly remove excess mulch and soil from the trunk and upper root system without doing more damage. The arborist will then carefully remove or sever the girdling roots to encourage a deeper root system. If this treatment is done in time, trees and shrubs will recover their health.

Before purchasing and applying mulch products, ask your SavaTree arborist for advice tailored to your landscape and the needs of your trees and shrubs.

 


Sources:

  • H. Christoph Stuhlinger and Tamara Walkingstick, University of Arkansas, Division of Agriculture, Research and Extension, “Ten Easy Ways to Kill a Tree (And How to Avoid Them),” accessed January 23, 2017, http://www.uaex.edu/publications/pdf/FSA-5011.pdf