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Beware of the over-mulching epidemic

A method of mulching, often referred to as “volcano” mulching, slowly impacts tree health and vigor.
A method of mulching, often referred to as “volcano” mulching, slowly impacts tree health and vigor. Photo provided

I saw it just the other day — landscape crews hard at work mulching trees, or should I say, over-mulching trees. Mulch is piled against the trunk of the tree rather than spread out in a thin layer. This method of mulching, often referred to as “volcano” mulching, slowly impacts tree health and vigor. Trees would be much better off having the same amount of mulch spread out across a larger area.

However, there is an epidemic spreading throughout this area and across the state that is slowly killing trees and shrubs in our landscapes. No, it isn’t another disease or insect we accidentally imported from another country, like Asian Longhorned Beetle, which is killing trees in New York City and Chicago, or Dutch Elm Disease, which changed our landscapes in the 1960s and ’70s. Unlike some of these imported pests, this epidemic can be prevented easily.

This epidemic is caused by misapplication of mulch around our trees and shrubs. We are over-mulching our trees and shrubs to death. Mulching is a terrific way to add organic matter and nutrients, conserve soil moisture and prevent lawn mowers from injuring trees and shrubs, but it is just being put on way too thick and piled too high on trunks and stems.             

When mulches are put on too thick and piled against the stems of trees and shrubs, they begin to suffocate roots and create a moist environment in which opportunistic decay fungi such as Phytophora, Armillaria and Leptographium attack the trunk and roots, causing root rots, a decline in plant health, crown dieback and tree failures. Besides causing the roots and stems to rot, over-mulching prevents the movement of oxygen and carbon dioxide in and out of stems. It can lead to rodent chewing and stem girdling, nutrient deficiencies and the production of toxic organic acids and often causes roots to grow up into thick mulch, only to dry out in hot summers, or form girdling roots that encircle the trunk.

A quick walk in the woods will illustrate how trees have a natural flare where their trunks meet the soil (visible even on young trees). It is important that we not cover that flare with soil or mulch. Spread the mulch out in a layer that is no thicker than 2-3 inches, and don’t pile it up on the trunks of trees and stems of shrubs.

Mulching your trees and shrubs can improve soils and grow healthy plants, but too much of a good thing can be harmful. So take a closer look at your mulch or your landscapers work this summer and make sure you don’t have mulch-mountains or “volcanoes” in your landscape. Please, let’s stop this epidemic before it kills more trees.

Dave Jackson is a forestry educator at Penn State Extension. Contact him at drj11@psu.edu.

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