(Spring 2004)

It’s impossible to say where this crazy practice of “volcanic mulching” began, but one thing is certain: commercial landscapers have been doing their best year after year to keep killing trees in home landscapes, corporate sites, even on public lands. Even if you tell landscapers to change the way they mulch your trees, you’re fighting an uphill battle. This practice will never stop until landscapers come to their senses and lay the law down for their crews.

First, let’s explain what landscapers have been doing wrong!

The area where the tree trunk enters the ground is critical to tree survival. This area must be kept devoid of mulch and soil to allow oxygen to enter tree roots where they diverge off the trunk and enter the soil. If we mulch this area around the trunk, or cover it with soil, we prevent oxygen from entering the roots, also the evaporation and dissipation of water. Over a period of five or six years, this rotting of roots ultimately kills the tree.

Other life-threatening problems happen because of over-mulching.

Root Suffocation. You wouldn’t think that mulching a tree trunk would suffocate tree roots, but it does. Roots absorb oxygen through the air. If anything prevents oxygen from reaching the roots, suffocation begins.

Disease. Because the mulch encircling tree trunks is always wet, bacterial diseases have the perfect environment to multiply and enter the bark and the white cambium layer of tissue where microscopic xylem tubes transport nutrients and water vertically throughout the tree, and phloem (floam) tubes move energy from tree foliage to the roots. Diseases can easily spread into the cambium layer, choking off the flow of food and water to tree leaves and needles, ultimately killing the tree.

This wrongful “volcanic mulching” of trees has been around so long that homeowners have accepted it as “bible horticulture,” thinking that if landscapers do it, then this is the way homeowners should protect their trees. As soon as possible, you want to correct these and other mulching mistakes so you grow healthy trees again. If you work for a company where contractors maintain the landscape, you want to bring this wrongful “volcanic mulching” to the attention of company officers so the practice can be stopped.

In the series of photographs later on this data sheet, you will recognize the proper and improper ways of mulching trees. We photographed trees at different sites, including shopping malls which are notorious for volcanic mulching. Every spring, landscape crews pile more mulch atop what they installed the prior year, this having given rise to the term volcanic mulching.

There is at least one photograph lacking mulch of any kind. Pay special attention to the “flare roots” exiting the trunk at the ground and entering the soil. You will find flare roots on most trees: thick roots on mature trees, thin roots on immature trees. In every case, these flare roots must never be covered (no mulch, no soil, nothing). Flare roots must be kept bare at all times. Oxygen entry to flare roots must never be prevented.

If you are to mulch your trees, then how should it be done?

Yes, you can apply mulch near tree trunks (never shredded mulch), but away from tree trunks. The area eight inches away from the base of tree trunks must be kept bare at all times. Measure this bare zone atop the soil, then you can lay down a two or three-inch layer of chunky mulch (no more) to encircle the trunk at the same distance. You can orchestrate the mulch so rainwater flows toward the trunk area, not away from it.

We recommended chunky mulch. Why?

Chunky mulch repels water, so rains will enter the soil at the point of contact. Oxygen readily flows through chunky mulch, thereby eliminating chances of disease outbreaks. The only shredded mulch recommended around tree trunks is cedar mulch because it naturally repels water. While costing more, cedar mulch will last for upwards of a dozen years before it decomposes and must be replaced. All other shredded mulch should be avoided!

Finally, many homeowners rely on landscapers to manage shrubs and trees. If you find “volcanic mulch” around your trees, don’t talk to the crew about how you want them to mulch your trees. Instead, tell the owner of the landscaping firm that your trees aren’t being mulched properly by his crew and that if the situation continues, you will find another landscaper who will set things straight.