Evaluating damage resulting from volcano mulching
2010 | Gary Watson, Morton Arboretum
This study examined whether “volcano mulching”—piling mulch high against a tree’s trunk—creates conditions that cause serious disease in landscape trees. Volcano mulching is widely considered poor practice because it keeps the trunk covered and may encourage fungi, decay, or damaging roots. However, the researchers noted that the claimed connection between volcano mulch and widespread tree disease had not been well tested.
Researchers first surveyed 837 public trees in 38 neighborhoods in the western Chicago suburbs. They looked for established trees with at least 8 inches of material mounded around the trunk, ultimately selecting about 83–84 trees for detailed study. The tree types included Freeman maple, river birch, honeylocust, linden, and some crabapples.
One unexpected finding was that many apparent mulch “volcanoes” were not actually made mostly of mulch. In many cases, the mulch layer was relatively thin and covered a mound of soil. These soil mounds appeared to result from trees being planted too high or from soil gradually accumulating when the surrounding turf was edged. Both the mulch and the soil beneath it were generally well aerated.
Approximately 80% of the surveyed trees showed no disease symptoms beneath the mulch. Seventeen of 83 trees showed possible symptoms such as cankers, discolored wood, oozing, wood rot, or fungal growth. Researchers collected samples from 12 symptomatic trees and attempted to identify the fungi present. Traditional laboratory identification was difficult, so DNA sequencing was used for 83 fungal samples. Fungi with some disease-causing potential were found in crabapple, maple, linden, and honeylocust samples, but no known pathogenic fungi were isolated from river birch.
The researchers then tested selected fungi by deliberately placing them into small wounds in healthy trees. Trees were grown either with proper mulch depths or with mulch piled over the trunk to simulate volcano mulching. Both container-grown and field-grown trees were included. After approximately one year, the trees were cut and examined externally and internally.
The inoculated trees did not develop the extensive cankers, oozing lesions, dieback, or death that would indicate severe disease. Most wounds closed within a year, and the trees generally appeared healthy. The most common internal response was wood discoloration. Overall, 205 of 362 experimental trees, or 56%, developed some discoloration.
Volcano mulch did appear to increase the likelihood of discoloration after the trunks were wounded. Among trees that were wounded but not intentionally inoculated with fungi, discoloration occurred in 59% of volcano-mulched trees versus 25% of properly mulched trees. When suspected pathogens were added, discoloration occurred in 70% of volcano-mulched trees versus 43% of properly mulched trees. The researchers could not determine whether the added discoloration came from fungi in the mulch or from the tree reacting differently to a wound covered by mulch.
Tree species also responded differently. Discoloration occurred in approximately 74% of maples and honeylocusts, compared with only 14% of lindens. Field-grown and container-grown trees produced similar results. Another notable concern was that about one-third of the surveyed landscape trees had girdling or encircling roots, which can constrict the trunk and may become a more significant long-term structural or health issue.
Most Important Findings
- Volcano mulch increased internal wood discoloration following trunk injury, especially when fungi were introduced.
- The fungi recovered from landscape trees were generally not aggressive pathogens on the tree species from which they were collected.
- Experimental trees developed few visible disease symptoms; none died or experienced dieback.
- Most surveyed trees—about 80%—had no visible disease symptoms beneath the mulch.
- Many apparent mulch volcanoes were largely soil mounds rather than deep mulch.
- Girdling and encircling roots were found on roughly one-third of the surveyed trees and may represent a separate concern.
- Volcano mulching remains an improper practice, but the study’s results did not support the claim that it commonly causes widespread, life-threatening disease in established landscape trees.
The practical takeaway is that mulch should still be applied correctly—spread in a relatively shallow layer and kept away from direct contact with the trunk. However, this research suggests that the disease risk from volcano mulch may be less severe than commonly claimed, although it can increase tissue discoloration and may contribute to other tree-health problems.
For more information on this project, contact the researcher via TREE Fund at treefund@treefund.org.