Growing Green: ‘Mulch volcano’
Overmulching landscape trees is common.
This is most obvious when mulch extends up the trunk, smothering the root flare and root zone.
This practice, known as “volcano mulching,” results in a “mulch volcano,” and is never recommended and should not be utilized.
As beneficial as mulch is, too much mulch is harmful.
Deep mulch may suppress weeds, but it wastes time and money and can cause major health problems that lead to tree decline and possibly death.
There are many problems associated with overmulching.
Tree roots need oxygen to grow and function properly. When too much mulch covers the soil surface, air may not penetrate the mulch layer and the underlying soil becomes depleted of oxygen.
In addition, excessively deep mulch can inhibit water loss through evaporation. Once soil pore spaces become filled with water, diffusion of oxygen into the soil is essentially blocked.
When soil oxygen levels drop too low, root growth declines, making it impossible for the plant to take up water and nutrients.
Plant death may result if too many roots decline.
The inner bark, also called the phloem, carries photosynthates produced by the leaves to the rest of the tree. When mulch covers the root flare and trunk tissues, they stay constantly wet. This tissue is much different from root tissue and cannot survive under these conditions.
Continuous moisture also interferes with respiration by limited gas (oxygen and carbon dioxide) exchange between living cells in the trunk and the atmosphere. If wet conditions continue long enough, phloem tissue dies and roots are starved of essential carbohydrates.
Most fungal and bacterial diseases require moisture to grow and reproduce. Overmulching creates conditions where trunk diseases can gain entry through constantly wet, decaying bark, especially if there are trunk wounds under the mulch.
Once established, these plant pathogens can cause fungal cankers and root rots. Cankers caused by these diseases can encircle the tree, killing the inner bark, ultimately starving the roots, and possibly killing the tree.
Mulch piled against the trunk favors moisture-loving insects, such as carpenter ants and termites, which could colonize and expand decayed areas of the trunk.
Voles and mice may tunnel under deep layers of mulch for shelter. These pests may gnaw on the inner bark of young trees, girdling them. If girdling is extensive, tree death may result. This often goes unnoticed until the following spring when the tree doesn’t leaf out.
Similar to composting, thick layers of wet mulch may heat up once decomposition begins. Temperatures within mulch piles may reach as high as 140 degree. This high heat may directly kill the inner bark-phloem of young trees or delay the natural hardening-off period that plants must go through in preparation for winter.
If you believe you have a problem with overmulched trees, carefully dig with a hand trowel to assess mulch depth. Remember two to four inches of mulch is sufficient on well-drained soils, less on poorly-drained soils. A light raking of existing mulch may be all that is necessary to freshen old mulch and break through the crusted or compacted layers that can develop.
If mulch is piled against the trunk, visually look for the presence of the root flare where the tree meets the soil line. If the flare is buried, it is essential to uncover it. Begin by carefully pulling mulch back from the tree’s trunk until the root flare is exposed, taking care not to damage the bark.
A good rule of thumb is to pull mulch three to five inches away from young trees and eight to 10 inches away from mature trees. Spread excess mulch evenly out to the tree’s drip line, checking to ensure the depth does not exceed four inches.
Research has shown that most trees respond rapidly with improved color and vigor once the root flare is exposed and excess mulch is redistributed.
“Growing Green” is contributed by Lehigh County Extension Office Staff and Master Gardeners. Information: 610-391-9840, 610-813-6613.