What Is a Mulch Volcano?

You've likely seen mulch volcanoes around your neighborhood, but in case you haven't, they're "a cone-like pile of mulch around the base of a tree trunk,” says Eric North, urban forestry program manager at the Arbor Day Foundation in Lincoln, Nebraska. “The mulch is touching the trunk with the trunk coming out of the top of the volcano.”

Mulch volcanoes are a relatively recent phenomenon, likely stemming from a desire to thwart weed growth. The reason, according to North, is a lack of education and training in the proper methods for tree planting and care. “Landscape companies that do not specialize in tree care likely lack the simple but important training on how to properly plant trees and care for them."

“Unfortunately, when homeowners see improper mulching from a landscape company, they may assume mulch volcanoes are the right way to mulch and repeat the practice,” he concludes.

How Mulch Volcanoes Kill Trees

Mulch volcanoes don’t kill a tree immediately. Instead, they contribute to a slow decline over many years. When mulch is piled against a trunk, it traps moisture, causing rot and inviting insects and diseases, which may be the ultimate cause of a tree's decline. Also, when piled against the trunks of young trees, mulch produces heat as it breaks down, injuring or killing the inner bark.

Additionally, mulch volcanoes can cause trees to develop secondary roots that eventually encircle the tree, choking off the main roots. According to North, this has the same long-term consequences as planting a tree too deeply, causing issues with root growth and tree stability over time.

“When trees are planted too deeply, they can form stem-girdling roots that can kill a tree, a process often taking several years to decades. I’ve seen large, mature trees fall on cars and houses during calm weather days due to improper planting and stem-girdling roots.”

How to Mulch a Tree Correctly

“The purpose of mulch is to retain water for roots, suppress weeds near the trunk, protect the trunk from lawn equipment, and provide organic matter for healthy soils,” North explains. The key is to make sure it’s done correctly.

“Improper mulching is an unprofessional and bad practice for all tree species,” North says. “Trees that are likely to exhibit the highest negative impacts are young, newly planted trees and species with a high tendency to produce sprouts and suckers (fast-growing new growth arising from the base of the trunk or roots).” Examples include apples, crabapples, maples, and linden trees.

The type of mulch matters. Shredded leaves are among the best mulches, as they imitate natural conditions. Wood chips are a good alternative.

“Organic mulches such as wood chips, bark or leaves, when applied correctly, typically offer more benefits for trees and landscapes than inorganic mulches such as rocks, plastics and manufactured materials,” North says. He points out that in some exceptions, such as deserts or fire-prone areas, inorganic mulches might be preferable.

Equally important is how the mulch is distributed. North advises applying organic mulches "at the base of the trees like a dog dish or donut” rather than mounding it up like a cone around the trunk. “No mulch should be touching the trunk, the mulch ‘donut’ center.” This ring of mulch will help direct irrigation water where it’s needed. As for the depth, North recommends a 3- to 4-inch layer for coarse-textured mulches like wood chips, 1 to 2 inches for fine-textured mulches like double-shredded bark.

Other Mulching Mistakes to Avoid

• Using landscape plastic or landscape fabric under mulch. “Mulch, especially organic mulches, should be in direct contact with the soil to allow water and nutrients to reach tree roots,” North says.
• Removing and replacing “old” mulch. “When organic mulch breaks down, it adds organic matter to the soil, improving soil conditions and benefiting roots,” North explains. Instead of removing old mulch, he recommends adding new mulch to the top, keeping the total depth between 2 and 4 inches.
• Allowing fine-textured mulches to “crust over.” This impedes the ability of water and oxygen to reach the soil. Apply in light layers and use a garden rake to break the surface of fine-textured mulches that bind together.
• Assuming landscapers know all the ins and outs of mulching. “My suggestion is to hire landscapers or tree care companies that employ ISA-certified arborists,” North says. “Companies that offer tree care services typically have the right education and training to properly plant and care for trees.”