Tree health problems might erupt from mulch ‘volcanoes’
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There’s a long list of good reasons to mulch newly planted trees and shrubs: Mulch conserves soil moisture, moderates soil temperature and keeps weeds and grass from encroaching.
“However, as with most cultural practices, too much of a good thing can have negative effects,” said University of Missouri horticulturist Chris Starbuck. “In many cases, the mulch is simply piled up around the trunk to form a ‘volcano.’ And while a mulch volcano is generally better than no mulch at all, volcano-induced problems can have serious effects on plant health.”
In mulch more than about 4 inches deep, roots tend to migrate up into the mulch when it is moist, he said. The cause is partly the temporarily favorable root growth conditions in the mulch and partly suffocation of deep roots from mulch-induced waterlogging of the underlying soil. “Then, when drought conditions occur, the plant may come under severe stress because many of its roots are growing in a material with much less water-holding capacity than real soil,” Starbuck said.
Mulch volcanoes also can kill new trees with their “umbrella” effect, he said. Fungal activity on the surfaces of volcanoes can make them water repellent, and they act as “very effective umbrellas, shedding water to the surrounding turf.” The problem is more common in high-carbon mulches like sawdust, wood chips or ground wood, he said, but can also occur in bark mulches.
“Until a newly planted tree can grow roots out of the original soil ball and into the surrounding soil, the tree is absolutely dependent on moisture in the original ball,” Starbuck said. “If the ball is kept dry by a mulch-volcano umbrella, the tree will suffer severe drought stress during the establishment period.”
Mulch volcanoes can also promote fungal canker diseases on the lower trunk, stress from poor gas exchange by the cells in the bark and “damage from rodents that may make take up residence in the volcano,” he said.
“Go for a walk in the woods to see how Mother Nature does it,” Starbuck advised. “Trees in their native habitats rarely have individual mulch rings.
Also, the mulch layer is rarely more than 2 inches thick — and never in the form of a volcano.”