Master Gardeners: Can trees and shrubs die from over- mulching?

It’s difficult to pinpoint when the beneficial gardening practice of mulching trees and shrubs started to become a problem for the very plantings it was meant to help. The two most common mistakes are top dressing old mulch annually when it isn’t needed and routinely mounding mulch up against the bark of trees. Communities are at risk to lose the tall tree canopies that we treasure because of these practices.

Richard Hentschel, University of Illinois Extension Educator in Horticulture states, “What happens with volcano mulching, which is particularly seen on single stem trees (where mulch is mounded up the trunk), is that the trees can typically plug along for 10 or so years before they really become distressed. They do show signs of stress along the way – perhaps they put on only 2 to 4” of growth per year when a healthy tree would have put on 12” per year. Tree trunks and their bark are meant to be exposed to air. Period. When trunks are mounded with mulch that is deep and typically never dries out, the trunk stays wet and rot begins.” With rot, diseases and insects are able to enter the tree, dramatically limiting its life expectancy. Deep mulch, greater than 4”, which is continually wet can lead to root rot of the underlying plant and limits oxygen penetration down to the soil.

Mounded tree trunks often exhibit the growth of secondary roots which can be seen growing throughout the mulch. If the mulch is wet enough to support roots, the bark is really in trouble. “When multi-stemmed shrubs are over-mulched the crown of the plant is covered. This will limit the number of new branches put out by the shrub and is particularly a problem on fine textured shrubs like spirea and potentilla,”notes Hentschel.

“To create a sustainable soil, one that best supports the plants by supplying nutrition to them from the soil, you need to feed the soil with organic matter. And that organic matter is mulch,” Hentschel emphasized.

Mulching done correctly will enable trees and shrubs to thrive for years. ‘Make a donut, not a volcano’ of mulch around the plantings is a wise saying. Limit the depth of the mulch to 3 to 4 inches and replenish only when necessary. Composted hardwood bark mulch is an excellent, efficient selection.

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Julie Moore has been a Master Gardener volunteer with the University of Illinois Extension in DuPage County for 10 years and has a degree in Ornamental Horticulture from the University of Illinois.