Applying too much mulch around trees can restrict oxygen in the soil just like planting too deep. In heavy clay soils, thick layers of mulch may result in waterlogged soils. Mulch piled against the trunk may cause it to decay, allowing plant pathogens to enter and produce cankers. Mulch depths of 3” to 4” are adequate. Avoid piling mulch directly around the trunk. Taper the mulch depth around the trunk to about an inch or so.
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