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Too much mulch can rot roots and stems, host insect pests and rodents, and cause nutrient imbalances in the soil. A layer 2-3″ deep is plenty. Over time, the layer of mulch disappears. Earthworms, freeze/thaw cycles, and other natural processes help to incorporate it into your soil, where it really works its magic. Compensate by topping it off with a new layer in winter.