The standard advice on winter mulch is: mulch your perennials in fall to protect them. This is not wrong. It is also incomplete in ways that can kill plants.
Mulch applied at the wrong time, at the wrong depth, or on the wrong plants is not neutral. It creates conditions for crown rot, delays spring emergence, and provides nesting habitat for voles. In zones 7–9, heavy winter mulch on some perennials causes more damage than the winter would cause without it.
The contrarian framing here is warranted by primary sources, not by contrary disposition.
What winter mulch actually does
The purpose of winter mulch is specific. It is not to keep the ground warm indefinitely. It is to prevent rapid, repeated freeze-thaw cycling of the soil — the physical force that heaves plant crowns out of the ground.
Per Missouri Botanical Garden's mulch factsheet: "Mulches reduce heaving of soils from frost. By loosening the soil structure, mulches lessen the tension between water molecules that form ice in the subsurface. Soil ice increases pressure which tears plant crowns and roots."
The mechanism is physical, not thermal. Mulch does not keep the soil from freezing in zones 3–5 — it freezes anyway. Mulch moderates the rate at which the soil freezes and thaws, preventing the repetitive expansion and contraction that would otherwise push shallow-rooted crowns above soil level where they desiccate and die.
The second purpose — insulation for marginally hardy plants — is relevant at zone limits. A plant rated for zones 4–8 being grown at the cold edge of zone 4 benefits from mulch insulation.
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