Why we mulch in autumn, not spring

Worms, frost protection, weed suppression — three reasons your borders prefer an autumn coat.

Most gardening books tell you to mulch in spring. And if your primary goal is to suppress weeds, that advice makes sense — a thick layer of mulch in March covers the soil before the annual weed seeds germinate.

But we mulch in autumn, and here is why. First, autumn mulching protects the soil from winter frost. A 10-centimetre layer of composted bark or green waste compost acts as an insulating blanket, keeping the soil a few degrees warmer than the air above. This matters for the soil biome — the fungi, bacteria and invertebrates that do the real work of building healthy soil.

Second, autumn is when earthworms are most active. They are pulling organic matter down into the soil profile at a rate that drops off sharply once the soil temperature falls below 5°C. Mulch applied in October gives them six to eight weeks of peak activity to start incorporating it.

Third, by the time spring arrives, autumn mulch has settled, partially decomposed, and formed a coherent layer that suppresses weeds just as effectively as a fresh spring application — but with the added benefit of six months of soil improvement already underway.

The one exception: we do mulch in spring around newly planted perennials, where the priority is moisture retention during the critical first growing season.