How does bark mulch affect soil temperature

Bark mulch works like a woollen jumper for soil, tempering wild swings of heat and chill. In summer, sun-baked rays strike the chips first; their loose texture and pale hues reflect a portion and scatter the rest sideways. Beneath the layer the temperature rise is slowed, so roots bask in gentle warmth rather than a midday scorch. The mulch also shades the surface, cutting evaporation; water held in pore spaces cools the profile further through slow evaporation when the air is hottest.

As night approaches the story reverses. Soil normally radiates heat into a clear sky and drops abruptly, but bark’s trapped pockets of air act as insulation, releasing warmth drip-feed fashion. On a cloudless April dawn, ground covered by five centimetres of mulch often measures two or three degrees warmer than bare beds—a margin that shields early seedlings from frost burn and keeps beneficial microbes active. Longer term, fungal threads hollow the chips, increasing porosity and strengthening this insulating quilt.

AHS LTD recommends refreshing the layer annually to maintain depth, as compaction reduces the thermal buffer. By keeping extremes at bay, bark mulch turns the upper root zone into a temperate chamber, smoothing the season’s highs and lows.