Hotbed

Structure

A hotbed is a covered growing bed that uses bottom heat from decomposing organic material or another heat source beneath the soil. It is an older method that can provide extra warmth for seed starting or early crop growth. In modern home gardens, it is less common than cold frames but still useful where earlier soil warming is needed.

How it works

Heat rises from the warm layer beneath the bed into the root zone and enclosed air above it. That added warmth helps seeds germinate sooner and keeps growth moving during cold conditions. The cover on top also traps solar heat and shields plants from wind. Because both soil heat and air temperature can change, careful monitoring is needed to keep conditions steady.

Best for

Hotbeds are best for starting seedlings early, growing cool-season transplants, or gaining extra time in cold climates. They are especially useful where cold soil delays spring planting. Gardeners willing to monitor temperatures closely benefit most from this method.

How to use it

Build the bed in a sunny, sheltered location and add the heat-producing layer below a suitable growing layer. Allow temperatures to rise and stabilize before sowing seeds or setting in plants. Vent the cover as needed to prevent excessive heat buildup during sunny weather. Recheck conditions often because the heat source changes over time and may become less active.

Tips

Uniform moisture helps organic materials decompose more evenly when that is the heat source. Use a soil thermometer to judge when the bed is ready for planting. Plan for extra ventilation because hotbeds can warm faster than cold frames.

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General guidance for US gardens. Pair with your local frost dates and hardiness zone. Page updated Aug 4, 2026.