Hotbed
Structure
A hotbed is a traditional early-season growing bed heated by fresh, strawy manure as it decomposes. Built beneath a cold frame, it gives roots gentle bottom heat when outdoor soil is still cold. It is a practical way to raise early crops without electricity, provided materials and regular attention are available.
How it works
As fresh manure rots, microbial activity releases heat. A deep, firmly built layer beneath the growing surface can warm the compost and air under a frame for several weeks. The cold frame retains this warmth and protects plants from weather. Heat is strongest at first, then gradually declines, so a hotbed is most useful for early sowings and young plants rather than a whole season.
Best for
Use a hotbed for early lettuce, radish, spinach, spring onions and carrots, or for starting module trays of brassicas and flowers. It can also bring forward tender crops such as courgettes and cucumbers for later planting out. Choose it where spring soil stays cold or where a greenhouse is unavailable but a sheltered frame site exists.
How to use it
In late winter, make a pit or contained bed around 60 cm deep and fill it with fresh, strawy manure, firming it evenly as you build. Water lightly if it is dry, then cover with about 20 cm of compost or good topsoil and fit a cold frame over it. Allow the initial fierce heat to settle for several days before sowing. Check temperatures and moisture regularly, ventilate on sunny days, and use the bed while warmth remains steady.
Tips
Use fresh material that has not already composted, but handle it with gloves and keep the bed away from watercourses. A thermometer prevents sowing into excessive heat. Once the heat fades, the settled material can be incorporated into composting or used as an organic soil improver.
Hotbed: frequently asked questions
How does hotbed work?
As fresh manure rots, microbial activity releases heat.
How do I use hotbed?
In late winter, make a pit or contained bed around 60 cm deep and fill it with fresh, strawy manure, firming it evenly as you build.
What is hotbed best for?
Use a hotbed for early lettuce, radish, spinach, spring onions and carrots, or for starting module trays of brassicas and flowers.
General guidance for UK gardens and allotments. Page updated 18 Sep 2026.