hot bed

A hot bed is a garden bed constructed over a heat source, typically decomposing manure or other organic matter, to raise soil temperature for early season growing. The heat generated by microbial breakdown warms the soil 10 to 20 degrees Fahrenheit above ambient air temperature, allowing gardeners to start warm-season crops weeks before the natural frost date ends. The bed is usually enclosed with a wooden or concrete frame and covered with a glass or plastic light to trap heat and moisture.

Traditional hot beds use fresh horse manure, which generates sustained heat as it decomposes over 4 to 6 weeks. A typical hot bed frame stands 2 to 3 feet deep and measures 6 feet by 4 feet; the manure layer occupies the bottom 18 to 24 inches, topped with 4 to 6 inches of topsoil or growing medium. Modern variants substitute heated cables buried in sand or soil, thermostat-controlled electric mats, or hot water pipes connected to boilers, giving the operator more precise temperature regulation and longer effective seasons.

The main constraint is temperature management. Without ventilation, internal temperatures can exceed 100 degrees Fahrenheit on sunny days and kill seedlings; growers must prop open the light or tilt the frame during warm afternoons. Conversely, sealed beds cool rapidly on cloudy days or at night, negating the heating benefit. Soil moisture also accumulates quickly under the closed cover, raising disease pressure from damping off and fungal rot if air circulation is inadequate.

Hot beds fall into decline in commercial horticulture because greenhouses with climate control are more reliable and scalable. However, they remain practical for small-scale market gardeners and home growers in cool climates who need to extend the growing season for tomatoes, peppers, eggplants, and cucumbers without the capital cost of heated structures. The term derives from the obvious warmth underfoot and the earlier British reference to a "hotbed" as any site of intense activity or fermentation.