In agriculture and gardening, a cold frame is a transparent-roofed enclosure, built low to the ground, used to protect plants from adverse weather, primarily excessive cold or wet. The transparent top admits sunlight and prevents heat escape via convection that would otherwise occur, particularly at night. Essentially, a cold frame functions as a miniature greenhouse to extend the growing season.
Historically, cold frames were built to be used in addition to a heated greenhouse. The name itself exemplifies the distinction between the warm greenhouse and the unheated cold frame. They were frequently built as part of the greenhouse's foundation brickwork along the southern wall (in northern latitudes). This allowed seeds to be germinated in the greenhouse and then easily moved to the attached cold frame to be "hardened-off" before final planting outside. Cold frames are similar to some enclosed hotbeds, also called hotboxes. The difference is in the amount of heat generated inside. This is parallel to the way that some greenhouses are called "hothouses" to emphasize their higher temperature, achieved either by the solar effects alone or by auxiliary heating via a heater or HVAC system of some kind.
