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Coldframes, or cold frames, are a small version of a greenhouse that allow a gardener to extend the growing season either end of the warmer, frost-free seasons. They are a transitional device rather than a permanent device, as it is expected to either remove the plants for transplanting to the garden or to protect the plant at the end of its growth cycle when the days get colder. It is called a coldframe because it is intended to be cooler than a greenhouse.
A coldframe is usually made from a timber frame from the ground up (using timer that won't rot in contact with the damp soil and elements), to which a top of framed glass is added (often a recycled window). The window top should be easy to open and a stick, woodblock or other stand is used to hold the window top open to allow for airflow when the day is warm.
A coldframe can be used to grow seedlings and to harden them off (acclimate them) before transplanting.
It is important to open the window top when the days are warm as the sunlight hitting the glass can create too warm a microclimate for the plants if left in place. It is important to close it at night too, to reduce convection. The gardener must include opening and closing the window top as part of the daily gardening routine when it is in use.