Cold frames are useful because they offer extra space near your greenhouse and are extremely useful especially in spring to harden off plants raised in the greenhouse but also during the other seasons to grow a wide range of crops. In the colder months they are used to protect winter flowers, to overwinter the seeds of hardy annuals sown in autumn and to shelter vulnerable alpines from the worst of the wet weather.
The most popular type of cold frame has glass or clear plastic sides as well as glass lights – frame tops with glass panes. Sometimes timber and brick are used for the construction of cold frames. The glass-to-ground models usually have a metal framework to sustain the glass.
There are different types of frame lights. Choose a frame that has removable or sliding lights for an easy access to your plants, or even a model that also have sliding front panels, useful for extra ventilation. Hinged lights that are wedged open still provide protection from heavy rain but sliding lights are in most cases removed completely during the day living this way the plants unprotected from heavy rains.
Another type of cold frame is the traditional timber frame. You might not find them nowadays for buying but you can build them your own from second-hand timber. This way you will have them even cheaper. The wooden sides retain heat well and it is not very difficult to fix inside the frame some soil-warming cables to provide extra warmth. To preserve the wood to make it last longer you should paint or stain it.
Another type that is rarely used nowadays is the brick frame. You can also build it at home especially if you have some old bricks leftovers and you are handy enough to make the lights. Brick frames have the advantage that they are warm and draught-proof.
Widely available and not very expensive are the aluminium alloy frames. They are usually sold packed flat and assembled on site. They allow more light than wooden or brick frames but don’t have such good insulation qualities. They also may not be as strong or robust as the other two. Being so lightweight you should use ground anchors to fix them.
The size of a cold frame should be about 1.2 m by 60 cm, but in most of the cases you must fit it into whatever space is available, preferable as close as possible to the greenhouse, so choose the largest affordable frame that fits the space you have. The height of the frame is important especially for plants grown in pots or for tall vegetables. Temporarily raise the frame on loose bricks if needed.
In very cold weather insulate your cold frame using sheets of expanded polystyrene or bubbler plastic. When sharp frost is predicted use some extra protection by covering the outside of the frame with layers of hessian or an old carpet. Remove the coverings during the day to allow light to the plants. If you cannot remove the cover over the day than use several layers of thick, clear plastic sheeting or bubble plastic for extra protection. They will not reduce the light to the same degree as a carpet or hessian.
In warm weather good ventilation is essential. Most frames have lights that may be wedged open to allow fresh air inside. Some frames have lights that slide along to allow more ventilation or are completely removable. This is useful for young plants that need to be hardened off.
Light is also important in a cold frame. If you have a wooden or brick frame is best to place it where it will receive the maximum amount of light in winter and spring. Aluminium frames may be moved around the garden to take advantage of the best light at various times of the year. In very hot weather, during the summer, cold frames need shading. For year-round use find one that lets in as much light as possible.
As glazing material you can use glass or plastic. Glass is the best one as it transmits light well, allow the frame to warm up quickly and retain heat better than most plastic materials. Always replace any broken or cracked panes. Where glass may be a potential danger for children or animals use plastic glazing material.