Cold frames are rather underrated by many gardeners. They are
not only useful in their own right but they can also be used for most of the jobs that are undertaken in the greenhouse. They are less
expensive than greenhouses, take up less space and are cheaper to keep warm. Their main disadvantage is that the gardener works
outside and not inside in the warm and dry as with a greenhouse.
In the vegetable garden, cold frames are
frequently used for producing winter or early crops of such vegetables as carrots. The frame can be in a permanent position in the garden or moved, rather like a large cloche, onto the vegetable bed itself. The vegetables can be grown either directly in
the soil or in growing bags. Later in the year, the cold frame can be used for growing cucumbers or melons.
Another basic use is to afford protection and warmth to trays of seeds or seedlings. Once the plants are ready to go out, the lights can be opened over a period of a week or two to harden off the plants before they are planted out.
As with greenhouses, the cheaper cold frames are made from aluminium. Their advantage is that they are light enough to move around, but they are not good at retaining heat. Wooden ones are better at this, and cold frames with solid walls made from brick, concrete or even old railway sleepers (ties) provide much better protection during the winter.
Aluminium-framed cold frames can be designed to include glass in the sides, which allows in more light. Solid-sided ones are much warmer but light can enter only through the glass above. It is a good idea to paint the inside of the walls white to reflect some of the light.
Lights (lids) that are glazed with glass are generally preferable, but plastic can be used where there is danger of accidents – if children or elderly people are in the garden, for example.
Most cold frames, as their name suggests, are not heated. However, it is easier and cheaper to provide some warmth than in a greenhouse, and if you want to propagate or overwinter tender plants it may be possible to supply some form of heat. Electric heating cables installed in the sand below the pots and around the walls of the frame is the easiest method. It is also the most efficient if the cables are connected to a thermostat that switches the electricity on only when heat is required.
A traditional way to heat cold frames is to set them on a pile of farmyard manure, usually horse dung. As the dung breaks down it releases more than enough heat to keep the frames warm. Soil can be laid on top of the manure and a wide range of vegetables grown in it during the winter. The manure should be fresh, and once it has rotted down and no longer generates heat, it can be spread on the garden and dug in.
It is easy to insulate cold frames because they are small. The simplest way is to throw an old carpet over the frame on cold nights. This may be sufficient to hold in the residual heat, so no extra heating is required to keep the frame above freezing. More efficient methods would be to cover the frames with bubble polythene (plastic) or even to line the inside of the lights with it.
When it is not necessary to keep the frames tightly shut to avoid heat loss, it is sensible to open them slightly, even if it is just a crack, to let air circulate among the plants. This helps prevent various fungal diseases, especially botrytis, which cause seedlings to die through rotting.