A coldframe is indispensible in the garden as it gives the gardener a means to gradually introduce plants that have been grown in a warm greenhouse to a much cooler outdoor environment, a process generally referred to as hardening off.
Cold Frames have been used for centuries,in fact the first glasshouses were no more than glorified cold frames.
These would often consist of glazed panels laid on stone, or brick walls to protect tender plants.
To supplement the heat in these enclosures the gardener would lay a layer of farmyard manure in the bottom of the frame.
The heat generated by the decomposing manure would often be sufficient to keep the frames frost free.
A coldframe is generally a permanent structure unlike a cloche,which is usually a tranportable form of protection.
Having said that, many modern forms of alloy coldframes can be moved around or readily dismantled if needs be!
Where practical, locate the coldframe so that the sloping side is facing south to allow maximum light into the frame.
The coldframe serves number of needs;
1) Is probably the most important function as it provides an intermediate stage between greenhouse and planting out outdoors, a process commonly known as Hardening off.
2) It can provide a sheltered, relatively warm environment for germinating seeds that do not require high temperatures to initiate germination.
3) It can provide a sheltered place to grow slightly more exotic plants that otherwise would not survive the variable UK weather patterns.
4) Slightly tender plants and hardy plants that do not like the wet UK winters e.g. Alpines,can be over-wintered in them.
Types of Cold Frame
Or they may use use salvaged materials such as old timber pallets and old window frames.
The advantages of timber sides is that the timber help to keep the warmth of the day in the frame overnight.
The Aluminium frame with glass sides.
The advantage of having glass sides is that this allows more light to enter the frame, thus avoiding spindly stems.
Sadly, unlike the timber sided versions, they do not hold the heat as well overnight.
The Aluminium frame with poly-carbonate sides.
*Polycarbonate comes in double and triple layer versions.
Polycarbonate does not allow as much light through as glass, but is far a better insulator than glass, and it is not as fragile or dangerous if broken.
Because the light is diffused by the material, the risk of scorching in sunny weather is reduced.
The polycarbonate types are generally accepted as more child friendly compared to glass versions.
Circa Week 46: Frames with glass or rigid plastic sides will not give as good protection against frost as solid-sided models, so it is wise to provide extra insulation for the winter.
Cold frames should be covered at night, or lined with clear plastic to allow maximum light penetration.
Thick sheets of polystyrene or similar foam materials are ideal and can usually be cut to shape with a fine toothed saw or bread knife.
If pots are to be plunged to their rims in the base, form an open topped box with 18” (45cm) deep sheets of insulating material, insert the pot into the box and surround it with sand or similar material.
Keep a sheet of agricultural fleece at hand to throw over the plants at night if and when a dramatic drop in temperature is forecast, alternatively place a layer of newspaper over them.
Circa Week 24: Utilise the space taken up by the coldframe/s to grow semi-hardy plants that may require some protection in the UK.