My colleague-‘Allotment Bob’ has kindly offered to impart some 
advice on how to use a cold frame.
Using a cold frame at this time of year is essential if you want to grow your own vegetables such as Tomatoes and Butternut Squash, or salads and harden off seedlings or plant plugs. The weather is still cool at times and frosts are still likely, plus the soil has not warmed up yet.
Cold frames are designed to provide extra insulation for your plants, which will really help them to thrive. They not only keep your plants safe, but also provide the ability to regulate the temperature and amount of natural sunlight to keep your plants at their best.
Seedlings or young plants taken from the cossetted greenhouse and planted into the ground can certainly effect the growth or even damage them. The hardening off process introduces the plant to the outside gradually, enabling it to adapt and thrive in the outside world.
Take the plant from the greenhouse and place in the cold frame and open the lid for a few hours in the warmth of the afternoon. After a few days, leave the frame open all through the day, just closing it up at night or if it gets really cold-do not forget to water regularly.
By the end of a week to ten days you should be fine to leave the frame open day and night, just sheltering the plants from the worst of the cold. After a fortnight the plant can come out and go into the ground or cloches removed.
The ideal location for a cold frame is a southern or south eastern exposure with a slight slope to ensure good drainage and maximum solar absorption. A sheltered spot with a wall or hedge to the north will provide protection against winter winds. Sinking the frame into the ground (if the soil has been pre-fed then even better!) will also provide protection, using the earth for insulation, it is therefore advisable to purchase a cold frame made from pressure treated timber. Most cold frames are light enough to be moved from one section of the garden to another.
There are four main types of cold frames:
Aluminium and Glass cold frame
Plastic cold frame
Cloches
You can of course have a go at making your own cold frame if you’re a dab hand at DIY!
