There is no difference in principle between a cold frame and a small greenhouse, except for the height of plants you can grow. Nevertheless, there are some special rules to bear in mind when using a cold frame.
– Position a cold frame facing south in full sun. Protection from north or east winds will keep it warmer in the winter.
– A 3-6in layer of good, quickly draining soil makes a suitable basis for growing. Fork over the soil inside and prepare a fine, crumbly tilth each time before sowing vegetables such as carrots, lettuce and radish. Incorporate peat and fertilisers to maintain the soil in good condition.
– Keep pots and trays containing young plants near the glass to prevent them becoming leggy. Set the pots and trays on old bricks to achieve this. Clean glass will also help.
– If possible, ventilate on the side away from the wind. Avoid high temperatures and musty air – both can encourage the rapid development of fungus diseases.
– Protect tender plants by draping sacking, rugs or ground sheets over the frame at night when frosts are expected. Cover up before sunset, rather than after dark, to help to trap the day’s warm air inside the frame.
– Use plastic netting, or a liquid shading paint which can be rubbed off in the autumn, to prevent sunscorch and high temperature damage in warm weather. Ventilate fully during hot weather – by taking the frame tops right off – but remember that even then you may have to shade the plants.
– Keep the frame in good condition and free from debris such as broken flowerpots and decaying wood. Otherwise woodlice, slugs and snails may congregate. If necessary, use slug pellets to protect susceptible plants.
– Root diseases or soil sickness may occur if the soil is regularly used for the same types of plants. To prevent this, shift the position of the frame each year if possible. Otherwise move the crops or replace the soil each year.