Cold Frames are very simple but extremely useful, usually compact in size but with many practical benefits for growers.
They could be described as either an accessory or an, albeit limited, alternative to a greenhouse. Whichever is true, a cold frame will earn its keep throughout the year.
So why is this box with a glazed lid, and its close relative the mini-greenhouse, so useful?
This depends on the time of the year. From February onwards, you can sow broad beans and sweet peas in a cold frame. Later, young plants, initially raised in the warmth of a greenhouse, can be hardened off before planting outside.
To harden off, the plants need to be moved into the cold frame about a week or ten days before planting out. On the second day, open slightly but close again in the evening.
Repeat this procedure, slowly increasing the plants’ exposure during the day and all but closing the lid in the evening. When completely open during the day, start increasing the amount left open at night until the lid is fully open at all times.
If they are not acclimatised by using this method, the plants’ growth rate and subsequent crop will suffer. The cold frame offers a halfway house, slowly lifting the lid on the tough realities of the outside world.
Cold frames can also be useful when acting as a cloche, warming the soil in the spring prior to sowing seed.
As the year progresses, tender vegetables such as aubergines can be kept warm so they ripen earlier and produce an improved crop.
Later on in the year, cold frames can protect young plants and cuttings from frost and other elements. Late summer and autumn sowings can be made of winter lettuce, spinach, kale even carrots.
Although you may have to keep your ambition in check, in theory vegetables can be grown throughout the winter.
Use bubble wrap to provide extra insulation and act as a draught excluder. However, ventilation should not be neglected.
Cold frames can become too hot during the summer or filled with stagnant air when closed in cold weather increasing the risk of disease. Open the lid now and again to freshen the air and reduce the risk of fungal problems.
As with your greenhouse, the cold frame should be checked regularly, especially in hot weather when watering is important and pests need to be spotted and eliminated.
By extending the growing season and providing protection for a large selection of plants at different times of the year, a cold frame or mini greenhouse will be an invaluable servant to the gardener.
Cold frames are not expensive either to build or buy. Sleepers and other pieces of timber, or bricks, are preferable to metal as they are better insulators and retain heat more efficiently.
They can rest on bare earth but the ideal base would be level paving slabs, and they must be robust enough to resist adverse weather conditions, hence why sleepers are such a good idea.
Panes of glass or rigid plastic should then be laid at an angle on top. Glass has the best light transmission but, if you have children running about, polycarbonate is a safer alternative to both glass and ordinary plastic.
During hot weather the cold frame lid can be fully opened but, as hardening off is a gradual process, you should be able to open the cold frame lid in stages to slowly prepare your plants for the great outdoors.
Bearing in mind that cuttings may prefer a slightly shady position, locate your cold frame in a sunny position that’s sheltered, so that it is warm and filled with light. Don’t forget that, similar to a wall garden, if placed adjacent to your house, the frame will benefit from its warmth.