Advanced: Using a cold frame
A cold frame is basically a box with a transparent lid, creating a mini-greenhouse that allows the hungry gardener to extend the growing season for a month or more at each end.
In spring, cold frames may be used to harden off tender seedlings and to start seeds, which often fare better than under lights indoors. In fall and winter, crops such as kale, spinach and chard will flourish in cold frames long after the rest of the garden has packed up.
You can plant directly into the soil or simply set flats/pots inside the frame, which should be sited in a sheltered south-facing spot. The minimum practical size for a cold frame is 60 centimetres by 1.2 metres by 30 centimetres in height but many DIYers recycle tempered glass patio or shower doors as lids, making a cold frame roughly 80 centimetres by 2.1 metres.
The frame structure can be made of wood, brick, concrete or plastic. Overall height may vary depending on what you’re growing, but the back wall is generally 10 to 15 centimetres higher than the front so the lid slopes to catch more sun and allow rain to run off.
Regular glass can be used in the lid but if safety is an issue, go with tempered glass or plastic. Success relies on keeping the cold frame not too cold, not too hot, and well ventilated; a hinged lid can be wedged open as needed to maintain an even temperature.
How to - Techniques
Extending your harvest
Make the most out of your edible garden when space is tight and the growing season is short.