Cold frames are like mini greenhouses. They generally take the shape of a rectangular wooden box with a hinged glass lid that can be left open or closed to regulate air temperature. Cold frames are kept outdoors where they provide a very useful halfway house between the warmth of a greenhouse, or a sunny windowsill indoors, and the cooler temperatures of a garden bed. We use cold frames for germinating seeds in trays and punnets as well as for preparing seedlings by ‘hardening them off’ before planting out in the garden.
A cold frame is usually placed in a sunny spot that is easy to regularly access. This means you won’t forget your tender seedlings that should really be checked daily. Because cold frames are usually portable they can be placed over planted seedlings in a bed to harden them of in situ.
Seedlings of many summer crops such as tomatoes, beans, zucchini and aubergine are grown from seed in the warmth of a greenhouse or a warm windowsill indoors. This allows us to get them ready for planting out as soon as spring weather warms sufficiently and stabilizes. As a result, these seedlings are accustomed to steady, warm temperatures and regular moisture. They thrive at this stage and they look good – however they are vulnerable. With developing root systems and tender foliage they can rapidly decline if conditions change dramatically. Hardening off is a process by which these seedlings are placed into a cold frame. On warm days the lid is lifted to allow seedlings to acclimatize as they continue to grow and at night the lid is closed to prevent them keeling over as a result of any dramatic temperature drops overnight. As the days pass, the lid can be left open on slightly cooler days as the seedlings become used to the cooler conditions – this is called ‘hardening off’. ‘Hardening off’ generally take a couple of weeks.
Cold Frames are traditionally made from a recycled framed window pane that is fixed on an angle across the top of a wooden box or casket. The angled window faces the sun. The window should not be too large as it should be easy to open and close. Glass can be replaced by a sheet of Perspex for added safety where children might be using the cold frame. You can buy corrugated clear plastic cold frames at garden centres.