Cold Play: Cool things about cold frames
Cold frames are fundamentally multifunctional mini greenhouses with a microclimate. They have four basic uses:
1. to give seedlings a head start
2. hardening off
3. extending the growing season
1. Giving seedlings a head start
A cold frame allows for sowing seedlings that do not require too much warmth to germinate a few weeks earlier than sowing directly into the exposed ground. The seeds can be sown directly into the ground inside the frame, or in containers. Once they’ve germinated the lids will need to be opened to allow for ventilation and prevent diseases such as Phytophthora that cause damping off.
2. Hardening off
Plants that are moved from life indoors or a greenhouse must be hardened off before planting out into the garden to avoid going into shock and sudden death from large temperature changes. Move them over to the cold frame once they have rooted well and have several sets of true leaves, and don’t pack too tightly, so there is still reasonable air circulation directly around the foliage. Lifting the lid on a cold frame increasing amounts and then leaving it off for longer periods in the day allows plants to acclimatise to life outside. It’s a good idea to start this once it appears the last frost is over. You need to keep a check on the weather forecast though, to avoid an unexpected cold snap.
3. Extending the growing season
A the other and of the summer, cold frames are good for extending the growing season, keeping temperature sensitive plants warm enough to keep on growing past the first frost. Most veg are day length sensitive though, so as the nights drawn in their growth rate drops, but the cold frame can still extend the last harvest.
The opportunity to open or remove the lid on a cold frame on warmer days allows for keeping frost sensitive but more temperate plants over winter. As cold frames are relatively small their temperature is more easily controlled that that of an unheated greenhouse. Starting off winter lettuce early and keeping them in a cold frame helps brings their harvest date forward.
The most satisfying thing about a cold frame like many aspects of gardening is that is makes life a lot easier. I’ve hankered after one for a long time, hardening off always seems to involved an awful lot of carting trays of young plants in and out of the greenhouse. Then there is the inevitable occasion when I would wake up at some antisocial hour in the morning only to remember that I have left the plants out on the one night that a cold snap has hit the planet.
Last month I discovered my father’s old cold frame from the ‘70’s languishing under a yew tree.
With some careful manoeuvring, I got it in into the car and up the M11. It took a lot of careful scrubbing and several new panes of horticultural glass and voila! Hort. glass is crucial, it is thinner than normal glass panes, but also incredibly strong. The lids are a little precarious on this one, but it’s been a really satisfying job, more rewarding than buying a new and more sophisticated model and ready just in time to protect my cuttings from future frosts.