For something that is so simple and relatively inexpensive, a cold frame can increase the productivity of your vegetable garden hugely simply by extending the effective the length of the growing season.
Historically cold frames were located on the exterior of a heated green house and comprised a glazed frame with hinged glass roof called a light. These frames were unheated, hence the term "cold" frame. The idea was that seedlings initially grown in the greenhouse would be moved to the cold frames to harden them off. To harden off a seedling, it is placed within the cold frame and over a period of weeks the roof light is opened more and more, gradually acclimatising the seedlings to the outside conditions. This process helps prevent any shock when the seedlings are planted out which can cause a check in growth. This is particularly important with cauliflowers where a check in growth can result in the heads never forming properly.
Cold frames today
Modern cold frames have eschewed glass in favor of modern materials, generally polycarbonate. Modern polycarbonates are as clear as glass and in the double glazed variety fitted to most cold frames provide thermal insulation as good as glass. Polycarbonate does not shatter, or break harmfully and is therefore a perfect material for use in gardens where there are pets or children.
Generally cold frames are designed either glazed down to the ground or with a solid wooden frame. There are pros and cons to both designs, although for the British climate I consider the solid wooden sides an advantage as they absorb warmth throughout the day and slowly release it during the night, leading to higher average temperature at night. However on an overcast day the fully glazed cold frames can capture more sunlight which can help prevent the seedlings from getting "leggy" due to lack of light.
Cold frames are incredibly useful for hardening off seedlings, which can be a real chore without one. However they can also be used like a mini greenhouse to germinate plants earlier than they would otherwise grow outside. With just a cold frame it is possible to have home grown cauliflower ready as early as late May. Simply sow your cauliflower seed in your cold frame at the end of October and the seedlings should be ready to plant out at the end of February or early March, easily gaining a month or more of growing time.
I use my cold frame nearly all year round, in fact during the course of a year, all of the following usually end up in the cold frame at some point:
- Onions (from seed)
- Sweet corn
A cold frame is one of those things that you don't think you need until you have one and then you wander how you ever managed without. Mine gets so much use it easily justifies its price. Although just the effort saved when hardening off seedlings would be sufficient to make it great value for money.
Two years ago I would not have bothered with a cold frame, thinking they were unnecessary, however I have become a true cold frame convert and really could not conceive gardening without one anymore.