A cold frame is an important piece of equipment, especially if you use the greenhouse to raise seedlings for planting outside . If plants that have been raised in the warm, humid conditions of the greenhouse are put straight out in the open ground, their rate of growth will be considerably reduced. This is especially so in colder climates in the early spring. The plants need to be acclimatised gradually to lower temperatures and increased exposure, a process called hardening off.
The best way to do this is with a cold frame. Most frames comprise a wooden, metal or brick box with a glass lid. In the past, cold frames were frequently made from an old window mounted (with hinges) on a wooden frame at an angle. The angle was aligned to the winter sun to capture additional sunlight and allowed water to run off. Because all plants in the cold frame must be at the same stage of hardening off, it may be more convenient to have two smaller cold frames rather than one large one.
Cold frames can also be used during the spring in colder climates to grow early vegetables of all kinds and such crops as melons. They also come in useful in the winter for growing vegetables that mature in spring, such as lettuces.
Ventilation is also an important issue. On warm or sunny days, it can become too warm in the cold frame, and the plants can burn. On days like these, the easiest thing to do is to hold the lid ajar by an inch or two, so that the warm air can escape from the cold frame. Keep an eye on the weather and your plants, if the plants begin to wilt, it may be too hot inside.
Plants that are good to start in a cold frame are lettuce, spinach, root plants or tomatoes. These plants will thrive in the cold frame environment. Tomato plants are a good idea because they need a good start before planting in early summer. If growing transplants, you can plan what you grow according to when they need to be started.
If you are a gardener who would like to get an early start on the growing season, or would like to extend the time that you can plant, a cold frame would be a good choice. They work for as long as you need them, and can be easily moved when not in use.