If you live in a cold temperate climate, or even somewhere that cold weather significantly slows the growth of plants during any part of the year, cold frames are a simple way you can boost your yields and increase the length of your growing season.
Cold frames are simply glass panels, windows or plastic with a frame that lifts off the ground to give space for plants to grow, like mini-green houses over your plants. They can be used to start seeds earlier directly in the garden before last frost, over-winter non-hardy plants, harden off plants to sunlight when they are used to the UV shield of your indoor windows, and, of course, to extend the growing season beyond the first frost.
Types of Cold Frames
Cold frames can be made from a variety of materials, including reclaimed windows, tempered glass or polycarbonate panels, or UV resistant plastic (other types of plastic will quickly break down and are somewhat wasteful), paired with wood, cinder blocks, straw bales or plastic or metal hoops in the case of plastic based cold frames/mini hoop houses.
They can be sunk into the ground to create more protection for plants, or can be placed over raised beds. They can be very low to the ground, suitable for starting seeds a few weeks before the last frost, or higher up to contain larger plants that already exist in your garden.
There are designs available online for many types of cold frames, from wood and reclaimed window-based cold frames , to brick or cinder block-based cold frames , to hoop cold frames  with the aforementioned plastic. A cold frame can be as simple as digging a hole in the ground or stacking straw bales  and putting glass or plastic over it.
Glass cold frames are, of course, more durable, while hard polycarbonate plastic will last several years at least, and other plastics will need to be replaced every few years on hoop-based cold frames. Plastic wrap also won’t typically hold in as much heat.
Another option is to insulate the interior walls of the cold frame, whether made with wood, cinder blocks or other materials, and to use thermal mass such as water buckets or cinder blocks on the north wall to absorb heat.
A further vital consideration is the ability to ventilate the cold frame, which can involve either simply lifting the glass onto a stake at the front, or creating an automated shutter and louver-based vent  tied to a thermometer that causes the vent to open and a fan to suck in, or blow out air when a certain temperature is reached.
Uses of Cold Frames
If you have limited space indoors to start seeds prior to the last frost, cold frames may be just the answer. You may especially find success starting cold hardy plants like Swiss chard, kale, arugula, lettuce, beets, carrots, cabbage, collards, and others up to 2 months earlier using cold frames.
Other less cold-hardy plants can also be started several weeks before last frost, but this will vary from climate to climate, so don’t be afraid to experiment to learn what works in your area, or ask more experienced gardeners in your area. If you lose some plants, you can always start over with more seeds, and then you’ll know for next year (though, of course, every year is different, so you’ll have to use your best judgement).
Over-Winter Hardy Plants
Tender perennials such as different types of hibiscus, perennial greens (e.g. types of kale or perennial cabbages), or passion flowers may benefit from cold frames for over-wintering. In short, you might try growing plants that are one or two growing zones less hardy than your climate requires.
Harden off Plants to Sunlight
Even if you have a plant in full sun inside your house, if you put it outside it could be burned from the direct sun, since it’s used to the UV shielding of your windows. Such plants can benefit from cold frames that are suited to their size, as long as they get enough ventilation during the sunny days, and will protect the plant well enough during cold or frosty nights. You can gradually expose these plants to more and more direct sun, starting with about 30-60 minutes and working your way up by another 30-60 minutes or so each day until they get a full day of sun. If the leaves appear droopy, or white or light green patches, spots, or entire leaves appear, pull back the sun exposure significantly or entirely until the plant recovers.
Extend Growing Season
Of course, one of the most important uses of cold frames is to extend the time of the growing season past the first frost date. This will often require larger cold frames, since plants at this time will be larger. You can also seed fall crops of the same plants you started early (kale, chard, etc.) and continue to get harvests well into the winter in many areas.
Whether you have time to build a sophisticated in-ground, automated ventilation cold frame, or a simple dug cold frame, the effort you put into creating these micro-climate mini green houses will reward you with more food for longer periods of time.
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