What is a cold frame?
Cold frames are an excellent way to give you more growing season. They shelter young plants at the end of winter and extend the season in autumn long after frost. In some parts of the world, people can use them all winter. Cold frames are a common sight in eastern Europe and Scandinavia. What is a cold frame, some may ask? It is basically the opposite of a raised bed with a transparent cover. The best cold frames are either a little below ground level, have insulated sides, or both. This isn't as hard to do as it sounds either. Cold frames are actually quite easy to construct and there are dozens of examples and plans on line. Or, you can even purchase ready-made ones on line. Many of the big box lumber and building supply stores have kits and plans, or a simple search on the internet gives you all sorts of retail options. Even Amazon offers a number of possibilities.
A cold frame insulates plants in winter
The way a cold frame works is that the soil and sides of the bed hold heat absorbed by the sun during the day and release it at night. The transparent top acts like a greenhouse panel so the plants receive ample light and just like your car, the glass helps the entire unit heat up. Cold frames are most often situated on a south or southwest facing wall. This allows the low, winter sun to heat it for the longest part of the day. A cold frame will keep the temperature inside between 5 and 10 degrees Fahrenheit warmer than the outside air at night and with a few extra additions such as a blanket covering it and jugs of hot water inside, you can raise the temperature a bit higher. However, this is still a cold weather tool and the plants that succeed in a cold frame are plants that grow well in cool temperatures anyway.
Cool season vegetables do well in a cold frame
Lettuce, kale, Asian vegetables, radishes and similar plants all do well in cold frames. You can also start cool weather flowers in a cold frame. Petunias and violas do great. The lid keeps the heat in at night and during cooler days. If you happen to have a warm winter day, you open the lid a bit to let in the cooler air. On a truly warm day, if there's no ventilation, you can fry your plants, so a thermometer inside is a good idea. Crack your lid when the temperature inside is between 60 and 70 degrees to keep it from getting hotter.
Use your cold frame in the summer
Cold frames can even do double duty in the summer. Instead of the transparent lid, replace it with a shade cloth and you've got an excellent place to root cuttings of perennials and shrubs. Cuttings do not need full sun since they do not have the root system that supplies water to the stems and leaves. This helps the cuttings survive until the root system evolves enough to take care of the young plant. You can tell when this is starting to happen when the cuttings start to produce new growth. Monitor the moisture levels in the cutting containers carefully since the soil needs to be constantly moist, but not wet. In the summer, even with a shade cloth, moisture will evaporate quickly.
Make a cold frame
You can quickly and cheaply make your own cold frame with just a few materials. Straw is a great insulator and between four and six bales will be enough for a nice sized cold frame. Use four if you have an existing back wall where you want your cold frame. Place two of the bales to make the sides and two of the bales end to end to make the front of the cold frame. Old windows are the preferred top for cold frames and you can often find them in yard sales, or even Habitat for Humanity stores that sell reclaimed and donated building materials. If you don't have access to that, a piece of Plexiglas from a big box lumber store will work. Even heavy plastic sheeting stapled to a frame works. However it isn't something that will last more than one season. Most people construct a cold frame so that the back side is about six inches taller than the front side. This will allow rain water to properly drain away from the bed. It isn't completely necessary, but does help with drainage. You can easily do this by making a frame for your lid with the back side higher than the front, or raise the sides of your cold frame in the back with bricks or an extra layer of straw. The good thing about using straw is that it makes great mulch once you do not need it anymore. Straw will also insulate an existing wooden or brick cold frame when the weather turns extremely cold.
Cold frame gardening is good for students
Cold frames are an inexpensive and quick way for gardeners to experiment with winter gardening. The smaller area and quick construction helps new gardeners learn as they go without being overwhelmed with a large greenhouse. Most people can easily have salad greens and cool weather vegetables growing all winter. It is also an excellent learning tool for at-home schooling. Even children as young as kindergartners can sow lettuce seed and take care of a cold frame with a bit of supervision. They learn about weather, how to protect their plants and even how to do the math to keep the temperature regulated. The added plus is that even picky eaters will often eat the results of their labor. If you have a spot that gets winter sun, you should consider building a cold frame.