A coldframe is essentially a type of low cost, highly efficient miniature greenhouse. To understand what a coldframe is, envision a very shallow box whose sides are made of transparent or translucent glass or plastic, and whose bottom is open. The boxes can be set over the top of newly planted flowers and vegetables to protect them from wind and cold temperatures. When used properly, coldframes allow you to get a head start on the planting season in the spring by setting plants out earlier in the year than would otherwise be possible.
The first step in using coldframes is constructing them. They are actually quite easy to build with some basic lumber and supplies from your hardware store. Begin by making an outer frame in whatever size you desire (you'll need to determine how much ground space you want to cover). The frame should be approximately a foot high. Then construct another, shallower frame of the same length and width, and fit it with a piece of plastic or glass. This second frame will be the lid and you'll attach it to the bottom frame with hinges on one side. Remember to add some small pieces of wood that will allow you to prop the lid up so it's slightly cracked. This will be important on warm days.
If you aren't someone who is especially handy, or you don't relish the idea of building your own coldframes, they can be just as easily purchased. In fact, commercially available coldframe greenhouses are often better quality and will last you much longer than anything you build yourself. If storage space is a concern, look for the type that collapse when not in use.
The best location for your coldframes is on the south side of your home, garage, or other structure. If you must place them on a different side, choose the west side where they'll still receive plenty of sunlight. Keep in mind, though, that you may need to provide some extra protection from the cold north winds that are prevalent in early springtime. Ideally, you should set your coldframes in place at least one to two weeks before you plant in them. This will allow time for them to warm the soil up, making a good planting area for new transplants or starting seeds.
While they are warming only the soil, your coldframes can be left closed all the time. However, once you plant in them, it will be important to vent the lid on warm days. As a general rule of thumb, if the daytime temperatures are forecasted to be between 50 and 60 degrees Fahrenheit, you can ventilate your colframe by simply cracking the lid and propping it up about 3 to 5 inches. If the weather is going to be between 60 to 70 degrees Fahrenheit, the lid can be completely removed, and if temperatures rise above 70 or 75 degrees, then you know it's time to remove the coldframe all together.
Be sure to keep an eye on nighttime forecasts, too. Late spring frosts are common in many parts of the country, and this is where your coldframe will really come in handy in protecting young seedlings. Make a practice of closing the lid every evening when it gets dark, and then raising it again in the morning, an hour or two after sunrise.
Coldframes are an excellent way to get the most out of your garden. Not only do they allow you to plant earlier in the spring, but they're also great for hardening off seedlings before transplanting into the garden. If vegetable or annual flower gardening is something you enjoy, but you don't want to go so far as to invest in a greenhouse, coldframes are a great next step toward improving your gardening skills.
