Building your cold frame cover
Cold frames can be built in several different ways, depending on personal preference and materials on hand. Starting at the top, you will need a covering that allows light in and can be propped open for ventilation when on warmer days.
Often, you can find free windows from old houses on your local free classified sites, such as Craigslist. These are great, but be careful about the possibility of lead-based paint in very old windows. In extremely snowy climates, glass may break when snow piles on top, so keep that in mind as well. Other covering options include plexiglass, greenhouse plastic, or fiberglass. Anything that will allow light to get through to the plants will do the job, so use whatever you can get easily and cheaply.
Building the frame of your cold frame
You also have options for your frame. Some people like to use old bales of straw or hay. Arrange them into a square or rectangle that will accommodate your cover. Cinder blocks, with the holes going vertically, or brinks can also be used. Many people have old bricks or cinder blocks laying around. If you don't have any, ask around or check on Craigslist or someplace similar. This could be another component to your cold frame that could be free.
Wood can also be used for your frame. Just avoid pressure treated wood so you don't end up with chemicals like arsenic in your garden plants. Use brackets or screws to hold the frame together. Then, you can even add hinges to the top for your cover if you like.
Extra building tips
When building your frame, try to make it so it slightly slopes toward the south. Make the back of your frame 4 to 6 inches higher than the front for the ideal angle. It needs to be angled enough for rain and snow to run off, as well as to allow the most possible light inside. Your cover will need to be propped open on days around 40 degrees Fahrenheit or above. You can use a stick or place a brick under the lid to hold it open. Just don't forget to close it in the afternoon so your plants don't freeze at night.
Very cold climates can add extra insulation by digging a hole about 8 inches below the soil level. Line the bottom with gravel and place your cold frame over it. Plants will be warmer when they are deeper. You can also use more straw, leaves, blankets, or old newspapers around the cold frame if you think you will need extra insulation.
Using your cold frame
Now that your cold frame is ready, you can plant lettuce, kale, spinach or other cool-weather vegetables in it. This will allow you to harvest fresh vegetables well into the winter. If you live in a cold climate, you can use your cold frame to grow vegetables that need hotter climates to grow well in the summer, such as tomatoes or peppers.
Another use for a cold frame is to use it to protect plants that are dormant for the winter. Often these plants are kept in a garage or other cool place, but if you don't have a suitable location, the cold frame is perfect.
You can also give your garden a jump start by using a cold frame. If you start seeds indoors, you can start them up to 6 weeks earlier and then transplant the seedling to the cold frame. Once the weather is warm enough to put them in the garden, they will be big healthy plants ready for their permanent home. If you have several cold frames in your garden, you can just remove the cold frames when the weather is warm enough and you'll have your garden already planted.
If you prefer, you can seed your plants directly into the cold frame instead of starting them indoors. This won't give you as much of a jump start, but even getting your garden started 2 or 3 weeks early will improve your yields.
With so many uses and the simplicity of building, a cold frame is a small investment that will give you big returns. Try one this year and you will wonder why you haven't tried it sooner.