If you have never used a cold frame this is the year you should try it. A cold frame is easy to build but what it does for you is more important than what takes to build one so lets go over the why you want to use one first.
A cold frame is essential for vegetable gardening in northern climates. You can get started growing vegetables in the Spring weeks before your last frost date. In the Fall you can still be harvesting at Thanksgiving that is the beauty of a cold frame, it will extend your gardening season and help you to provide more fresh produce for the dinner table. The construction is not hard and you can build one that is either permanent or portable.
The first thing to do is to determine where your cold frame will go the location should have a southern exposure so that you be able to capture as much sunlight as possible. The area should be well drained and in the full sun if possible. Before you build you should loosen the soil ten to twelve inches and work in four or five inches of compost this will give your seedlings a great environment to grow in. When building your frame you will want to use rot resistant wood such as cedar or cypress but do not use chemical treated wood these chemicals could leach into the soil. The frame should built so it is 12″ to 18″ taller in the back than in the front and the sides should be at least 12″ high in the low point. You will need to get a piece of clear glass or Plexiglas to use for the top. I prefer Plexiglas as it is more resistant to breaking than regular glass. Mount the material in a frame large enough to cover the complete top of the cold frame and if possible attach hinges so it is easier to raise and lower the top or prop it open on very warm days. When completed ideally you would like to have a minimum 10″ of clearance from the top of the soil to the glass in your cover.
Once the weather starts to get a little better in the spring and you can start to work the soil in your cold frame loosen the soil and get it ready for planting. Once the temperature in the frame reaches 50 degrees you can start planting your early lettuce spinach, and radishes. Generally salad vegetables will do well and start giving you fresh produce weeks before the garden is even ready to plant. The cold frame could also be used to start your seeds for transplanting outside after the frost. You can use it like a mini greenhouse. It is important though to keep an eye on the temperature if it rises to more than 75 degrees inside the cold frame you will need to raise the top for ventilation to allow some hot air to escape.
When growing in a cold frame the soil will dry out faster so be sure to always have a watering can nearby. Also for sudden drops in temperature it is good to have a heavy blanket or burlap to throw over the frame for extra insulation. Once you start using a cold frame I am sure you will want to garden with one from here on out.