Building a cold frame is a cheap and clever step to improve your garden in its earliest stages. Planting flower and vegetable seeds in a cold frame, where they are protected from cold spring winds, helps speed their germination. Also, the cold frame serves as a way station for tender seedlings, started indoors, which can't stand the shock of going directly from a warm house to a cold garden.
The most valuable element in the cold frame is an old wooden storm window with its glass intact. Look around your neighborhood, especially if a homeowner has recently added aluminum windows, and beg or borrow a discarded storm window. (It's even worth it to replace a window's broken glass, considering you will be using the cold frame for years.)
All you need to do is build a wooden frame beneath the storm window, attach a few hinges, and you are ready to grow.
The frame itself is a rectangular structure and should be constructed to fit the dimensions of the storm window, leaving a 1/2 -inch overhang in the front so you can easily lift the window. The rear wall is a piece of 1-by-10, the front rail is a piece of 1-by-4, while the sides also use 1-by-10, cut at an angle so that the four corners match up.
Scrap lumber will do. For a cold frame to match a 4-by-2 1/2 -foot storm window, you need to scavenge 10 feet of 1-by-10 and a 4-foot piece of 1-by-4.
After cutting the boards, secure each of the four corners with a set of 2 1/2 -inch hinges, with removable pins. Place the hinges on the inside of the frame to keep them from rusting.
Next, staple a strip of felt insulation all around the top edge of the frame, upon which the window sits, to seal the cold frame and protect it from drafts. Attach the storm window to the rear wall with a set of wide strap hinges and weatherproof the frame with a coat of exterior house paint or weatherproofing. The total cost ranges from nothing, if you have sufficient scrap material, to about $25 if you have to buy all the lumber and hinges.
Set the frame in an area where it will get plenty of direct sunlight, making certain that the bottom of the frame is sealed to the ground to avoid drafts. Then let nature take care of the rest.
After setting out your seedlings or seed flats in the cold frame, be careful to check the weather daily. In the morning, if the temperature exceeds 40 degrees, prop open the frame a couple of inches with a piece of board so that the frame does not overheat.
If freezing temperatures are forecast, cover the cold frame at night with a blanket to protect your plants.
After the planting season, when the frame is empty, dismantle the pieces by pulling the pins from the hinges and stack them in a storeroom or garage.
And be happy. You have saved about $100 -- the cost of a manufactured cold frame.