It gets quite cold here in Iowa during the winter time. A cold frame is one of the cheapest and easiest ways to extend your growing season right on through the frost and snow. Imagine a very tiny greenhouse made of a recycled lumber frame and an old window - you've got yourself a cold frame.

Step 1: Materials and Tools

Materials
The only thing you really need for a cold frame is the window. What you construct the frame out of is entirely up to you; it could be metal, wood, stone, old drywall, anything you can make a frame out of really. We ended up using materials we picked up at a reclaimed and recycled building materials center called Allen's in Ottumwa, IA (thanks Allen and Beth!):
-one old double paned window measuring 32"x39" (make sure you don't get a window that is polarized or has an anti-UV coating, sometimes called low-e, as this will deprive your plants of the sunlight they need).
-three 2"x4" boards approx. 4' long
-two 2"x6" boards approx. 4' long
-two 2"x8" boards approx. 4' long
-two 2"x2" scraps roughly 6" long
-two hinges that will support the window
-two sticks to hold the frame open for working and venting the cold frame
-wood screws

Tools
Use tools appropriate to your materials. This project can be done with fewer tools than we ended up using (i.e. you can build it entirely with hand tools which we could not do due to time constraints). We ended up using our school's shop so we used:
-circular saw
-band saw
-measuring tape
-carpenter's square
-electric drill
-pencil
-paper for sketching and writing down measurements

Step 2: Measuring Your Window and Cutting Your Lumber

First Rule: measure twice, cut once. Really, just do it.

So, first measure your window (in our case 32"x39"). We decided to have an 8" inch drop from the back of the frame to the front so that we could have the window at a nice angle to catch the low winter sunlight. This means that the shorter dimension of our frame was actually shorter than 32" because of the angle. To solve for this dimension use the Pythagorean Theorem (dig deep back to that middle school math class!). So take the length of the sloped side of the window (hypotenuse) squared minus the drop (8") squared. Then take the square root of this number and you've got your frame measurement!

Since we built the back of our frame by stacking two 2x6 boards we decided to secure them together with two little 2x2 supports.

We made the rest of our frame with three 2x4's for the base and one 2x8 cut to the proper slope (to create the proper window slope). Once you have the frame mocked up just place the 2x8 where it is needed and draw a line from the back of the frame diagonally to the front of the frame (this takes care of the measuring). You can then cut the 2x8 and use both pieces that came from one cut.

Step 3: Put the Frame Together

We used wood screws, an electric drill, and a square to keep us lined up. Because we had much stronger and thicker material than was structurally necessary we did not use any reinforcement in the corners of our box. You may want to depending on the strength of your materials.

Step 4: Mount the Window

Make sure everything lines up nice and flush. The measure out even intervals and mount up your hinges to both the frame and the window.

Step 5: Revel in the Completion of Your Cold Frame

We did not put handles on our frame because the lock mechanisms that came on the window made great ones. This building process took two people (who did not really know what they were doing) roughly two hours. If we did it, so can you!

Step 6: Grow Stuff!

This is why we did all this, right? I would recommend that when you put your cold frame in your garden you add some thermal mass either in the frame or supporting it underneath. This will allow the structure to hold heat better and will reduce the need for covering the frame with a blanket in severe cold.

Remember to put your cold frame on some good soil and facing south so it gets the most sun in the winter.

Here is a list (courtesy of Eliot Coleman via Sheila Higgins) of some of the things that can be grown in your cold frame for some fresh and delicious treats even during the winter (it really does vary so consult your local nursery or a friendly book to find out what grows best in your area):
- Arugula
- Carrot
- Lettuce
- Radish
- Parsley
- Scallion
- Endive
- Mache
- Spinach
- Swiss Chard