Build a Cold Frame

What is a cold frame? In its simplest form it is just a box with a transparent cover, which allows you to start seeds or plants outside when it is still too cold to plant in the garden. The transparent cover lets in sunlight, which warms up everything inside of the box and the soil. While the sunlight is not as direct or as strong as it is in the main growing season, it is enough to get plants started or even to grow cool season crops like lettuce, spinach, radishes, and onions, to name but a few.

I have wanted a cold frame for many years. My dad had one when I was a kid but I don't remember how successful he was with it. I used plastic sheeting in The Netherlands to create something similar to a cold frame and it was very effective.

Being a bit of a cheapskate, I wanted to build one for as little cost as possible. Below is my project.

Keeping cost in mind, I choose to use 5/4 inch deck boards for my box. 2x6s or 2x8s could also be used and would have a better insulating value but they would cost twice as much. I bought seven 5/4"x6"x8' boards for my project. What I have found over the years is that these boards are saturated and if you build something and let them dry quickly outside they will warp and split something terrible. So I now stack them in my garage with slats between them and the top board weighted down so that they dry slowly and evenly. The weights, in this case pavers, keep them flat as they dry. It took three weeks to dry them and I put a fan on them for the last couple days.

To cut down on waste and to get the biggest number of square feet inside the box, I cut my boards so I had a five foot piece and a three foot piece. That would give me 15 square feet of soil surface once the box was put together. If I had cut the boards in half, two four foot pieces, I would have 16 square feet of soil surface but it is very hard to reach to the back of a four foot box. You should always cut treated lumber in a well vented area so that you aren't breathing in the fumes and saw dust. I used my garage with all the doors open. It gave me a flat work area and plenty of fresh air.

Always use a square to get 90 degree cuts and accurate measurements. It is not fine cabinetry but it will certainly look better and be stronger with accurate cuts. I used my battery powered circular saw, one of the best free scavenged items I have found. It was perfect for this project. But you could do this with a plug in circular saw, a table saw, or even a hand saw. I did finish a few cuts with a hand saw so that I wouldn't have over-run cuts.

I used two inch deck screws to attach all the parts. Treated lumber, and being outdoors, will rust plain screws or nails in a short time. I prefer screws to nails because they hold tighter, stay tight longer, and can be taken apart easier if you make a mistake (which I did) or you need to make an adjustment. Deck screws are self tapping but I always pre-drill screw holes. That eliminates splitting. I used a bar clamp to ensure the boards were tight together. The box should be as air-tight as possible to retain the heat.

Each corner was braced with a 2x2 instead of trying to screw the flat boards together. This is much easier and far stronger. The long sides had a third brace in the center, which you will see in the next pictures. My boards were not perfect, they seldom are, and so I chinked between the boards with sisal twine, much like wooden ships were chinked, or lapped, in the old days. When the twine gets wet it will expand and completely seal the small gaps. I used a putty knife to force the twine into the hair line gaps.

 As you can see in the photos I set the box on a foundation of brick. It is not necessary but I like the look and it helps to protect the bottom of the box when I am working the soil with tools.

Here you can see the two center braces to strengthen the long side of the box. The front brace is on the inside and the rear brace is on the outside. The rear brace is on the outside because I installed a long mirror inside the box, as you'll see below.

For this spring I just used a plastic bag that I had in the garage to cover my lid. It came from a large poster my son bought. Perfect fit. The lid is another 5/4x6x8 that I ripped to two inches and then cut to length. The center piece is one inch wide. The top frame is glued and screwed together and is quite sturdy. I attached the lid to the box with four small hinges. You can see the mirror inside. This will reflect light from the back of the box to the front during the early part of the day. This lights up a shadowed area.

And here is the finished Cold Frame. I will install a latch on the lid so the wind doesn't blow it open and I have ordered an automatic opening device so that the box doesn't over heat and cook the plants. Everything combined was less than $50 plus the opening device was another $44. It wouldn't be necessary if I lived there full time; I could raise and lower the lid as needed.

Last weekend, after placing the Cold Frame in my garden, I planted onion, radish, lettuce, and spinach seeds using the square foot gardening method. I should have some plants growing by next week. I'll keep you posted.