I made another cold frame yesterday, using the same design I’ve used for the last several ones I’ve made. They are 4 feet by 4 feet square, about 8 inches tall in front and tapering to 12-1/2 inches in back.
This size is perfect to fit the beds around my greenhouse, which are about 3-1/2 feet wide. And the height is tall enough for what I usually grow in those beds, things like lettuce, spinach, Asian greens, radicchio, endive, and kohlrabi. It’s also tall enough to protect early plantings of broccoli and cabbage.
What follows is not intended to be a step by step tutorial on how to build these cold frames, but I’ll try and give enough details so that anyone with basic carpentry skills could adapt them to fit their own needs.
The base is made of 2″ thick untreated dimensional lumber. I’ve made cold frames from exterior plywood in the past, but I find the lumber is easier to work with. Plus, if I get the pieces in 8 foot lengths it works out well for the 4×4 foot overall dimensions of the cold frame. The front piece is a 2×8, and the back is a 2×12. The side pieces are 2×12’s cut using a circular saw to slope from 7″ at one end to about 11″ on the other end.
The corners are reinforced with a short block of 2×3 lumber. The side pieces are screwed into the blocks with #8 exterior 2-1/2″ screws. I drill pilot holes for all the screws used for the sides. Then I use my cordless drill to screw them in.
Once the base is assembled, I add a strip of 2×2 lumber to the bottom of the base lumber pieces. That keeps the side pieces from sitting directly on the ground, which means the 2×2 pieces should be the first thing to rot. It is easy to replace the 2×2’s, and it is cheaper than replacing the larger lumber. It also adds about 1-1/2″ to the height of the cold frame. The 2×2 strips are screwed to the base using the same exterior screws used before.
The top is constructed of 2×3 untreated dimensional lumber. I lay out the pieces on a flat area, using a simple butt joint. Then I use angled and flat corner braces to join the pieces together. I don’t use any additional nails or screws in the top pieces.
The top construction is quite adequate for my lightweight covering materials, which are usually polyester row cover material or plastic poly sheeting. Another design would be needed for plexiglass or glass covered tops.
I use two 3-1/2″ or 4″ hinges to secure the top on the base. No need to mortise the hinges – this isn’t fine cabinetry work. The screws that come with all the hardware are adequate for my needs.
I like to add a couple of handles to the sides. That makes it easier to move the rather heavy cold frame around.
Here’s a photo of the new cold frame in place next to the two older ones. I bought enough materials to make one more frame, which should be all I need for the time being.
The last step is to add the cover material. In this case I’m going to use Agribon row cover material, stapled to the top.
If you’re not using cold frames, you might want to give them a try. They are a great way to to extend your growing season. They’re easy to construct, and fairly inexpensive to make. The materials for this one cost about $35 per cold frame, and I put it together in a little over two hours.