Cold frames are something of an afterthought for most gardeners, and their various designs are indicative of what some would call Yankee Ingenuity. The simplest might be a hole in the ground with a glass cover – usually an old window frame – though it might have sides raised by hay bales destined to provide mulch after the plants are in the ground. The general idea is to be resourceful, and provide a place to harden off your seedlings while protecting them from a late frost.
I didn’t have any hay bales or glass, but I did have some lumber, plastic, and a frost blanket, and a door that I’d salvaged during a remodel. I decided to use the door – painted white – as a reflector and built a 2×4 base to support it and the rest of the frame.
Underneath the frame I laid a sheet of black plastic to absorb as much solar radiation as possible. Next year I plan to add a layer of insulation board and cover it with sand so I can use a heating cable to bottom heat my seedlings.
When the frame is complete the plastic will be folded up and stabled to the base to keep out drain water, and the frame set against the building to provide additional shelter.
I actually bought a couple of 2x2s to make the top doors of the cold frame. I used 1x2s for the sides and the corner braces. The top is covered with clear plastic and the inside is covered with frost blanket to hold the heat. Using staples to secure plastic is far from ideal. Rather than using a furring strip to secure the plastic I apply a bead of silicone to the frame and then staple it into place. The silicone not only provides a seal but also acts as an adhesive that easy to remove if the plastic weathers and needs to be replaced.
I use the same technique when applying the frost blanket to the inside of the frame.
Since the cold frame will be out in the weather year around I decided against using metal hinges, and instead employed an old bicycle tube. After cutting away the stem I split the tube long ways and cut 8 inch sections that I nailed and stapled to the frame.
I then nailed the tube to the door that forms the back of the cold frame.
Now I’ll have to admit that a piano hinge would look better but I doubt that my plants will notice. Since punctured tubes are fairly common, you shouldn’t have too much trouble picking up a few for weather stripping and hinging. One drawback is that this set up tends to be pretty flexible and the lid does move around a bit, so I’ll probably have to rig a catch of some sort to keep the lid snug and in place.
Most people build cold frames using old windows, or the odd piece of glass scavenged during a remodeling project, so there’s no set size. Since I built mine around a door my cold frame measures approximately 7 foot by 3 foot. While it faces south it gets a lot of shade in the summer and my hope is that I can use it to grow lettuce in containers into the summer months. That’s a second reason for the frost blanket as it moderates the inside temperature. Of course nothing prevents you from adding another layer over the plants.
So there’s still a bit of trimming of the excess plastic, and picking up some hay bales to insulate the front and sides but by March I should have lettuce growing, and seedlings hardening off before planting.