For some years, Linda has wanted me to build her cold frames. For those of you living in more temperate locations, where you can start your lettuce in January and begin harvesting before March, this might not be much of a concern. However, here in the Appalachian Mountains, and other regions north, April frosts and killing freezes are common night events. However, if we wait until the risk of frost is over (officially May 20th where we are) to plant cool, Spring crops (lettuce, cabbage, broccoli, peas…) the sprouts will be toasted within a week when the temperatures shoot up to 85F for a day or so in early May. Thus, April brings cold frames, which I have finally gotten around to building.
A cold frame is a box with a window placed on it. The window on top will be angled toward the south and should be hinged to allow it to open as the day warms up. The box and window trap warm air and warm the soil to keep the seedlings from freezing at night. With the window opened during the day, excess heat can flow out.
Start by harvesting some spare windows that you happen to have stored in your garage or shop, along with some hinges. What, you don’t collect all the neighbors’ cast off windows when they upgrade and remodel? Get out and do some dumpster diving. Remember, those windows required energy to build in the first place. No need to expend more energy hauling them away to crush them up in the land fill.
If you happen to keep a stock of scrap wood around, you might make your cold frames for nothing, other than time and some screws… Of course, you have boxes-and-boxes of screws left over from prior projects. This is a budget-neutral appropriation. Well, I will admit that I had used up all my 1 x 4 x 8’s, so I did purchase those on a recent trip to town.
With two windows of similar size selected, I constructed frames around the windows’ frames and held them together this lots of screws. This is pretty crude construction technique, but it is going to sit in the garden, not the living room.
Then I took some measurements to create the front and back. To keep the dimension simple, I used two 1 x 4’s for the front and three for the back. That gave me about 7 1/2’ of front height and 11 1/4”, allowing for the slope of the window.
I constructed the sides with five 1 x 4’s, held together with a ridiculous pile of odds-and-ends from cutting all those 1 x 4’s. The middle 1 x 4 got cut along the diagonal from corner to corner, thus matching the pitch from three to two 1 x 4’s on the back and front sides. I set the depth of the skill saw blade just a tooth more than the depth of the 1×4, so that my cut left the two support pieces to hold it all together. Then I flipped the sides over and paralleled the cut, taking off an extra inch of the supports to allow the window frame to slip down snuggly. Make sure that you remove screws that end up in line with any of these cuts.
Before assembling all of this, Linda sealed all the wood that was exposed. That will help them last longer, and not require that I rebuild them so soon. Once the four sides are excessively screwed together, I attached the window/frames with the hinges, on the back side.
Unless you are home daily and compulsive about checking for when your cold frames need the windows lifts, you need to find a heat sensitive lever arm to raise the window. You can purchase these from companies which sell green house supplies. These devises have a pneumatic piston which expands when warmed. As the piston pushes out, the hinge lifts the window up. It reverses at night, when the piston cools and contracts, closing the window. Pretty spiffy. I’ll have to update your on when the seeds sprout.