We did some remodeling projects around the house this summer, and I am left with two sliding doors that I don't want to pay to take to the dump. I thought that they would make a nice top to a coldframe. Is there any place that has plans for one, or do people just improvise a box for whatever size window they have? Also, if I build one in early January, when could I put plants (lettuce, spinach, etc) inside?
Michael Cicha Iowa City, Iowa Zone 5A
Just improvise. If you can remodel, you can certainly make a decent coldframe. How early depends on a number of things. The deeper the coldframe, the better insulated, of course. The more leaves or snow you add to the sides, the better insulated. If you plant in the soil inside, which never freezes, it is better than if you plant in small sixpacks that could freeze. Will you place some sort of heat sink, such as water containers, or hot manure to generate heat, etc. Since I put my lettuce seedlings out under the tunnels on March 15, and also seed the spinach at the same time, you can use your coldframe (with much better insulation under all circumstances) at least as early as that. I would roughly guess March 1, keeping in mind that Ann Arbor is just a bit milder than Iowa.
I make them from scratch. Here are a few pictures of some I built over the summer with scrap wood, spare white paint, Thompsons wood sealer, and a few hinges. The glass (of varying sizes and shapes) was purchased at garage sales. Some of these are for sale if anyone is around nw NJ to pick them up :)
When starting, the only accurate measurements you have are the length & width of the glass top, the angle of the glass slanted toward the sun (usually 20 or 25 degrees), plus how high the cold frame "box" part will reach (higher boxes will trap heat too high up for plants, but a low box will be very cramped after pots & soil are added).
Now you need the width and height measurements for the cold frame. I will make this "example" cold frame with a 20-degree slope. I make cold frames in 5-degree slope increments because my circular saw only has 5-degree sloped increments...if you have a circular saw that can perform more accurate slopes, a 23.5 degree slope is the best angle for most cold frames. I'll make the glass plate 24 inches in width for the following example.
To make a long story short, you obtain the cold-frame WIDTH with the following equation (the length is however long the window is).
CosA° = adjacent / hypotenuse or Cos20°= width / 24 inches (the glass plate width = the hypotenuse of the triangle) A = slope, which in this case is 20°
width of cold frame = 22.55 inches
The cold frame REAR HEIGHT is made with the following equation. The FRONT HEIGHT is however high the box part is. In this case, I'll make it 12 inches high.
SinA° = opposite / hypotenuse or Sin20° = rear-height / 24 inches
rear-height = 8.2 inches, pluse the FRONT HEIGHT (12") = 20.2 inches
Now you've got all the cold frame dimensions you need. Cutting at angles with the circular saw is tricky, but Im sure you'll get the hang of it. Remeber to factor the slope of the cuts into your measurements, plus _thickness of the wood_ (a 24-inch long cold frame using inch-thick wood only has to be cut 22 inches in length). A cold frame with these dimensions and 3/4 to 1-inch thick wood will be extremely snug and retain heat extremely well. Adding a layer of insulation, inside or out, surrounding the frame never hurt, either.
Depends on where you live, and what your winter temperatures are. You can always risk it in colder weather, the soil might be able to retain enough warmth during the day to keep the plants alive at night. Gallon-jugs of water placed inside the frame will also retain heat during the day and release it at night (water has a very good capacity to store heat).
Dan
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