Just in case
people are similar to what I was a year ago, a cold frame (to me) is a box with a transparent lid and no bottom, which one can set onto the ground. It is a short
greenhouse, sort of.
I am committed to building 4 cold frames (of the Fine
Gardening design), which is nominally 3x6 (or 18 square feet). Or rather, I can't afford to put acrylic or polycarbonate "glazing" in the top, so I will be using 6 mil PE (polyethylene). I might be able to figure out double or triple PE glazing if I need to.
That design, uses 2x6
wood. I am only planning to make the bottom 2x6 out of western red cedar. The Fine
Gardening design has a second full 2x6 stacked on top, and then a half layer (wedge) onto which the top is mounted.
My initial needs, are for trying to grow
trees from
seed. Once I have
enough trees, I could use them for other things.
Most trees grown from seed don't get very tall, very fast. So, a box which is nominally 11 inches tall (2 times 5.5 inches) is probably fine. If you put 3 inches of soil in the bottom, you have 8 inches for the seedlings to grow to before they run into the glazing. I am guessing a person
should go "heavy" on the sand fraction of the soil, so that it is easier to separate the soil from the
roots come time to transplant them?
But, if at some point I don't need to grow trees; starting vegetables (and other non-trees) might be useful. Or even just leaving the vegetables in the cold frame for the season (as a shorter alternative to a hoop house)? How tall does a person want the cold frame (minus the wedge layer)?
Operating
I will guess that if the object is growing trees from seed, a person starts by putting many layers of
newspaper on the ground, and then plopping the cold frame onto the newspaper (after the ground has been levelled). Or, maybe that gets rephrased to, if you are growing trees from seed on an ancient fescue pasture, ....?
If after N years of growing trees, you decide to move to vegetables, I
think what a person might want to start with is potatoes. I have always heard they were a good preparation crop. If the newspaper hasn't disintegrated by then, maybe fork the bottom a bit, so that the potato plants can get to "the real soil". And after some undetermined number of years, the potatoes may have broken up the top enough, to start planting other things? But, could a person plant tillage radish (aka daikon radish) or one of the big turnips, or ... to now get 1 to 2 feet of the underlying soil broken up enough that lesser crops (like carrots?) could grow there?
I have tillage radish seed. I have yet to actually get tillage radish to do anything useful. Soaking the seed overnight before placing it on the ground does seem to help. But, if you have too high a seed density on the ground, it doesn't seem to grow.
I ran across a generic howto on hoop house building.
High Tunnel Hoop House
Construction Guide
Steve Upson
https://web.extension.illinois.edu/bcjmw/downloads/54183.pdf