Building a Cold Frame for Season Extension

Cold frames are a technique for extending the growing season in the garden. In general they can effectively make it so the microclimate within them is like that of one USDA Hardiness zone greater. In practice, this means about 2-4 weeks extra on either end of the warmer months for more tender annuals, and some more hardy biennials (like kale, cabbage, winter spinach, etc.) can be grown within them all year round (depending on your location).

Building a cold frame can be simple and cheap or as complicated and expensive as you like. The simplest method is to build a box of wood about 8-16″ tall all around and just place the window on top of it. Since the Southside Permaculture Park is angled north, we wanted to correct for that so we made a pair of simple angled cold frames for free using scrap and upcycled materials. We’ll show you how we did it.

First, we found a pair of double-pane glass windows in a dumpster. Remodeling and construction sites almost always have perfectly good windows in the dumpsters, and they rarely care if you hop in and take them away. We highly discourage the use of new materials, as it is wasteful and there are plenty of perfectly good materials available in the waste stream already.

Perfect cold frame windows sitting in a dumpster waiting to be rescued.

A comrade scraping caulk off of scrap wood to make it flat and usable for the cold frames

Next, we measured the inside of the frame so that we could fit a rectangle of wood flush with the glass. We then cut up some scrap wood to size and screwed the boards together into a rectangle.

Once the rectangle that holds the windows was made, we had to create some trusses to angle it up towards the sun. The Southside Permaculture Park slopes north at about 10º, and since we want the angle of the cold frame to face south at about 20-30º, we decided to angle it up at 35º. If you are building cold frames on a site with a different slope, adjust the angles so that the window faces towards the south sky at an angle of 20-30º. Effectively, we are building triangles to attach to the box we just made.

Box made in step one with window resting on it and trusses attached. Note that the box made in the first step is not the bottom of the triangle, but the angled face attached to the window.

To do this, we cut one board to a length equal to the length of the long side of the box divided by the cosine of the angle, in our case 35º. Since the length of the box was 29.25″, we cut this board to 29.25/cos(35) = 35.7″.

Formulas for lengths of the different sides of the triangle. The side labeled ‘X’ on the bottom is the side in contact with the window (gray rectangle), and will vary based on the size of the window you can find. Note that this side will not actually be in contact with the ground, as shown here.We then cut off the corners, one side at an angle equal to 35º, and the other side at an angle equal to (90 – 35) = 55º. This forms the hypotenuse of the triangle.

To make the short leg of the triangle, we cut a board to a length of the long leg multiplied by the tangent of the angle, plus a little extra for the overlap with the long leg to fasten them together. Numerically,  29.25*tan(35) + 3 = 24″

With all the pieces cut, it just took a drill and a few screws to put it all together.

To close up the gaps, we used some old greenhouse plastic that we got for free from a scrap yard, but you can use any material from plywood to 2x4s to garbage bags or whatever else you can have on hand!

Once they are sealed up, flip them over and let them do their thing!