Making a cold frame to start spring seeds in winter

So you already know I am super excited for spring planting. I have my seeds germinating and a bunch of little sprouts. To start them off early and get them hardened off a cold frame is a way to have your seedlings outdoors but protected in case of one more frost.

I wish I had known about this last year since I moved in at the start of winter and my spring plants didn't get a good showing with the late season frosts stunting them as sprouts. Basically a cold frame is a mini greenhouse.

The top though has to be see-through and placed at an angle for the sun to warm up the sprouts that will be inside. I took some plexiglass out of a free large plastic picture frame for this part. The bottom stays open to the dirt, or it could be closed I suppose. The top needs to be able to open as well so you can use hinges or like I did, put a 1x2 piece to rest on the bottom. Unfortunately it means I have to use two hands to open it but I didn't have the right kind of hinges in my collection. I still have some chip board from my neighbor and some 2x6 scraps from my deck project I will post about later to make the sides out of.

I have heard too about a heat activated window opener, like a small shock, that many people use so if it gets too hot in the cold frame, the heat makes the opener open the lid to the cold frame and let some air in to keep the plants nice and healthy. I don't think I have too worry about things getting too hot in the winter here though. I was worried about snow load on the lid though so I designed it so the snow will slid off my lid.

The size of my cold frame was dictated by my plexiglass sheet. First I made a frame for this. I did only three sides however and left the bottom off so the snow will slide off. I used a table saw to cut the groove so the plexiglass will slide in. I used screws on the bottom edge with the heads keeping the plexiglass in place but glue in all the grooves will work probably better. It was too floppy leaving the bottom off completely though so I put a piece of wood underneath a few inches up from the bottom, as short as I could make it while being able to screw onto the wood frame to act  as a block to keep the lid on instead of hinges. Later I did it on the top as well.

Now I know the length of my cold frame. I cut my two front and back pieces to length. One I made about half the height of the other since I want a steep angle on mine. With a little difficulty I laid the lid on top to see what my width would be and cut the two sides of my box.

The angle on top is simply the height of the front piece to the height of the back piece. I now used 2x6s for the corner supports. If  you don't have scrap dimensional pieces, 2x2s would work great. Since I have an arbitrary angle, I made them a little long and penciled the angle on the piece and cut them that way. Then I screwed them to the side pieces while laying flat and then raised them vertical to screw the front and back pieces on.

If you are doing hinges, mark your holes and pre-drill the holes for the screws and do that, but like I said, I just put blocks under my lid to hold it in place. Then I painted it all dark brown because the plywood was very conspicuous in the place I put it, which is in a sunny location with the angle facing the south so it will get the most exposure. Plus the dark color should help absorb a little more heat as well.