A cold frame is a bottomless box with a sky light that creates a micro-climate by trapping solar energy. If possible you will want the top to slope so you catch the slanting winter sun.
More sun will enter the higher the angle of the slope.
STEP ONE: Select location
The best location should have good drainage and be facing south. Prepare the soil to a depth of 12-18 inches if you are going to plant directly into the ground. Mix in some compost or manure.
If you don’t want to plant in the ground you can add a layer of gravel for good drainage, and set your potted plants on that.
STEP TWO: DIY Cold Frame Supplies
The back of the cold frame should be higher than the front if you want to create a slant. So I made the front of my frame two boards high (12”) and the back three boards high (18”).
I cut the front and back boards, five in all, to a length of four feet, same size as my plexiglass top.
I screwed the long pieces together at the inside corners using a piece of one inch wide scrap wood and two screws per board.
STEP THREE: Cut the sides
Mine are two feet long, again to match the plexiglass top. To cut the slanted top board, measure the correct length and cut that first, then lay a straight edge from the bottom corner tip (see arrow on photo) to the top corner, draw a line and cut. You will have two identical pieces, one for each side. Nail or screw these together using corner blocks like you did the other sides.
This photo shows the corner blocks I used to hold all the side pieces together. I pre-drilled all the holes to prevent the wood from splitting, a good idea whether you use nails or screws.
The straighter your cuts the tighter and warmer your box will be, very important here in Michigan.
STEP FOUR: Insulation
I lined the inside of my box with foam board insulation. I also plan to add a hay bale to each side of the box for added warmth and to help block cold winter winds.
Because my piece of plexiglass was so flimsy, I used an old wire shelf I had laying around to give it support. I drilled holes in the plexiglass and zip tied it to the wire shelf.
Then using construction adhesive, I glued the rest of the plexiglass edge to the same 1 x 1 board I used for the corner blocks. That allowed me to add a handle, which is optional.
STEP SIX: Add plants.
Remember to remove snow from your box with a broom so you don’t break the glass or plexiglass.
On really cold nights you can toss an old quilt over your cold frame, but remember to remove it in the morning.
When the temperatures are above 45 degrees you should vent your cold frame, either by propping open the lid a few inches or opening it all the way.
Don’t forget to water your plants as needed.
If you have no building skills or tools, you could make a fast cold frame with an old storm window laid across the top of straw bales arranged in a square.
While rummaging around in the garage looking for scrap lumber, I found these two window well covers, and a light bulb went off in my head! I placed the covers back to back and drilled five holes, three across the top and one on each side and then zip-tied them together. This has got to be the quickest and lightest cold frame I’ve ever made.
ABOUT THE AUTHOR: Karyn Flynn is a life long gardener, mother of two & grandmother of three.