Here at Stoney Acres we have three cold frames. Each is 4 foot by 8 foot. The plan I used came from “Four Seasons Harvest” by Elliot Coleman. Although I did adapt the plans a little to my use and the tools I had to build them.
Construction takes a while but is not overly complicated. I used Douglas Fir to build my frames. One of my frames is 4 years old, the other two are 3 years old and all seem to be holding up well. I chose Douglas Fir mainly for cost. Cedar and Redwood would both last a lot longer but cost about 4 times as much. I hope the frames themselves will last 5 to 7 years and the lids much longer, we’ll see. I would never use pressure treated wood because the chemicals used to treat the wood can leech out of the wood into the soil and contaminate my organic soil.
The wood itself represents only about 20% of the cost. In 2012 dollars about $30.00 should cover it if you’re using the Douglas Fir. The biggest cost is for the plexi glass I used. Each 4×2 sheet costs about $25.00 and I needed four sheets. So the total cost for each frame was about $130.00. If you chose not to use plexi glass and instead just used 6 mil plastic your cost would be a lot less but you would also lose a lot of heat retention.
The box itself is made up of one 2×12 and one 2×8 with another 2×12 cut in half and then cut at an angle from 12 inches to 8 inches. The angled boards are the sides of the box and add the slope to the box. I also added a stretcher to add stability and the stretcher also makes moving the box much easier.
Some people paint or stain the wood but for me that just adds one more thing to have to take care of (repainting) and it also adds another chemical source to my organic garden.
I did add a strip of 1×2 to the bottom of all my boxes. This wood is super cheap only about $4.00 per frame. It puts a layer of wood between the actual frames and the soil. This will greatly increase the life of the frames. When it gets old and rotten you can simply pull it off and add some fresh wood, in this way the wood on the bottom of your frame rots a lot slower.
The lids (or lights as they are called) are made out of 2×2’s. I cut a small slot in the long sides of the boards to allow the glass to slip in and hold. I then added some screws to each end to hold the glass in place. My cold frames sit in a part of my yard that is protected from most winds. Only once in 4 years have my lights actually blown off or even moved in the wind. If you don’t have a similar spot in your yard you may also want to add some kind of hook or other latch to keep the lids from blowing off in the wind.
Over all each box took me about 3 hours to build. I have built several frames for neighbors as well and now I’m quite the pro and I could probably get one done in two hours or so with a little help from my son.
Now with all this said you don’t really need to go to this much work. My cold frames are kind of the “Cadillac” version. Many people will use an old storm door or window on top of a wooden frame. I have even seen people have success with the box being formed by straw bales and the top just being 6 mil plastic. The point is to protect your plants from the cold wind and freezing temps. My design works well and looks good but it is not the only way to protect your crops.
Next time we will look at hoop houses.