A dear friend of mine, Kim, asked me if I would explain how to build a coldframe while there is still time before planting time in early Spring. There are many designs available online to make a coldframe or hot bed. Here is the coldframe plan (as a pdf) that I basically used when building my coldframe with a few small exceptions. Now mine isn’t super refined as you can see in the photo but seems to be working! (Be sure you look at the pdf because there is a lot more information in it). As you look at the plan, it’s pretty self explanatory but here is what I did for the sides and the bottom inside.
I used 2 inch x 10 inch lumber for the bottom section all around and another 2″x10″ section for the top side slanted pieces. Cutting the diagonal piece is easy, I drew a line from one corner across diagonally to the other corner and cut on the line then I used one piece for each side on top of the bottom piece. The only thing I did differently is I put one more 1 x 2 inch piece vertically in the middle on each side (screwed in-see photo not diagram) and in the back to join the top and bottom piece together. I added a 2 x 4 (long) piece on the lid that I screwed in (not tightly) so I can raise and lower the lid and prop it up vertically (see the piece holding up the lid on the right side). In the photo you notice I taped BIG bubble wrap (not small bubbles) with duct tape on on the inside of the plexiglass lid to add extra insulation in the dead of winter. I also divided mine into two sections.This isn’t in the plan but in the left section I started with hardwire cloth on the bottom to keep out gophers. Then I added dirt and compost on top of the hardwire so I could plant in it but the soil isn’t heated. The right side has a garden heating cable. I attached the cable with twist ties to the hardwire cloth that is cut out to fit the bottom. Then I turned over the hardwire cloth so now it is on top and place it on the bottom. I did this so I can’t put a spade through the cables while digging around in the dirt when planting. Then I put dirt on top like the other side. So the right side is a contemporary hot house when I plug in the heating cable (which I haven’t done yet). I will use it to heat the soil to a temperature so the seeds will sprout. A traditional hot house has a dug out area where ‘hot’ green manure is placed into. Then on top of that is the dirt that you would put your plants or seeds in. The manure gives off heat as it composts, heating the soil just like the heating cable. Now I love the idea of the manure and doing it naturally but I don’t want to have to replace it every year so I chose the heating cable. I will use an extension cord to bring power from the house. I’m experimenting with the non-heated and heated side to see if it really makes a difference in early Spring. If it does, I’ll heat the left side too next year.
Last time I reported, the right side which I planted with transplants in November, is still doing well while the left side where I planted transplants in January all died in the -20°F we had one night (except the parsley which survived) which is interesting cause I don’t even like parsley! I think the left side all died while the right side didn’t because they did not have any time to grow roots while the right side planted in November did. Remember I haven’t heated the right side yet so I know that wasn’t a factor. Anyways I’m sure the plans will help you more than my description!