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By extending the season, you can garden year-round if you live in a southern zone and you can add months of productive time if you live in northern climates. Greenhouses and cold frames are structures that trap energy from the sun to create heat. They warm up the air inside the structure as well as the ground below. Even in cold temperatures, the light from the sun can provide enough energy to grow warm weather crops in a greenhouse.
What makes cold frames so effective is the amount of energy they collect to get seedlings started in the spring. A cold frame will allow you to get a 2 to 4 week jumpstart on your spring plantings. Here's how to build your own:
Materials:(All lumber should be pressure-treated pine, redwood or cedar.) 2"x12" 6' feet long
2"x12" 3' feet long
2"x8" 6' feet long
1 1/4" and 3" galvanized screws
8 L-brackets
6 straight brackets
3' x 6' Plexiglas sheet
2x4 6' feet long
2x2 3' feet long
Construction adhesive, 2 tubes of clear or white
3" Utility Hinges
4"x8" sheet of insulation board
Circular saw
Drill with wood and Plexiglas bits
Level
Tape Measure
Speed Square
Pencil
Safety Gear - The cold frame is 6 feet wide and 3 feet deep. Use 2 x 12's on the sides and back, and a 2 x 8 piece of lumber on the front. Measure, and then cut the boards with a circular saw. Lay the pieces out to form the cold frame. There's a step down between the height of the back and sides and the height of the front of the cold frame. In order to catch the energy from the sun, the top of the cold frame has to be angled at least 1 inch in drop per foot of depth. The box is 3 feet deep, so you need at least a 3-inch drop. There's a 4-inch difference between the heights of these boards so you can use that as the measurement to angle the sides. Make a mark for the lowest point, where it lines up with the 2x8. Now take the level and draw a line from this mark to the upper back corner. This is the line you will cut to give the cold frame lid the sun exposure it needs.
- Using the circular saw, cut along the angled line marked between the height of the 2x8 and the height of the 2x12. The straighter the cut, the tighter the fit will be between the lid and the frame. This slope will support the lid of the cold frame.
- Once all of the sides are cut, assemble the bottom of the cold frame. Use galvanized screws to attach the sides together. Pre-drill the holes before putting in the screws to prevent splits.
- To be able to move the cold frame around it has to be sturdy. Add metal L brackets to the inside of each of the corners of the box; put 2 per corner, 1 near the top and one near the bottom. This will keep the corners from splitting apart when it's moved.
- In order to retain heat inside the cold frame, insulate it. Buy insulation boards used to face houses. Measure the insides of the cold frame box and transfer those to the insulation sheet. The board cuts easily and you can use a sharp pocket knife or box cutter to make the cuts.
- Once the pieces are cut to size, install the insulation to the inside walls of the cold frame. Use construction adhesive to attach them. Add a few 1 ¼" inch screws to each piece to keep the insulation in place until the adhesive sets up.
- The bottom of the cold frame is complete and it will provide support for the most important part of the project, the lid. Use clear Plexiglas for the top, you could also use an old window or door as the top. The most important thing is that lots of light can come through, otherwise you won't get the heat build-up you need.
- Use 2x2's and 2x4's for the frame. Measure and cut 2 2x2's to 39" and cut 2 2x4's to 69 3/4" to make the casing for the cold frame lid. Also cut a 2x2 32" support brace for the center of the lid to add additional strength. Make sure that the outer finished dimension of your lid is the same size as the cold frame base or your lid and box won't line up.
- To assemble the lid, use 3" inch galvanized screws. It's important to pre-drill the holes because the wood is narrow and will split easily. Once you have a screw in each corner lay your Plexiglas on top of the frame. Square your frame to line up with the edges of the glass.
- Once the lid frame is assembled, run a line of construction adhesive around the top edge of the frame. Then place the Plexiglas on top. Using a bit made for Plexiglas; drill holes every 4-6 inches around the frame and secured the cover to the frame. The adhesive and screws together will make the lid more water and airtight.
- To put the top and bottom together, use utility hinges to hold the top in place. You could also use window hinges or gate hinges.
- To attach the hinges, center the hinge joint between the lid and base at one end of the cold frame and mark where to pre-drill. Repeat step at the other end. Then pre-drill the holes for the screws; put the hinges in place and attach them with 1 1/4" screws. Next, measure in 22" inches from the center of the outside hinges and attach the last two hinges.
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