In recent articles we have discussed the benefits of having a greenhouse to grow plants and vegetables during the winter months. We also discussed how to add
more heat to an existing greenhouse and how to construct mini greenhouses using PVC pipe, picture frames and plastic bottles to protect your plants or to start growing plants that could be transplanted into your garden when the weather permits it.
There is another structure that can extend the growing time of your garden through the winter months. It’s called a cold frame and it is very easy to build.
The purpose of the cold frame is to take advantage of the rays of the sun to warm up a closed compartment where plants can grow.
The frame is rectangular in shape and includes a glass or Plexiglas top that is angled toward the sun.
Tools and Materials
You will need to collect various kinds of tools and materials to use to construct the frame. These include:
· Box Cutter
· Circular Saw
· Tape Measure
· Construction Adhesive
· 2×4 boards
· Speed Square
· L Brackets
· 3-inch Galvanized Screws
· 1-1/2-inch Galvanized Screws
· Foam Insulation Board
· Plexiglas or Glass
· 2×12 Lumbar Boards
· 2×8 Lumbar Boards
Constructing the Frame
For the purpose of this article, the cold frame will measure 6-feet wide and 3-feet deep. However, you can construct a frame to your own desired measurements.
The 2×12 boards will be used to build the sides and back and the 2×8 boards will be used to construct the front of the frame.
Measure one 2×12 board 72-inches long to serve as the back panel. Measure one 2×8 board 72-inches long to serve as the front panel and measure two 2×12 boards 36-inches long for the sides. Use the circular saw to cut the boards.
Lay the cut boards out in the form of the cold frame. You will need to angle the top of frame at least a1-inch drop per foot of depth so that it is properly slanted to catch the energy from the sun.
In this case, the box is 3-feet deep, so you need a drop of at least 3-inches from back to the front of the frame. In this construction, the height of the backboard and front board is a difference of 4-inches. So use this as the measurement to angle the sides. Use the pencil to mark for the lowest point where the side lines up with the front of the frame. Use the level and pencil to draw a line from this mark to the upper back corner. Repeat this process for the other side.
Cut along the angled lines on the sides of the frame. Try to cut as straight as possible because the straighter the cut, the tighter the fit between the lid and frame.
Pre-drill holes for the screws then use the 3-inch galvanized screws to attach the sides. Add the L-brackets to each of the inside corners of the box. Place two brackets per corner, one near the top and one near the bottom.
Measure a length of 39-inches on each of the two 2×2 pieces of lumber and cut. Measure a length of 69-3/4-inches on each of the two 2×4 slabs of lumber and cut. These pieces will serve as the casing for the lid.
Measure 32-inches long on each of the 2×2 boards and cut. These boards will be the support brace for the center of the lid. Form the pieces into the frame of the lid. Make certain that the outer dimensions of the lid are the same as the outer dimensions of the base of the cold frame so they line up.
You can use a recycled window or glass-paned door to use as the lid if you prefer. The purpose of the lid is to permit a lot of light from the sun to pass through the lid to warm up the interior of the frame. If you use a recycled window or door with windowpanes, make certain that you construct the frame using their dimensions.
Pre-drill the holes on the lid’s frame and then place 3-inch galvanized screws to connect the lumbar together.
Lay the Plexiglas on top of the frame and square up the edges with the frame to assure a good fit. Remove the Plexiglas and squeeze out a line of construction adhesive around the top edge of the frame. Put the Plexiglas back on to the frame.
Use a Plexiglas drill bit and drill holes every 4-inches to 6-inches apart around the fame and secure with screws. The screws and adhesive together will help assure that the lid remains water and airtight.
Center the hinge joint between the lid and top of the frame box at one end of the cold frame and mark where to pre-drill and then do the same on the other end. Pre-drill the holes for the screws and then attach the screws to the last two hinges. Use a total of four hinges to secure the lid onto the cold frame.
A cold frame is ideal for giving seedlings an early start before transplanting them into the garden. Pots or trays holding the seedlings can be put inside the cold frame.
If you want to use the cold frame strictly for preparing seedlings for transplanting, then you may want to consider a gravel base. If you start the seedlings off indoors under light, the seedlings can be put on trays and moved to the cold frame about a week before transplanting them to the garden after the threat of frost has subsided.
(Images courtesy of diynetwork.com)