Basic cold frames can keep your spring seeds warm enough to germinate and grow into seedlings. You can build cold frames for your own garden, allowing you to grow any time of year. To grow spring plants in your cold frame, simply wait for mid to late winter before planting seeds directly into the soil or seed pots filled with soil.
Measure one side of a 2 foot square piece of plywood and mark the edge of the board where it is 16 inches tall. Draw a diagonal line from the opposite edge’s top corner to the 16-inch mark.
Saw along the diagonal line and keep the larger wedge for the frame. Repeat with your other 2 foot square piece. Stand the newly cut pieces parallel 3 feet apart on their bottom edge with the fresh cut edges running diagonally.
Place the 2-by-3-foot plywood board on its 3 foot edge between the two end pieces to form the back of the frame. Make the 2-by-3-foot board flush with the 2 foot end of the sides. Attach the side boards to the back board with four evenly spaced screws inserted through the sides and into the long board.
Set the 16-by-36-inch board between the side boards and bring it to the front to close the box. Make the edges flush and screw three evenly spaced screws through the side boards and into the front panel to complete the cold frame base.
Dig the outline of a 2-by-3-foot rectangle in your garden bed to create a shallow trench three inches deep and two to three inches wide. Set the cold frame into the bed area and work the bottom of the frame into the trench. Pack the soil in toward the plywood on the inside and outside of the frame.
Place an old window over the frame on the angle. Adjust the window to bring the back edge of the window flush with the back of the cold frame. Center your hinges a foot apart along the back. Attach the hinges to the cold frame and the window by screwing through the holes in the hinges.