Propagating plants is an inexpensive way to fill out your garden and create new plants using a cold frame. You can also extend your gardening season in spring and fall.
A cold frame acts as a mini-greenhouse constructed of four walls and a movable transparent or semitransparent lid. The walls provide shelter from wind and cold and the transparent lid lets sunlight in. With a cold frame, simply add your rooting medium and then insert the plant cutting to the proper depth.
When planning your cold frame, remember it should be 12-24 inches high in front and 36-48 inches high in back. The slope makes it possible for sunshine and heat to enter.
Make the transparent lid out of old salvaged windows, translucent fiberglass, Plexiglas or plastic sheeting. The size will depend on your individual needs and available space. (Note: If you plan to use an old window for the top, its size will dictate the size of your cold frame, so measure and build accordingly).
To build a 24" x 48" cold frame with a Plexiglas lid:
Use 2x4s for the frames. Cut two pieces to 46 3/4-in long and two pieces to 19 1/2-in long. Screw the pieces together so the ends of the 19 1/2-in pieces are between the 46 3/4-in pieces.
Cut two pieces of 3/4-in plywood 22 1/2-in wide by 24-in tall. Mark the plywood 12-in tall on one end and 24-in tall on the other. Cut the plywood diagonally between the marks. The cut should run at approximately a 45 degree angle. Position the 22 1/2-in edge flush with the bottom of the frame and screw the plywood to the frame. Repeat this step on the other end of the frame.
Cut one piece of 3/4-in plywood 48-in x 24-in. Position the plywood so its bottom edge is flush with the bottom of the frame and its outside edges overlap the plywood end panels.
Cut another piece of 3/4-in plywood 48-in x 12-in and screw it to the front of the frame the same way you did the back panel.
Screw L-brackets into each corner of the cold frame 1" to 2" down from the top to hold the corners together.
Cut a 2x4 to 48-in long and secure it to the outside of the back panel flush with the top.
Secure hinges to the ends and center of a piece of 48-in x 29-in Plexiglas. Screw the hinges to the 2x4 at the top of the back panel.
Place the box in a shady, well-drained area, fill it with rooting media and add your cuttings.
1. Face the frame south to get the most benefit from winter sunlight.
2. Fill with potting mix and plant seeds directly in the cold frame. If you live in a colder climate, you may prefer to start the seeds indoors and transfer the seedlings. Follow the planting instructions on the seed pack.
3. Raise and lower the lid to different levels as temperatures increase and decrease to "harden off" the seedlings. Remember, although it's called a cold frame, cold may be of less concern than heat. Too much heat from the sun will "cook" the seedlings (even in February), so make sure you do not leave the cover closed too long during the day. If you are really serious, buy a thermometer for your cold frame to monitor high and low temperatures.
4. Cover the frame with a blanket at night to insulate from extreme cold.
5. Smaller cold frames can be moved or disassembled at season's end.
6. Plants in a cold frame require extra attention and can be time consuming, but if you value early flowers, vegetables (and bragging rights), give it a try.