Every year I try to plant a fall vegetable garden to take advantage of those last few warm days. But even in the mildest climates, temperatures can drop suddenly and cause big problems, so I'm always looking for solutions.
I came up with a simple design for a cold frame. It sits right on top of the framed bed in my vegetable garden.
I've also found I can use this cold frame to protect plants during winter by arranging bales of straw in a square and placing it on top of the bales. This creates a protected spot for plants I want to overwinter in containers.
The idea here is really simple, it's just taking energy from the sun and warming the soil and air temperatures. It is like creating a mini-greenhouse. Just remember to remove or prop open the top of your cold frame during the day because it can get pretty hot in there.
There's really nothing to building this handy device, some 2 x 4s, plastic and concrete reinforcing wire are all you need to get started.
The materials that I have listed below will build (1) 4' x 4' cold frame.
(4) 2" x 4" boards 4 feet long
4 mil plastic sheeting
(2) 24" wide x 7 foot long pieces of galvanized welded wire (concrete reinforcing wire)
staples for staple gun
(8) 2 1/2" nails
The first step in putting this cold frame together is to build a wooden base. To save time have the boards cut to size at the home center or lumberyard. Attach the boards together using a simple butt joint to create a square. Use 2 nails at each joint to hold the frame together.
Galvanized welded wire or concrete reinforcing wire is strong stuff and it's the key to supporting your plastic sheeting. It can be purchased in standardized rolls, usually about 24 inches wide and 25 feet long. Cut the wire long enough to create an arch that's at least 24 inches high at the highest point. This gives you plenty of room for ventilation, and enough room to reach under the cold frame and work in your vegetable bed. And be sure to factor in a few inches on each end to staple the wire to the wooden base.
Attach the wire to the boards using a staple gun. Then drape your 4 mil plastic sheeting over the top of the wire frame and secure into place by stapling to the wooden base. To prevent the plastic from tearing I double or triple roll the edge that is going to be stapled. You can find plastic sheeting at any home improvement store sold in rolls or sometimes in the gardening department as frost protection. Allow the plastic to hang over the open ends of the frame so that you can pull it back during the day for ventilation and close it up at night for frost protection.