Building a Simple Cold Frame

A cold frame is a bottomless box with a skyward-facing window. Like a miniature greenhouse, a cold frame lengthens the gardening season by protecting plants and seeds from the moderately cold temperatures and drying winds of late fall and early spring. With the addition of a simple heater, a cold frame can be used nearly year-round to grow cool-season flowers and vegetables, and to give summer plants an early start.

The low-cost, easy-to-build cold frame presented here takes one or two weekends to build and uses widely available materials. You can place it on a deck or patio to grow plants in pots, or you can place it over a garden bed.

• 4-x-4-foot sheet of 1/2-inch exterior plywood (or 2 2-x-4-foot panels) plus scraps of 1/4-inch plywood for corner braces
• 18-gauge perforated steel angle -- approximately 3 feet
• 2 1x3x8-foot clear pine for the top frame
• Hardware: 25 1/4-inch-diameter by 1-inch-long hex bolts and nuts; 16 washers sized to fit the hex bolts; 4 4-inch steel L-brackets; 45 3/4-inch No. 8 wood screws; 3 3-1/2-inch steel or brass hinges. Expert tip: Buy a few extra bolts and screws.
• 6-mil poly sheeting, clear, to cover the top of the cold frame
• Tools: Long ruler or tape measure; saber saw or circular saw; hacksaw; power drill and 1/4-inch bit; screwdriver (or screwdriver bit for drill); staple gun, or a hammer and 3/4-inch roofing nails
• Optional materials: 1 or 2 4-x-8-foot sheets of 3/4-inch rigid foam board insulation (Foamular or similar). 2 or 3 100-bulb strings of miniature holiday lights. Vinyl tablecloths or blankets.