Use a cold frame to extend the growing season fall into early winter and late winter into spring and protect plants in summer and winter. Insulate the cold frame with sheet plastic or thick rigid urethane to protect plants in mid-winter; replace the cold frame lid with lath in summer to create a lath house.
A frame 3 by 6 feet (1 by 1.8 m) will fit easily atop a raised bed. A frame this size is portable but large enough to reduce temperature swings inside the frame.
Use a lid made of plastic, plexiglass, acrylic, or plastic sheeting. A double layer of plastic sheeting will keep the frame interior 10 to 20 degrees warmer than the outside temperature. Add additional insulation—a tarp or old blanket–atop the lid on nights when the temperature is frigid.
The seal between the lid and the top of the frame should be as tight as possible. Cover the frame edge with weather stripping or strips of carpet underpadding to close gaps. Be sure to prop the lid open on mild days for ventilation.
The height of the frame should be slightly taller than the plants growing inside at maturity.
A low frame—6 to 8 inches (15 or 20 cm) is all that is needed for seed starting in spring. Frame units of the same size can be stacked on the bottom unit to raise the height of the frame if more height is needed. Use pegs that fit in the unit below to secure the frame. The top unit should have a sloping lid.
The slope of the frame lid should be 7 to 15° to admit light and allow rain to run off. To maximize sunlight and warmth in the frame—especially in early spring or late autumn, a lid slope of 30° is desirable.
Frames or frame units set on a raised bed can be removed and stored during the summer or the lid can be covered with cheesecloth, plant fabric, or shade cloth to protect young seedlings from summer heat.