How to Build a Cold Frame
A cold frame is a structure that traps heat from the sun in order to grow warm-weather crops, even in cold temperatures. Get a jumpstart on spring seedlings by building your own.
Find a location near a water source on well draining, slightly sloping soil. Avoid sites prone to flooding or strong winds, or sites that are shaded by nearby trees or structures.
Dig a rectangular pit one foot deep to hold the fresh manure and topsoil. It needs to be below ground level to help retain the heat in the hot frame and make it lower than the planting level of the seedlings. Make the outside perimeter of the pit one foot narrower than the glass door or window.
The bales should sit at ground level, not in the pit itself. Butt them together so they fit tightly with no gaps in between the bales. They will form the insulated wall of the hot frame.
The key ingredient to making the hot frame work is fresh manure. As it decomposes, it gives off heat. A good manure to use is from cows or horses and can be obtained for free or at nominal cost from a local farm or stable. You can also use poultry manures or bat guano, which can be more expensive and hard to find. Avoid using dog, cat or human manure, particularly when growing vegetables, as they can transmit disease. Wear old clothes and fill the pit with a six-inch layer of manure. Break up any clumps with a hard rake and tamp it down lightly with a shovel. Moisten the manure with water to help build up heat inside the frame.
Add a six-inch layer of topsoil on top of the manure and rake it smooth. This creates the planting bed for new seedlings. The surface of the topsoil should come up to the bottom of straw bales stacked around the pit. The plants need a buffer between their roots and the manure. If planted directly in the manure, the seedlings will burn up from the heat and an overdose of nutrients.
Plant tender seedlings of vegetables or ornamental plants in the hot frame and transplant them to the garden after the last frost. In milder climates, grow greens and other winter veggies here for harvesting during the cold months.
Place the recycled door or window on top of the straw bales so its weight is evenly distributed across all of them. The glass panes will allow light inside the hot frame. The ground, straw bales and fresh manure will work together to warm the inside of the hot frame. On very warm or cold days, keep an eye on the temperature inside the hot frame; it could help to add a maximum-minimum thermometer to monitor the high and low temperatures inside. For added ventilation, prop the roof open during warm temperatures. If too much sunlight is baking the seedlings, place shade cloth designed for greenhouses on the roof and secure in place.