Cold frames are boxes made of wood, cinder block or bricks topped with glass. The bottom of the box is open and rests directly on the ground. The cold frame keeps the interior of the box about 10 degrees Fahrenheit warmer than the weather outside the box. Cold frames allow gardeners to start seedlings outside before the average date of the last frost in spring and growing a few weeks after the average date of the first frost in fall.
Place the cold frame in an area that receives eight hours of sunlight and is sheltered from the wind. Next to a wall or stone fence is good because the wall retains heat.
Place a layer of newspapers over the grass or dirt of where you will place the cold frame. The newspapers will kill the grass or weeds by depriving them of sunlight. Eventually the newspapers will decompose and add organic matter to the soil. If you are planting seeds directly in the cold frame, add 3 inches of compost, 6 inches of potting soil and slow-release fertilizer per package directions. Mix the ingredients well.
Grow cool-season vegetables such as spinach, chard, kale and leaf lettuces in the cold frame. Plant the seeds an inch apart and cover with 1/4 inch of soil. Water so the soil is moist all the way down to the newspaper layer. Harvest when the leaves are 3 to 4 inches long.
Plant broccoli, cabbage and head lettuce seeds in pots. Place the pots in the cold frame. When the last average date of frost has passed, transplant the seedlings into the garden.
Plant tomatoes, peppers, eggplant and squash or other warm-season vegetables in pots in the cold frame six weeks before you would normally plant in the garden. Place a black plastic garbage bag on the newspapers. The black attracts and retains heat. The cold frame also keeps the seedlings warm. If nights are forecast to get below 50 degrees Fahrenheit, put a few gallons of water in plastic jugs inside the cold frame.