A cold frame is a simple device that allows you to start the gardening season earlier in spring or extend it further into fall. Consisting of a wooden frame with a sheet of clear glass or plastic on top, the cold frame protects plants from cold temperatures and acts as a miniature greenhouse. Starting tomato seeds in cold frames allows you to plant these vegetables in early spring when frost is still a danger, allowing the plants to reach maturity sooner for a sooner harvest.
Place the cold frame on the south side of your home or another structure. The south side receives the maximum amount of early spring sunlight, and placing it near a structure helps protect the area from wind and frost, as well as making it more easily accessible.
Fill a seedling tray or a seedling cell-pack with a moist potting mixture. Use a soil-less mix, as these are sterile and better suited to seed starting.
Plant 1 to 2 tomato seeds in each cell of a cell-pack or sow seeds 2 inches apart in a tray. Plant the seeds to a depth twice that of their width, or approximately ¼-inch deep.
Place a 2- to 4-inch layer of straw on the bottom of the cold frame to provide insulation. Set the seedling tray or cell-pack on top the straw and place a thermometer next to it. Close the lid to the cold frame.
Place a blanket over the cold frame glass on nights when frost is predicted. This helps retain the heat collected in the cold frame during the day.
Open the cold frame to vent when the temperature inside reads more than 75 to 80 degrees Fahrenheit. Close the cold frame if the temperature dips below 70 degrees Fahrenheit. Cold frames heat up quickly on sunny days, even if the outdoor temperature is low.
Water the potting mixture in the trays as needed to keep it moist, but not soggy. Tomato seeds germinate in 7 to 14 days, but may take longer if temperatures are particularly low.