No matter where gardeners live, the growing season is never long enough. Right? It wouldn't be hard to extend the growing season to earlier in the spring by constructing a cold frame.
My grandfather always used a cold frame and had lettuce and onions weeks before anyone else up in Morgantown, West Virginia. He used some old wood and a wooden door to make his.
A cold frame is really no more than a glass or plastic covered box that stores the sun's heat while giving the gardener some control over temperature through the use of ventilation. The home gardener can also start seeds in the warm daytime temperatures that the mini-greenhouse provides. The cool nights inside the cold frame allow seedlings to develop healthy, rugged growth in about six weeks.
The size of the cold frame is dependent on the amount of lettuce and onions you want. However, a 4 foot by 6 foot box is plenty large enough for an average family of 4-6. All you need are two boards 4 feet and two 6 feet long and about 12-15 inches wide. Then, hinge a top made of wood strips or even place a wooden door over top like grandpa did. How far you set the cold frame into the ground actually depends on the local weather factors.
You might want to slope the box towards the South to take advantage of most of the sun's rays. The temperature should stay around 75 degrees during the day, but if it heats up, just prop up the top. It wouldn't be a bad idea to have a thermometer inside the cold frame itself. As for night time temperatures, it should be around 45 degrees for best plant development.