I'm preparing a raised bed for planting some cool weather vegetables by adding fresh compost and covering it with a 6 mil thickness of plastic. The frame cover is made of PVC and is also used later in the season as my "critter cage" to keep out animals. The plastic will provide a greenhouse effect and warm up the soil in the bed and loosen it for planting. This method is one way Square Foot Gardening extends the growing season. My plants will be growing before the regular ground is even thawed.