Geodesic Dome Greenhouse - milk jugs right next to the plants in the spring to pick up heat during the day and a garden hose from the water pump around the outside of the greenhouse, right next to the plants. The hose is much warmer than the air at night.
21 DIY Greenhouses with Great Tutorials: Ultimate collection of THE BEST tutorials on how to build amazing DIY greenhouses, hoop tunnels and cold frames! Lots of inspirations to get you started! - A Piece of Rainbow