Greenhouses require constant and rather high heat during the winter to keep your plants alive. Since heating bills already go up in the winter, avoid going over budget by creating your own, free greenhouse heat. The concept is simple: water barrels set against the inside of the north wall of your greenhouse absorb solar heat through the south wall during the day. The water releases the heat at night, keeping your greenhouse at a constant warm temperature throughout the winter months.
Measure the height and width of the north wall in your greenhouse. Measure the height and width of your water barrels. Divide the height of the wall by the height of the barrels, and the width of the wall by the width of the barrels to find out how many barrels you need. For instance, a wall 9 feet high by 8 feet wide needs 12 water barrels 3 feet high by 2 feet wide.
Dilute about 2 tablespoons of bleach in a pint of water and rinse the inside of each of your barrels with the solution. This prevents mould growth and keeps fungus from spreading in your greenhouse.
Fill each of your barrels with lukewarm water and stack them against the inside of your north wall. Do this early in the cold season, around late September, when the days are still relatively long. This ensures your greenhouse warms quickly, and stays warm throughout the snowy months.
Choose barrels with a medium to dark colour, such as royal blue, navy, hunter green or black. You can also spray paint them if you can only find light colours. Use a compass to find the north wall in your greenhouse. Simply stand inside the greenhouse and turn your compass until the needle points north. If your north "wall" turns out to be a corner or narrow wall, choose the northeast or northwest wall for your barrels. They'll still get plenty of sun.