Plastic greenhouses are ideal for starting seeds for spring planting. They're also a great way to store and protect fragile outdoor plants from cool weather. The best thing about plastic greenhouses is that they are designed to absorb and retain heat, so heating them can be done safely and cheaply. It only requires a little knowledge about the sun and how greenhouses work.
Find the south side of your property. Placing your greenhouse in a place where it will get southern exposure guarantees that it will benefit from the most light and heat energy from the sun. Place your greenhouse in a place where it will not be shaded by trees or buildings if possible.
Vent your greenhouse at the top. This will allow air to circulate. Plants need good air circulation to thrive, and soil needs good air circulation to prevent mould and bacteria growth. Vent the top just enough to let air in without letting too much heat out.
Seal or close up your plastic greenhouse. Greenhouses are made to allow sun energy to enter freely, but to be unable to escape. Simply placing your plastic greenhouse in the sun will provide most of the heat you need, and best of all, sun energy is free. Monitor the internal temperature of your greenhouse, and adjust your air vents as needed.
Add heat mats underneath your most fragile plants. Heat mats are waterproof and resemble electric heating pads. They provide a 10- to 20-degree increase in soil temperature and are relatively inexpensive. Heat mats will protect your delicate plants from any fluctuations in temperature and ensure consistent, even heat.