While a greenhouse is a natural solar heater there are many situations where the optimum temperature still cannot be reached. Colder environments require a greenhouse heater to retain their temperature. Heat escapes through the glass very easily at night, and many greenhouses are used in the winter to allow plants to grow all year long.
Most greenhouse heaters are electric, heat the air and circulate with a fan. They are cost effective, and many are small and portable allowing them to only be used when needed and stored during summer and warmer months. Very humid environments can cause problems with the electric heating element, but they are all grounded to prevent shock.
Another type of greenhouse heater looks more like an extension cord with the accompanying plug at the end. The cord is buried in the soil, or installed underneath the greenhouse benches, and keeps the surrounding soil at a constant 70-80 degrees. They plug into a regular socket, are grounded against electrical shock and have built in thermostats to automatically turn on when soil temperature drops.
