There are many ways to heat your greenhouse and few of them are truly free. Here are some great ways to heat your greenhouse without an electric or gas bill. A good greenhouse temperature is between 75F degrees to 85F degrees. Maintaining this temperature year round can get expensive if you are using gas or electricity to heat and cool your greenhouse.
Important Notes:
Any electrical heat source can be a fire risk.
If you use electric or fuel heat in a chicken coop you should not use flammable bedding. When using and heat that could be a possible fire risk bedding should be play sand and nests should have rubber nesting pads which are not made of straw.

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Featured Video: Better than a rocket stove greenhouse heat

In this video you will see how leaves are used to produce heat for a medium sized green house. If you measure the temp in a leaf pile you can easily see how decomposing leaves generate heat. Check this out

List of Ways You Can Heat Your Greenhouse-Free

• Reuse plastic bottles and plastic barrels filled with hot water. Place them on the sunny side of your greenhouse (inside). This works to help supplement your main heat source.
• Use compost piles. Putting a compost pile inside your greenhouse can get pretty smelly and possibly toxic. Put your compost outside your green house and make a vent going into your greenhouse.

Build leaf compost piles outside your greenhouse and make a vent going inside. I actually put all my raked leaves on my green house floor in the fall (about 14 inches deep) and they generate heat all winter long. I then clean them all out in the spring and use them on my garden. However, on really cold days when temperatures get in the teens the leaves are not enough and I have to supplement the heat source.

• Ceramic wall heaters. (Most can be ran on solar generators). Of course there is the initial investment of purchasing ceramic heaters and a solar power generator.
• Heat Lamps:

On cold sunny days I  have successfully and safely used heat lamps to heat the greenhouse and chicken coops without spending a dime. I use solar power kits or solar powered generators as a source of power to operate heat lamps, heavy duty cords and contain the heat lamps in a cage where birds cannot roost on top of it or knock it down. Safety Tip: You can safely use heat lamps with this trick. I wire the heat lamps into metal bird cages with a metal floor and then securely hang the heater with chain attached to the bird cage. Be sure that they are at least 3 feet distance away from anything flammable. How long you can run them depends on how many solar panels you use and how many solar power storage batteries you have.

• Barrel wood stove: I have a friend who has a very large greenhouse and heats it with a barrel wood stove. He made it himself with a wood stove barrel kit. I personally have not used this. He also uses the barrel wood stove that he has channeled into his vent system of his house to generate heat but that will be in another article later on.
• ​Hot bed greenhouse gardening (See video below)
• Chicken coop heater

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Solar powered generators can be used in remote areas where you have no electricity on your property. The size and power capacity of your solar generator and backup batteries determines how much electricity you can produce, store, and use to operate appliances and lights.

Featured Video: Hot Bed GreenHouse Heat. Can Compost Heat A Greenhouse?

She's got some really great ideas for hotbed greenhouse gardening in Zone 4 (very cold can reach temps of -35F), including something she calls "rabbit tea". A must see...

How to Make a Hot Bed For Your GreenHouse Plants in the Winter

Build a box to contain the composting manure and bedding. You can make a box from cinder blocks, bricks or use cardboard boxes.
Fill box with compost made from manure, bedding, and hay 
Water the compost
Cover with landscape fabric
Cover the box at night to trap heat 
​See two  of the best printable designs for hotbeds I have used by Penn State Extension are here: https://extension.psu.edu/two-designs-for-low-cost-hot-beds-for-small-scale-production

The larger your greenhouse, the more heat you will need to produce. Some people prefer a small greenhouse but me, I prefer a large greenhouse I can walk into. It is easier to keep cool in the summer months and depending on what you are using for a heat source can also be easy to heat in the winter.