>>927770Yes, you can do this for winter gardening for the veggies you want to grow.
-Use normal electric space heaters.
-Don't use gas heaters unless they are forced air and have a heat exchanger so that gas fumes don't enter the greenhouse.
-Use large-bubble bubble wrap on all the windows in the winter. You may double this up, two layers thick if you want (though Zone 8 should only need 1 layer). This will take care of heat leaving too quickly. Press the bubble side to the window. Mark each panel of bubble wrap with marker so that when you take them down you can put them back up in their assigned place for easy work next winter season.
-Place as many containers of water in your greenhouse as possible. Paint the outside of the containers black (sandpaper them then paint). Fill every space you are not using with water containers (2-liter soda bottles are great, plastic 55-gal drums can be used as tables with plywood on top). These will absorb heat during the day and release it at night helping to keep temperature drops from happening.
-Install fans for moving the air around both for better heat use and for making a breeze on your plants to help keep them sturdy.
-Remember to clean the windows, inside and out, before putting up the bubble wrap. Any light dust/dirt/algae on them will reduce the amount of sun that enters the greenhouse.
There's no need to put black bags on the windows. I'm not sure why you'd do that. The cost in electric will be greatly lowered with the steps used above. Initially, more money will be spent heating up all that water, but will then lower once it has reached the room's temperature. Then they will act like a buffer for heat storage, reducing costs.
