But for now, my greenhouse uses the sun as its sole source of heat. (And no electricity or auto -venting on too-warm days). Of course the automatic watering consists of me turning the hose on.
So this leaves the problem of maintaining sufficient warmth during the cold months and having a water source that doesn't freeze.
Note: I live in southern Virginia. Having previously lived in Minnesota, I do understand that cold is very relative. Our lowest temperatures of the year *usually* fall at 10-15 degrees during a two week period in January. Even on those days, if the sun is out, the greenhouse will be at 80 degrees, during the day. Your mileage if you live father north will be much different and this particular sort of solar greenhouse wouldn't be worth your time. But you can check the DIY plans at Build It Solar. It can be done.
Keeping some warmth in it at night is the problem. The walls are half-wood, giving it some shelter, and the outside paint is dark green, which helps absorb some heat. The interior walls are white, so they reflect as much light during the darker winter daylight hours. The upper walls/roof area is a heavy duty plastic (an excellent product purchased from Northern Sun in North Dakota -it's lasted three years even in our southern sun, with no visible deterioration or brittleness).
The northern upper wall/roof is also covered inside with bubble wrap - the large bubble air pockets give it some extra insulation. And believe it or not, the floor is carpeted. As of this last week, there are two-three layers covering the dirt floor. Long-term plans include a brick paver floor. Meanwhile the carpet retains some warmth, and minimizes mud.
However, to solve both the heat retention problem and the winter water source, I have several large tubs of water (dark colored tubs, bought at walmart for $5 each) that I fill late in the fall (say, today for instance). The hose connections actually run across the yard, being buried approx 2 feet down, but the connection is outside the greenhouse (it was there first), and so the connecting hose is exposed and vulnerable to freezing.
After filling the tubs, I fill as many plastic jugs as I can save up during the summer, and use through out the winter as needed. Milk jugs can be used, but will only last one winter -the heat in the greenhouse makes them brittle. Ice tea jugs are intentionally heavier, due to the tannic acid found in tea and will hold up much longer.
Most of the solar greenhouse sites I see recommend painting the water reservoirs black. I've tried this and found no discernable difference in heat retention, plus the paint eats the plastic while it peels and chips off.
I have 5 large tubs of water, and 36 waterjugs - this will probably get the crops almost completely through the winter. In the last three years, the water has never froze or even iced over.
The coleus are on borrowed time, but I'm hoping to baby the green peppers through at least December since they are loaded with little peppers.
During the day, both the peppers and the coleus are covered with a gauzey sheet, to keep the heat from burning up (average daytime temps now are 100 degrees,even when it's 50 outside), and during the evening hours the gauze holds more warmth in close to the plants.