I'm in PA also and have been starting my seeds inside under lights then moving them to a small greenhouse when they get their firsttrue leaves. I keep a small heater in there too, but only turn it on when it gets cold. I haven't tried starting any seeds in it because I think it would just take longer for germination. I've got a few hundred tomato & pepper plants in it right now. I'm going to move them to a larger, unheated greenhouse at the end of the month.

If you've got a little money for a propagation heat mat, you could start seeds now. Heat mats use very little energy (like 25-100 watts, depending on size), and then you wouldn't need to burn up a lot of energy heating the whole greenhouse. I use 50-cell trays, with clear plastic domes to do my veggie starts. The combination of the mats and the domes allow me to keep my propagation trays at about 80F. You would need to hook up the mat to a thermostat to avoid overheating. You should be able to get a large mat (18" X 48"), a thermostat, and trays and domes for about $100 total. This would be big enough to handle four standard size propagation trays, and you could start 200 seeds. I also use my heat mats in the winter to cut down on heating costs, while still providing warmth to some of my tender, potted greenhouse plants.

Thanks ninj and kudzu for your replies! Great suggestions.

Bad news about the temperature overnight - the thermometer shows it went down to 19 degrees inside the greenhouse last night. I think that was the temperature outside! I thought it would at least be 5 or so degrees warmer in the greenhouse. I think I will definitely have to have a heater in there.

I have been monitoring the temperature in my unheated glass greenhouse. Generally at night, the outside and inside temperatures are the same. I got excited when the greenhouse was 4C warmer than outside (still -20C); but, the differential disappeared with the snow cover.

I'm winter sowing some seeds in it now. The problem is keeping it cool during the day. :) There are two winter sowing forms on Garden Web if you are interested.

Thanks everyone for re-assuring me that your greenhouses are keeping overnight about the same as mine. I thought mine was not working correctly. First year for it and me. We covered a 10x20 canopy with greenhouse film.

Another question, what type of heaters do you use? I was thinking of a 4000 watt electric heater that I have available. I also have 3 1500 watt ceramic heaters also.

Our first winter also with a new GH and we're in PA but Zone 4-5. Our GH is heated but we've had salad greens, cukes and some herbs going since early Feb...somre more successfully than others....all from seed. Our GH is all-glass so we've actually struggled a little more to not fry the greens than we have to not freeze them. We have a 35K BTU propane-heater and a 5K BTU electric. The propane heater is turned down pretty far but we did see a few temp dips over the winter...once down to 36F when our outside hit -18F...so not bad considering we didn't bump the propane heater up to compensate ahead of time.

We're building out all of our summer stuff now, but next year we plan to install a solar pool cover to wrap the GH and make more efficient use of the heating.

There are on-line caluclators that will tell you base don GH material and desired temps how many BTU's you'll need. We intended to use only electric, but the requirements would have required more heaters than out electrical breakers would have been happy with...so we opted for propane.

I just put some water in my greenhouse. I realized my rain barrel wouldn't mind being used to hold water :) and I had other containers. I placed a layer of 4l ice cream pails under my flats, which should moderate temperature fluctuations. Altogether, I put about 450 litres of water in. US gallons ~120, imperial ~100. It sounds better in litres. Hopefully, this will be enough to keep the night time temperatures above freezing.

The water seems to be having an effect. Lows have been -1,-0.3C,2.3C. Outside temps were -8,-8,-3C. It seems the water is gradually getting warmer and that when the temp tries to go below freezing the heat capacity of the water holds the temperature steady. On the cold nights the green house temp went below freezing about 7AM and stayed there until the sun gets around to the greenhouse, 3-4 hours.

gardenobsessed,

I am in DELCO, I've had my tomatoes and peppers in my cold frame (with water jugs) during the day for about 2 weeks now. They are very stocky, but growing nicely. The day temps here have been between 45-60, with recent night temps between 35-45. On the nights that the temp stays above 40, I've left the tomatoes and peppers outside...on the nights below 40, they have come in. They had been outside for 5 straight days, until last night, because we got hit with a bit of a drop in temp (35 last night). I started the plants inside (next to my furnace and hot water heater, and the tomatoes sprouted in about 4 days. They stayed inside until a single flourescent light set-up I bought for $10 at HomeDept ($15 if you count the two bulbs), and potted them up and started putting them out when they developed their first set of true leaves.

I also have planted out my cabbage, spinach and lettuce. A week ago I had to cover them at night, temp fell to the high 20's, but they didn't seem to mind when I took the cover off in the morning!

I would expect that in your greenhouse, even unheated, you could be planting greens and coles. They don't mind a little cold. Inside I would be planting tomatoes and peppers and even some others, with the idea of moving them out to the greenhouse as soon as they sprout. They should be fine. If the temp are forecasted to drop too low (based on the temp differential with water jugs) just bring them inside the house overnight, and then put them back outside. If you are using seed starting trays like the Jiffy peat pellets, that would hold 72 plants easily until time to pot up after first leaves...by that point, you shouldn't have to worry about bringing them inside the house.

Good luck!