You'll heat them up and warm the soil, but you'll get little protection from freezing temperatures without additional insulation. An old sheet will get you maybe 3-5º additional protection. This winter I used heavy row cover and an old flannel sheet set and everything survived perhaps a dozen nights below 0ºF. You'll want to vent if it gets above, say, 55ºF with no clouds. Takes a bit of fiddling, but you'll get it and never go back.

Dan

Thanks Dan for the encouraging words!

Here are all my goals (tell me if I'm missing the mark on any, or unaware of other benefits the hoophouse will provide for me):

1) I wanted a bigger cold frame than my small one, built from old wooden pallets.

2) I wanted to warm the soil faster, in an effort to get my tomatoes and peppers in the ground sooner...maybe a full two weeks earlier. (Average last frost date is April 14th...I'm hoping to be able to put the toms and peppers in around April 21st.) So, I'd be hardening them off in the hoophouse during the days from April 14-21. - I'm thinking that the tomato/pepper plants would be fine in the hoophouse, even if air temps drop to the low 40's (for a night or two), IF the soil they are in is warmer than that. Also, I could throw a row cover over them to add a second layer of protection.

3) The hoophouse was constructed very cheaply (as noted above), and the end panels are just 3.5 mil plastic, held in place by large paper clamps. I figure I could just fold down the tops of the sides on sunny days to vent and get some cross ventilation.

4) Onions/greens/cole crops could be transplanted into the hoophouse in the next week or so (currently are hardening off in the hoophouse), as a freeze is not expected in the near future. If an unexpected freeze were to occur, I would still be able to put them under a row cover inside the hoophouse, or even a sheet. (Current lows are expected to range from high 30's up to 50 for the next 10 days).

I have greens under my hoops all winter, ~1 dz below 0ºF nights this season, but my cold frame is much better. The warm soil helps as long as the air is slowed down from leaving under cover. Row covers over the top are best. Make your own clamps with black poly irrigation pipe, same dia as PVC. Cut about 20% of the diameter out lengthwise and push on. Sanding-melting cut ends is even better. All your onions & greens should be fine under there with some protection on coldest nights.

The materials and methods coming out recently for extending the season are a nice little revolution and not too expensive, either. You'll be hoo-oooked!

Dan

I have a 10' x 18' cold greenhouse covered in firerglass panels. It is March 16th today and still going below 0 Celsius at night. When can I begin to plant my seeds to grow in there? I have no room in my house. Besides the cats will tramp through seed flats just because they can.

You should see temps spike whenever the sun shines. A good way to test the waters is to sow some spinach, lettuce or radishes - they'll come up when they can. Warm temp plants will need some care, you might have to wait until overnight lows are in the 50's, but the cool weather crops will be ready to rock before then.

Be aware you'll need to vent because plants can take the cool but not the heat.

Thanks. I will be experimenting this year I guess. The greenhouse is already vented at the top as the panels are wavy and don't fit tightly and I will be able to open the doors in the daytime so the air will have flow through. I can also put in a fan if the temps are too high.