I had too many growing areas going, under light, in windows and Wsed. So I decided to combine most of the seedlings to one area , on my unheated west facing porch on metal shelving. 2 nights this week temps were 24 and 28 degrees, so as soon as I got home from work I covered the shelving with old curtain sheers and later a tarp to retain heat. Recorded temps in there were 57 at 11pm and 47 by sunup. Everything survived and are happy campers. I still have my impatiens in the bedroom, pinching and propogating more of those, as well as the seed geraniums in a small patio greenhouse in the front of the sliding glass doors. If you are referring to those greenhouses that have clear plastic, be careful, that clear plastic can fry plants in the daylight in a matter of hours. When I had one of those I used it inside in front of the west windows, even inside it was difficult to maintain without frying and temp control when I was working and couldn't watch all day. Using it outside was a disaster, either frying plant or drying out too fast or getting blown over by wind. That mini greenhouse went to dumpster land 2 seasons ago. Any of the seedlings I plan out putting outside now would remain in their trays and pots take outside during the day and bring in at night. I finally smarted up and got shelving with castors!

soycandle, I might not even ought to post because I am not in your climate zone (I am in zone 8B) and have no experience with the type of winter you have, but...that said... I have had a portable greenhouse for 5 years. Its the same basic type as yours, from a different company. Mine is called a "Groenhaus" and it came from a company in I believe Norway (may have been Sweden, but its one of the Scandanavian countries where they have an actual winter with snow, ice and wind.) Mine has a steel frame in a hex design, air vents, and zipper door. Its made of 12 mil woven poly fabric, and conducts about 83% of available sunlight. The cover is thick and insulates very well. I myself live in Northern Florida where there is no snow, no ice, but it does occasionally get down to 20F once or twice a winter. I have had GREAT SUCCESS with my portable greenhouse. I have used it to overwinter tropical plants that were containerized (plants like heliconias, Ti leaves, gingers, aroids like anthuriums, orchids, bromeliads....) I have also used it one winter to cover a 6 x 6 raised pond that had tropical water lilies in it, I also stacked in a bunch of containerized plants in with the pond and they loved the humidity...currently, I leave the base of the frame on the ground year round, surrounding an outdoor inground planting of tropical plants that would freeze even in our relatively mild winters (Alpinia purpurata, Heliconia, bromelaids, Ti leaves, birdsnest fern, etc) and erect the cover over the bed in winter, then take it off in spring. These little portables work GREAT. All you need is to put a portable ceramic space heater (the kind with a thermostat) inside the grennhouse. For your size house, 6 x 8, one 5120 BTU heater will make it toasty for all your stuff. I have the type that you can set the temp, the range is 55,60,65,70,75 and 80. The heater goes on and off automatically to keep the GH at the temp you choose. The heater has a built in fan and blows warm air out, and it oscilates to help distribute the heated air more evenly. Its also helpful to have a small oscillating fan that you can hang from the frame in the roof to circulate air and heat. This cuts down on consolidation and also on algal growth and pests. I see you are in zone 5...it will of course take you more electricity usage to heat your greenhouse than it does me, but it also depends on what you are growing....I keep mine at a minimum of 60F, but the plants you are growing may only need above 32, or the 40-50 range. I would think you could go on and set the thing up and start using it now. The only advice I would offer additionally is, if your winter storms include WINDY CONDITIONS, be certain you use any tie down apparatus (stakes, guy wires etc) that they offered you. Without them you run a risk of the whole thing blowing over in a high wind. I think of mine as a big, almost clear TENT that my plants are camping out in.

I grew lettuce outdoors last fall, just trying to keep it as long as I could for a mid-November market-and I had it until the first of December by just putting a floating row cover on a frame over it outdoors. If you put a frame of some sort over the top of it, then frost settles on the row cover and doesn't touch the lettuce at all and it stays good longer. For a frame support I used one cement block lying on it's side with another one upended standing on top of it. Then I put conduit pipe over the top and used the frost cover in an upside down v shape-like a teepee-over the top of that. If you do that under cover in a high tunnel, then you'll not have problems with it all blowing away like what finally got mine. Fix air leaks in your high tunnel by using spray foam insulation under the baseboards or shoveling dirt around them to stop leaks and keep the doors shut during the afternoon before cold temps are forecast and you'll get a little better heat retention. We got down to 24 night before last and I had banana plants still in my 26X36, with no heat and they didn't get frosted at all. I did get them out of there yesterday though!

It's warm enough to harden them off outside now without a greenhouse. The key thing is not to expose them to full sun for more than an hour or so the first few days. They will cook, much like people sunburn if they hit the beach for hours after no full sun exposure. Baby them, as it sounds like you have. Gradual exposure is key. These cloudy days may be a pain to us gardeners, but it's actually ideal for acclimating plants to the outdoors. There is also less transplant shock to the plants when we place them in the garden with cooler temps and not a lot of sun. Hope you have much success!