Hi. I am new to this forum, (been lurking for sometime). Just wondering where you are located and what are you growing? I too have a cold greenhouse. Temps here at night get down into the 30's inside my greenhouse as well. I have germanated yellow squash, cucumbers, tomatoes, rudbeckia, snapdragons, lupines, zinnias, sweetpeas and more and they all seem to be doing just fine. I try to keep the daytime temp about 80 by opening up the vents and door if nessesary, and close it up as the temps start to drop in late afternoon. My cukes, squash, rudbeckia, and sweetpeas are all popping they're second set of leaves. I also dampen down the floor to help keep the humidity up,(I also have a dirt floor.) I have had my greenhouse for about 1 month. At this point I don't plan on changing anything. If it's not broken, don't fix it I say. Good luck.

How about giving us a state or a zone . It would help to answer you questions if we knew where you are located.

His zone is displayed to the right of "Posted by:"

I too have a cold greenhouse. The only time i see freezes is when the outside temps drop to the low 20s.

Outside was 32F last night and inside at bench level it dropped to 37F. No damage whatsoever to cukes, squash, potatoes, tomatoes, peppers, eggplant, artichokes, peanuts, etc. etc. I even left the vents open.

Do not try this at home.

Yes, keep the seedlings inside for now...you won't have to worry about frost settling on them, and there is less risk of them getting set back.

I am in a more temperate Zone, but have to worry about my greenhouse getting too cold in the winter, and in the spring when I want my seedling starts to grow happily. I do two things: 1) I have an electric heater that is thermostatically controlled and comes on if the temp gets down to 37-38F, and 2) I have propagation mats that keep the soil in my seedling trays at a nice 80F regardless of the ambient temp. There is a modest initial cost to these things, but they do not cost very much in terms of the small amount of extra energy I use, and they do make my greenhouse much more efficient.

Heat mats are great, but with no electricity, that won't work for the OP. Too bad, as they are a great solution for the cold parts of early spring.

With an unheated greenhouse, you can look for or create microclimates within your greenhouse. For example, the floor is warmer than the benches during the night, but the opposite is true during the day. So at night you could move your flats onto the floor and cover them. I use the clear plastic domes to cover the flats and then I put a blanket or two over that.

During the day I don't use any covers because I have killed more than my fair share of seedlings by cooking them inside the clear plastic domes. :(

You could also consider creating some thermal mass inside your greenhouse. There is much debate as to the effectiveness of this, but you could try some plastic buckets filled with water and painted black. Often I put the seedling flats next to the buckets and put blankets over the buckets and the plants. That way the heat that is stored in the water is insulated under the blanket. Does this work? Who knows, I have not measured it. But it's easy enough to experiment with if you are so inclined.

Happy growing! This is a fun time of year with all the seedlings holding such promise.

You also need to be aware of which plants can take how much cold. Last night dropped to 30F. HAd i left tomatoes or cukes out they's be toast, but lettuce, anything in the cole family, pansies and some herbs are fine even when they had a heavy coat of frost.

Here in western MA, or traditional last frost date is may 15th. I will not set tender plants out until then.

peas, onions, spinnach, cabbages and their like, lettuce and carrots are all out in my garden.

here is a pic of the seedlings in my cold greenhouse. these were all started in the cold greenhouse within the last month.they are doing just great. Temps at night down in the mid-upper thirties, daytime 50's-60's, (inside the greenhouse daytime 75-85

Here is a link that might be useful: squash & cukes

With no electric you are limited. Thermal mass does help, but it takes a lot of water to do much. How about and old kerosene lamp or lantern on cold nights?
Mine? small electric heater and window swamp cooler. Never below 45 or over 90 degrees.

I'm in the same Zone, and my greenhouse does have auxiliary heat ... but I hate to use it on general principles !

I get around the 'nights in the 20's' problem by using raised planter boxes ... with gallon jugs of water filling the lower shelf. This keeps the soil temperature in the planter in the mid-40's for my seedlings even on nights when the outside temperature is in the mid-20's. And it also keeps the soil temperature in the planter from rising super-high during sunny July days, allowing me to grow peppers, tomatoes, cucumbers etc. in the same planter through the summer and fall !

Here is a link that might be useful: Raised Planter Box

Try the greenhouse-within-a-greenhouse trick - rig a plastic tent over the plants you are most worried about. I've heard a very rough rule of thumb that each layer of plastic moves your plants about 100 miles south, or about one zone.

Maybe also try adding some water containers for thermal mass?