Since you did not specify the seed you will try to germinate/grow .... As a GENERAL rule most seed will germinate at 60F the major exception will be the "cole" crops.
You may wish to germinate the seed indoor & as the SOIL temp reaches 45F move them to the greenhouse.

Well as I stated I grow quite a few veggy and flowers. I am playing with some seed from Australia now for a sweet mild pepper. But they germinated fine. I germinate my seeds between 70-80 degrees soil using a frame i made. Helps especially with my hot peppers. As I do a lot of them and tomatoes. I am asking basically how cold can a plant get at night with out stunting or killing it. Seems I read as long as it doesnt freeze or go below 32 it would be ok. But I was figuring 40 degrees . Just hoped some one knowing more about green houses then me would have a general temp.guide line. Im in Ohio zone 6. Usually above 0 here at the lowest. But can be 35 at night and 70-80 in the day some times.

In commercial greenhouses every effort is made to prevent going below 40 and 45 minimum is preferred. In our greenhouses, which are predominately pepper and tomato plants with some mixed perennials all the auto thermostats are set to trigger the heaters at 45 with audible alarms triggered at 40 so we can intervene.

Tomato and pepper plants both can be stunted below 40 F.

Optimal germination temps are much higher of course so all germination is done in separate insulated and heated compartments.

Dave

Thank you! I'm germinating seeds in a framed wooden box I made. Has a string of C9 Christmas lights in side and a PID controller on the lights. I can maintain any soil temp I want while germinating.I did around 78 degrees on the peppers Im playing with. I germinate covered in the basement with a 60 degree temp. Soon as they germinate I uncover and add lights. The peppers are 3 weeks old and maybe 1-1 1/4 inch tall. Concerned they may be growing too slow. But I figure the warm soil and cold air would make better plants.Soon as they get a little bigger i'll add a light breeze.When they get there first real leaves I figure I'll transplant to 3-4 inch cups. I was hoping if timed right I will be able to sit in a green house. But it wont have any heat. figured when nights stayed above 40-50 degrees. Does this sound good? Or am i doing some thing wrong?

Or am i doing some thing wrong?

Personally I can't ever advocate continuing to heat soil once the seeds have germinated. It is far too easy to stress and even cook the young roots and it only encourages tap root growth. So if you are continuing the soil heat after germination I would discourage it. Instead remove and transplant those that have germinated and leave the rest on the heat. Young seedling can be carefully transplanted at any time after germination. It just takes a bit more care. And be sure to transplant them deeply to just below the cotyledons.

Lack of sufficient light isn't the only thing that causes leggy plants. Heat does too. Cooler temps - both air and soil - are much preferred for growing on after germination. Ideal air temps are 60-65 (70 max) and soil temps in the same range.

Since wet soil will always be cooler than air temps avoiding over-watering and using a very well draining mix is crucial.

Hold off on the nutrient additions until after they fully develop 1 set of true leaves.

Dave

an alternative is to winter sow the seeds letting them set outside. I've also done it inside an unheated greenhouse. Either way is much easier than indoor with lighting, heat mat, etc.

The winter sowing forum has more details. The link below has photos from seed sowing to flowering in the garden.

Here is a link that might be useful: easy seed sowing

Well I have found 78-80* seems to work good to germinate. Then I move them to my basement into a stead 55-60* temp.I also maintain the soil at 68-70* which seems to be great. Plats are slower growing. But quite dark green and heavy stemmed. At 12 weeks My Sofia Peppers are approximately 3" tall and looking great. Hoping to have my green house up when temps warm up. Figure when night temps are about 45* or better I'll move to the Green House.

My 400 watt HID lamp is over my bottom heating plate, and my newly emerged seedlings need the light but not the heat. My solution is to insert overturned pots under the seedling trays, moving away from the heat and closer to the light. A fan is always running over my seedlings. My unheated greenhouse seldom falls below 45 at night. Al

I start all seedlings in the house and grow under lights.I then move plants to the GH end of March,early April,but I always heat it at night (small electric heater),never know when it may fall below freezing.