Need more info, When is last frost date (2)how big are plants (3)how cold does it get in hot house with heat mat and heater (4) what will be your coldest (est) nite temp outside. With this info you will get good advice on your problem. GOOD LUCK Bill

Agree with Bill that need more info. Based on the info so far I'd have to say no. Even this far south we are still having to heat the greenhouses at night so in your zone I probably wouldn't risk it yet.

While the outside temp is somewhat relevant it is the temp inside the greenhouse that is important so we need to know that - specifically the temperature at the plant height. And that all depends on how your GH is constructed/insulated/oriented and what type of air circulation it has. You might also want to consider adding some heat sumps to the GH if you don't already have them. They can make several degrees difference inside on really cold nights.

You may have to do a couple of night of dry runs with no plants - just thermometers, the heater, and a fan to circulate the air. This is where MIN/MAX thermometers come in hand in the GH. But I sure wouldn't use heat mats on established plants as that is a good way to cook the roots.

Dave

If you have the heat mats already you may just want to drape row cover fabric over your plants at night within the greenhouse. Row covers alone are usually sufficient to protect plants from frost when temps are in the high 20s.

Many people use propane heaters to get through the colder nights in their greenhouses. I use propane as a backup heat but I have all three of my greenhouses setup with woodburners. It is cheap heat and fairly easy to operate. My tomatoes are loving the days despite the cooler nights.

I am using propane this year.
It is MUCH cheaper and more effective than space heaters.

a 2000 BTU unit heats a drafty, extremely poorly insulated
greenhouse like thing to +20 degrees over outside.
17 LB of propane tank lasts... I dunno... 4-6 days at 8 hours a night.

Propane has other advantages:

- It produces CO2 which plants like.
- It produce water vapor(moisture) also good.
For every pound of propane one pond of water vapor will be produced.(apprx)

Back to the OP's question.
It depend what are the things that you want to put in GH.
It will be perfectly fine with cool crops(onions,
broccolie, chives, lettuce, etc. Later on you can take your
tomato seedlings in ther. An further down peppers, eggplants. Definitle 27F can cool of the GH quite a bit. but 38F should be fine. I hav a cold frame. During the last week we have had lows in mid 30s-F. and my tomatoes and peppers(cold sensative ones) have been fine. But then they did not grow much.
Of course a small cold frame ( 50" x 36") can cool of much faster than a bigger GH.
At any event, you don't want to risk. But you can test try some.

Keep them away from the outside walls/glass as much as you can.

I would like to know more about using propane or wood for heating my GH. What kind of heaters are they and where can I find them? I've been using electric and my energy bill has been huge this year. This is my second year having the GH and Try to keep it above freezing only, should it be warmer? We don't have too many cold nights in the Dallas area,but when it does it's cold for several days at a time. I have a fan in there but I don't think I'm using it the right way. It's well sealed plus we put plastic around it this year and all is well and I have had my tomatoes in there for a few weeks I just put most of them in the ground. Hopefully it will stay warm so I don't have to worry about them.