Last night we had our first hard freeze. The temperatures were in the low 20's for over 9 hours. I just checked the minimum temperatures on both my greenhouses and neither of them got below 42°. I also checked inside and had no freeze damage even to the leaves that were touching the sides.

How has your greenhouse fared in this unseasonably cold weather?

With my solar blanket insulation and two 1500 W heaters in each GH/porch (with fans moving the heat), my GH's stayed 50-55 F. I really don't want less than 50 F for my orchids'sake.

That's great. I'm always terrified of a power failure. My next GH is going to have natural gas heat. I use at 1500 w heater in each of my GH's and fans as well. Anything that can't tolerate low 40's stays in the house.

When I got up at 6 this morning it was 24 outside and 49 in the GH. I have the same solar blanket that Ken has. Had a 1500 watt heater and a Mr. Buddy propane heater going. Fan runs 24/7. The propane heater went out at some point after midnight though. I don't know what it's problem is. That's 2 nights in a row that it went out during the night.

Whew! That's scary .. good thing you had a backup!

Holly.

I don't know why a propane heater would go out. All it needs is LP gas in the tank.

Here in central TX we have had 8 straight nights of freezes. One night was 27. My 6 x 8 GH stayed between 48 and 56 -in cycles. I just have a cheap ceramic heater in there - one night I also left a 40 watt light bulb on over some of the plants. I have bits and scraps of bubblewrap, cardboard, egg crate mattress over the door-but still have some exposed walls. And mylar blankets surrounding each grouping of plants (most of the wall space has some mylar blanket down it). Will be installing clear solar pool cover as soon as it arrives.
I also have small oil filled heater that should arrive on Thursday -- with "freeze protection" built in (comes on at 44) and programmable levels of various heat and timing functions. Has a manual on/off switch (as opposed to electronic) so if there is a short power outage it would automatically come back on. It was about $70 at Amazon - so I hope it will be well worth the investment. Last year we had record colds down to 22 - but that's very unusual for us.
I just added a new electronic thermometer (which I posted separately) which has two temperature alerts - so I have one set for 44 degrees and one for 89. So if things malfunction at least I'll get a warning ... guess I could light some candles in there then. Reading ya'll comments makes me think I should dig that little propane heater out so I know where it is -- just I case.

The propane is the backup/extra heat for us. Our power outages here are mostly summer/tropical storm related. Ken, it's going out because the heater is designed for use with small screw in one pound canisters but can be connected to a 20lb tank with an adapter hose. We use the 20 lb tank and the hose but there is some sort of pressure problem between the hose and the heater that is fixed by a filter attachment. Husband's a chemical engineer so insert here lots of engineer guy speak that I don't translate well. At any rate...he's going to pick up the gizmo that we need.

Our electric heater is this one http://www.wayfair.com/Dr.-Inf... and it sure seems to be a workhorse. Comes is a larger/higher btu version also.

Since we can have prolonged power outages ( the worst was almost a week), I opted for a LP gas heater that will operate with out power. My research showed I needed 14k BTUs so I bought a 20k. I had one night early last year when the heater failed to light. Our GH hit the 30s briefly. Most of our tropicals died back to stumps, but over the course of the next 3 months all survived. The only thing that we lost were some cuttings and a couple of adopted orchids. I added 2 small electric heaters in hope that it would not happen again.
What I need to do is build a power free temp alarm. It's pretty easy, all you need is an older style thermostat, door bell, battery, and bell wire. put the thermostat in the GH, run a wire, attach the battery and bell in the house, and you have it. It will not fix a problem, but at least you know you have one.

I use wireless temp alarms for my greenhouses - runs on batteries and just bought a 5, 500 watt generator. With the prediction of colder than normal temperatures and higher than normal precipitation for Winter - meaning snow and ice storms I'm just not willing to take any chances. I also have a power failure alarm so if it happens at night it will wake me up and I can fire up the generator.

I haven't used any heaters so far and despite having some outside temps in the low 20s, we're still in good shape. Some of my sweet potatoes have gotten a little brown on the tips of their leaves, but other than that everything is doing fine. I'm thinking the big aquaponics tank does a lot to keep the temperature a little bit warmer, perhaps. The humidity probably doesn't hurt anything, either.

In my next greenhouse I'm considering heat cables under the floor.

I considered under-flooring heat cables when I built my last GH. I was worried about that heat reaching all the way to the ceiling of the GH though, since my GH, though completely wrapped in solar blanket material, can't possibly be as well-insulated as my home. I know that the heat would rise, but...............

Great to hear Dave.
Ken, I gave that some thought when we poured the original floor. I considered putting a coil of tubing in, just in case I wanted to connect to a solar panel.
I would think in an emergency you could not raise the temp very quickly but as a secondary source possibly, especially if your running an off peak electric meter.

I don't even know what an "off-peak electric meter" is, Ric. Sorry, I just have a meter.

That's Okay Ken, in some areas they have programs where they want you to conserve domestic electric use during business hours, so they install a meter that records timed use. The rate per KW/hr. goes down after the peak of business and industrial needs. People in that situation usually have things like timers on water heaters, drop down thermostats, heaters like ceramic storage units, and I could see mass storage heaters, like floors. There is a pretty good incentive to it. The idea is to stabilize the mean use on the grid. It's current availability is unknown to me, but you used to have to have a total electric home. The other thing they are putting in the new meters in some areas is that they emit a low level RF signal. The meter reader just points his RF device "gun" at you meter and downloads you reading from the truck.

We have it here and I run my pool pump mostly at night for this reason. It makes a big difference in our power bill.

Here in the Austin area we've had pretty mild temps this last week. GH temp hasn't dropped below 58 and about the same temp outside. Tonight it is going down to 50 so I am running one of those oil filled electric heaters. Probably will only come on a few times. The popup GH on the patio is keeping toasty with two 65 watt light bulbs. We will get some really cold winter before it's over and already had 8 freeze nights. But not down in the 20s yet. We're pretty mild compared to what most of you are posting. We've had overcast days this week but a little sun yesterday and today so that helped.

Now that I have gotten off the Obsessive Train in regards to the temps in the GH, I can't tell you much about the temps except that I have the heat thermostat set to 63º. I still try not to "clock watch" the GH temp indicator in the house. I am happy to report that everything lives and thrives in my little GH. They seem to be happy and might even get more attention now than they do in the yard! I have some new seedlings that continue to put out leaves. In fact, I might have to repot them soon.

Before I speak too soon, our winter weather doesn't usually come until mid-January through February. I have leaky deck floors (that will be rectified next year!) and leaky windows. I have caulked, weather stripped, and placed insulation boards on the north side. Doing the best I can, I can only hope for the best. For now, I deal with leaves that stick on the roof and leave icky messes.. I will have to get a ladder to reach one side tomorrow.

Despite my poor inexperienced gardening skills, I have "some" successes in the GH. The patio GH has produced some wonderful ripe cherry tomatoes and a tomato plant I transplanted (and broke off - so it had only one leaf left) is making a comeback. In the HFGH I have some nice arugula, a small tomato actually trying to produce a tomato, and the newer radishes I planted (Chinese red) seem to be taking off finally. So, I guess my GH temps must be working okay. Apparently the GH is able to have some success even for "newbies" who are totally inexperienced!