Last Spring when I planted 3 extra early tomato plants in March I used cheap candles to heat the makeshift enclosures I built around them. The candles in glass bowls work best and seem to be safe. When using light bulbs I used a temperature sensitive outlet (turns on at 35F / off at 45F) and it would save some energy when using a larger heat sources like an oil filled radiator. Amazon has those temp outlets, $14 if memory serves? Another investment were remote outdoor thermometers to keep an eye on things.

That (35/45) temp control outlet is Great, I also use one in the chicken coop for the heat lamp and water heater, and another one in the well house for the heat tape and light bulb hook-ups.

Yeah I was a little wary of using a candle for heat too but it can be done safely. When we lose power I'll use one in the well house too.

If you have or can still find them, the old fashioned C9 Christmas lights should do the trick. Each bulb produces 7 watts and a full string is capable of keeping an 8'x10' otherwise unheated greenhouse above freezing.

That's a good idea about C9 Xmas strings, and allows for some heat distribution. Now, most of the C9 size strings I now see listed are actually LEDs, and the C9 incandescents that I see advertised are largely replacement bulbs. So C9 incandescent strings are similarly scarce. I see a very few such strings advertised at Big Box stores, and some are REALLY CHEAP (not surprisingly), but those are also listed as "unavailable". Might be able to chase up some C9 incandescent strings on Craigslist from people who are upgrading to LEDs, though.

Not the best time of year for it now, but I'd check out garage and estate sales. C9's last forever and are often unloaded for next to nothing when downsizing or clearing out a home. My folks had some that were a good 30+ years old and still fully functional when we kids cleaned out the place.

(Wish I had kept the string of bubble lights. Serious Christmas nostalgia and worth a few $$ now.)

They are a little higher but you can still get the reptile lights that give off various wattage that you can use. They also sell lights for chickens but they are usually anywhere from 125 to 250W, with 250W being way too high.

Yes, I used to have a couple of reptile heat lamps that I used for a project. They're ten or fifteen bucks each or something like that. That's a possibility, but incandescent bulbs were less than a buck each. Converting line voltage into heat is technologically trivial. I have to wonder why they make it expensive.

Check out freecycle, craigslist and that type of place for free or cheap Xmas lights! People have them hiding in their attic as they've gone on to more modern lights!

Re Xmas lights, you really need C9 incandescent bulbs (7W each). Most strings are the itsy bitsy 0.5W bulbs. It appears you can still get strings of C9 incandescents for $20-40 (25 bulbs on a 25 foot string = 175W total). But they're going away ... Now, that's still pricey compared with old A19 incandescent lamp bulbs. My perusal of Craigslist shows a couple of offers for Xmas lights, but they don't specify what kind of bulbs they are.

Besides the Xmas lights, at my old place I had a HFGH and would stack gallon jugs of water on the sunny side of the greenhouse and it would help with keeping the GH warmish.

If it's 20F at night, jugs of water won't do much. No sun shining on them then. The lowest nighttime temperature is at the end of the night, by which time, even hot water will have chilled.

This is the set up I use on below freezing nights. C9 Incandescent light string and lights. I used to buy them at home depot but I did not buy any this year so I do not know if they are still selling them.

These light strings (Home Depot) and 16 mil plastic on a hoop house keep the inside of a 4' x 8' hoop house 10 degrees warmer than the outside. The 10*F difference in temp with and without the lights is when the outside temp drops to between 32* F and 25* F. I don't know if it would warm it this much if the temp was colder outside the hoop house than 25*F.

My neighbor uses a 5 gallon bucket of water with an aquarium heater in it. He says he can very accurately control the temp this way, and he crunched the numbers and figures it costs him less that 20 cents a day,,,we're in upstate NY and it is COLD !!!

That's an interesting idea, but if I'm going to shield plants from the cold, and ideally throw some insulation around them, why would I want to take up space with a five gallon bucket? I don't think the issue is accuracy of temperature control. Just want to keep everyone above freezing.

Good question, they sell plug you screw a light bulb into with different on off temps. For example pick a 40 degree on and when temp goes below 40 it turns the light bulb on until it goes above 45 (or something like that). Saves waste. I like that idea (incandescent bulbs). Note you can buy "light bulb sockets" that plug into an outlet and take a bulb. Imagine simply "plugging" a light bulb into any outlet. They cost about $1 each.

Yes, temperature controlled sockets were discussed below. Never seen them for $1, though. You have a source?

^^^ That's the thermoplug I use to keep my chicken coop and well house from freezing up. Works like a charm, just plug it into a normal outlet for 2 temp control lines. Cost ~$14 on typical websites like Amazon.

Yes, that's the cost I saw. Of course, the issue is not so much temperature regulation as it is how you get your heat. Incandescent lights are getting hard to come by, and tape heaters are pricey. You'd like to believe that an ac heating element (as in, plug it in and it gets warm) has to be just about the cheapest thing you can imagine. But nope. Ain't so. Even ceramic reptile heat bulbs (as in, a brick with a filament) are $10-20 apiece.

I saw a 4 pack of 100 watt ceramic reptile heat bulbs for 20 dollars. That makes it a little better. They put off a lot of heat. (yeah, burned myself on a 15 watt bulb)