Must have a thermostat. A bird bath feeder looks like my best option?

All sorts of soil heating cables and/or mats are available for coldframe use. Seed starter folks use them all the time. Some have built in thermos but they are preset to specific temps or optional plug in thermos are also available that allow you to set the trigger temp on the cable/mat.

Even plain old incandescent bulbs can be used. Plus there are all sorts of passive methods like just building a tent over the cold frame. I don't see how a bird feeder could work and bird bath heaters have to be immersed or floating in water.

Dave

soil heating cables

You can buy any cheap small electric heater (or a bunch of incandescent bulbs) at a home store, and attach it to a settable thermostat plug-in. You can get the latter for $20 or so. It will simply turn the heater on and off, depending on the temperature and the set temperature.

A few years ago I built a smallish cold frame to house a small Meyer Lemon tree outside over the winter. I used 2 incandescent bulbs (75W each) and a remote thermometer to keep an eye on the temp. It worked quite well until the 2nd winter when we lost power at 4am while the outside temp was 10F. Within 30 minutes the tree was frozen.

eBay has a nice selection of inexpensive "outlet thermostats". The cheapest ones are very small and have fixed setpoints, and might be all that you need. The nice thing about incandescent lamps as a heat sources is that you get some light out of the deal. For a small cold frame, I suspect one bulb would do the trick. But the advantage of two bulbs is that you could tolerate one bulb expiring, which is a real threat for incandescents.

Home improvement stores sell a double light bulb extension, that with an electric eye outlet added for the 2nd bulb works well as a backup. I use one for my well house where I have one light bulb on during below freezing temps and if it burns out a 2nd light bulb automatically comes on. If the power fails I have a back-up candle. A candle wouldn't have worked with my lemon tree though, I'm not that fast at 4am during a power outage :-(

If you have a thermostat controlling them, you don't need an electric eye. Just have a two-bulb heater, and if one bulb fails the other bulb will just stay on twice as long to keep the same temperature. Total electricity usage will be the same in both cases.

Good ideas. I was thinking the bulbs would waste electricity vs other options - but now i think I am wrong. Also I the bulbs could provide a tiny bit of supplemental light during the short daylight winter days.

Bulbs used for heat don't use any more electricity than heater wires used for heat. To the extent that not much light escapes from the cold frame (and I suspect rather little will) a 100W incandescent bulb produces as much heat as does 100W heat tape. A watt is a watt. They are both hot wires. Though one is a lot smaller and hotter than the other. But yes, the light from bulbs is nowhere near as intense as real sunlight, so the amount of supplemental light they produce is quite tiny. Do be careful that the plants aren't too close to the incandescent bulbs.

I was thinking about doing a frost-proofing frame myself, and after looking around a bit I stumbled on Thermocube TC-3s, which are fixed setpoint (on at 35F, off at 45F) thermostatically controlled outlets that look pretty nifty, and are compact and cheap. Now, I've never used these myself, but I think I'm going to get one. I believe they chose 35F as a turn-on temperature to allow for a bit of miscalibration without allowing any freezing.

Dan, I use one of those 35/45F outlets to control the heat lamp in my chicken coop and it does work as advertised. It's main function is to keep the hen's drinking water from freezing, if memory serves it cost about $14 on Amazon. After 3 years it still works ok.

Thank you for the review, vgkg. They're cheap, small, and simple, and appear to be designed, as you say, just to keep things from freezing with no effort or supervision. I see them around for $10 these days.

I got a TC-2 turns on at 20 off at 30 - I an trying to keep rosemary and a few other things through winter. This things seem like a great idea and should work great. I likely will get a 35-45 one and build a cold frame for kale, lettuce and chives.

Well, that's a good point. Rosemary can handle freezes, but just doesn't like very hard freezes. So a TC-2 should work OK for cold-hardy cultivars. Not sure what kind I have, but they do 22F just fine. If you do not have a cold-hardy cultivar, a TC-3 would be a better choice.