Your experiment has already been done by others with success but they throw a tarp over the lights.

You can buy a thermostat for your Christmas tree lights so you don't have to watch the weather and maybe get tricked by an inaccurate weather report. My lights go on automatically as soon as the outside air gets down to 40 F.

I always go to the after Christmas clearance sales and load up on the large bulb strings of lights. The large bulb types are getting harder to find, so I've got a good sized collection of them sitting in storage.

I also buy the replacement bulbs in the after holiday clearance. They are really cheap then.

It never occurred to me to throw a tarp over the tree. I'd never be able to get a tarp over my trees and if I did somehow, I'd never be able to keep it on with the winds in my area. Just lights gives some protection.

Those using a tarp, are you putting it on at night and taking it off during the day? I'd think that once the tree is blooming, it would need light.

There have been lots of reports of these lights helping from people who don't know the critical temperatures for flower buds or fruit. I've never seen damage from any temperature above 30F. Turning them on at 40F is a waste of electricity.

If it drops to about 32 and the fruit survives someone thinks they have saved their crop. I've seen many partial crops after temperatures as low as 13F at full bloom. That was with no protection on a Surecrop peach in Amarillo TX. It is not unusual to get some fruit after 24-28F during bloom or later.

Sorry if this doesn't agree with someones pet project. But I've seen many many dozens of freezes on fruit. West Texas usually gets two or three each year and I've been here 40 years.

I certainly think lights might save the crop if you threw a tarp over them, but I'm only speaking theoretically. All I've used that I've proven to myself this year works, is a tarp. Fig with tarp totally protected, one nearby without totally fried. Lights under tarp might well protect another couple degrees worth or more- have to test it to know.

Now that fruitnut mentions it I doubt lights without a tarp to catch heat would have much affect or such a system would be used commercially. They use a whole lot more heat to protect trees than what's generated by a few small light bulbs.

Like to have a peach tree that reliably crops after experiencing temps under 28 degrees. 29 wiped out much of my crop this year and I have lots of frost resistant varieties. Wiped out my apricots too, and they were against a south wall. Only reached that temp for about an hour too. They were all at least a week past petal fall.

It's not just about temps, but duration, wind, gound cover and the temps the day leading into the frost.

I saw a project that Purdue was doing one time with huge heat lamps over a peach orchard. This was serious heat for commercial application. But it just took too much heat and was too expensive to be economically viable. Often on things like this I ask my self why isn't this used commercially if it works so well. In most cases it doesn't work or is too expensive. Xmas lights aren't too expensive at least for a home owner. But I think you're looking at 1F or in that range of protection.

I agree that a tarp and heat can be significant help.

OK I never used Tarp. From where you buy it. My trees are dwarf so the tarp can work along with the Xmas lights. I am just assuming I should have a post to hold the tarp. My trees are 6-8 feet so 8 feet post will work.

I get a lot of nights down into the 20's while my trees are in bloom. I still get nice crops of apricots, peaches, and nectarines.

I have the lights go on at 40 degrees, as the temperature is going down, so the lights have time to heat up and build up some heat before it gets to 26,or 24, or whatever it is going to get down to.

It seems to me that the blossoms can survive some where between 26 and 28 F and still produce fruit. The lights give me a couple more degrees leeway, which is often all it takes to make a difference.

I don't light the cold weather fruit. Cherries, apples, pears don't get lights. They do tend to bloom later, so they get slightly better weather, plus they are more cold hardy.

I got Blenheim apricots this year even though it got down to 20-21 F degrees at night for 2 solid weeks while the tree was in bloom. Not a large crop, though. The fruit was right around and directly above the light bulbs.

The Blenheim is my earliest bloomer. The rest of the trees got weather no lower than 26 degrees while they were in bloom.

For my rescued fig, which is only about 7' tall, I quickly drove in (shallowly) 3, 8'stakes just outside the tree. Foliage touching the tarp burned, the trapped heat saved everything else. It was cold enough that night to destroy most of my mulberry and paw paw crop as their foliage was completely burned.