I would never have considered the science around water freezing. Although the sprinklers idea is very clever, I think the 100 watt bulb sounds the easiest to set up. Since the area underneath the frame is going to be around 400 cu/ft, I assume I will need a few bulbs? If so would Christmas tree lights do the trick?

Most trees are still pretty young and have a trunk diameter that is well short of an inch.

Very interesting ideas here as I am searching for insurance against cold damage, too. I noticed that here in Japan, tea farmers have their fields fitted with small fans several feet above the bushes. I imagined this was for air circulation but never thought about frost damage prevention, as with orange groves. After researching, I found this photo on line and learned that they also use sprinklers for frost protection! Doesn't seem like the fans could possibly do much good at those heights.

Christmas tree lights work for me in Northern California, but it doesn't get that cold here. With the cold frame I would imagine they would work great for you! They only work if they're old style (C9) lights. :)

Looks festive wrapped around the trees too!

You're officially (usually) only supposed to connect 2 strings of lights together from 1 outlet though (fire risk), so keep that in mind. :)

Thank you for pointing out that the Christmas tree lights need to be need to be C9. I was trying to figure that out.

Cori, that is good, specific advice. I have given Christmas tree lights advice on a couple of occasions- but I didn't know there was a specific type. Yikes!

They don't need to be just the C9 style bulb, they can be the C7 or mini lights, BUT they have to be the old kind, not LEDs-- LED won't generate the heat that you are looking for.

Raee_gw is right. They can be C7 too... C9 just tend to be easier to find (for me at least). Here's a link online at amazon. I'm pretty sure these are the same ones I have.

https://www.amazon.com/gp/aw/d/B002AQFPVG/ref=ya_aw_od_pi?ie=UTF8&psc=1

Use your judgement on when and how this may be safe in your application but I have used little votive candles on really cold nights to add some heat under my covered trees. Just don't set your covering on fire and plan for how to contain your candle when the wick burns down to the bottom of the candle. It's usually quite wet here so I can probably use this a little safer than you can over there in Austin.

I ordered some lights and wrapped them around my little trees last night. Unfortunately they do not give off much heat at all. Most of the glass on the bulb stayed cool. How many bulbs should I be using per 2 foot tall tree? I have them strung up across 8 trees and have 50 bulbs.

Hmm... I use 1 strand per tree in my yard. That's 25 outdoor transparent C9 lights per tree. They didn't get warm at all? That seems strange... mine actually get hot to the touch. Yours aren't LED right? Here's the type I use....

They are meant to be incandescent and I cant believe they would be LED at that price. The glass immediately around the light is warm. The rest of the glass on the bulb is cold. We had a frost last night so I may be in trouble.

If your tree is visible then cover it with a lot of led string bulbs. 100 watts worth of led bulbs put out the same heat as a 100 watt incandescent.

6b Steve

Cold to us is still warmer than freezing, be careful wanting too warm as you might burn the plant.

steve is right that 100w is 100w but make sure it's 100 actual watts if you use led, you do not want 100w equivalent.