Keep Seedlings Warm Outside With Holiday Lights

comments (4) January 16th, 2013

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yourownvictorygarden Greg Holdsworth, contributor
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Inside the cold frame, with lights attached.
The seedlings are ready for the test, including two maverick pepper plants.
The lighted festive cold frame was indeed a sight to see! Note: I ended up not using the plastic thermometers shown in the photo.
The below-freezing outdoor temperature reading.
And the inside temperature...
Yep, the cold frame is officially snow-proof.
Almost looks comfy in there, no?
Click To Enlarge Photo: Greg Holdsworth (All photos)

The holiday season seems to officially end when you have to tackle taking down the lights and decorations put up only a few weeks earlier. While I was doing so a couple of weeks ago, I thought about how I might be able to repurpose, or find additional uses for the lights. I had used some rope lights in a lined configuration, attached to a board, to heat indoor seedlings. This "holiday light heat mat" did the job admirably. 

I set my sights higher with a bigger challenge. I wanted to see if outdoor holiday lights could sufficiently heat an outside area enough to protect seedlings and transplants from freezing overnight temperatures. The location? Inside my newly-rebuilt wood cold frame.

Here's what I needed:

1. Lights - incandescent, at least 15-18 feet long, outdoor rated, larger bulbs the better
2. Staples and staple gun
3. Thermometer
4. Extension cord
5. Timer

Step 1
I stapled the lights along the edge of the cold frame, about 3-4 inches from the bottom. I had three six-foot lengths that were connected together.

Step 2
Ran the extension cord from the house to the cold frame, and connected it to the lights. All systems go.

Step 3
That evening was the ultimate test, as it was supposed to be at or below freezing. I purchased a good-quality digital thermometer that had a wireless remote. This was well worth the price, as I could monitor the temperature from a distance (in other words, inside a warm house).

Step 4
Put in the plants, and placed the thermometer outside the cold frame. Waited a few minutes for it to register the correct temperature. It finally read 29 degrees Fahrenheit.

Step 5
Put the thermometer inside the cold frame, and again, wait a few minutes for it to register the temperature.

The results were amazing. A relatively short length of lights successfully raised the temperature over 20 degrees! It topped off at 50 degrees Fahrenheit, which would be sufficient to keep cold-sensitive plants like peppers and tomatoes from damage.

Fast-forward to yet another victory. I had some seedlings in the cold frame two nights ago, when we received an unexpected half-inch of snow in the early morning hours. The "holiday light heater" did its job and kept the inside warm.

The project was a success, and it will definitely help my Spring veggie and herb transplants transition themselves to the outside world.

 Watch Greg give you a tour of his holiday-light-warmed cold frame here!

After you try it, show it off to other members in the
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posted in: diy, cold frame, transplants, cold protection, lighting

Comments (4)

Coette2 writes: Don't do it! I followed these instructions and have spent the last half hour cleaning up the mess of melted glass and the charred remains of the cold frame and the fence behind it, after our local fire squad came to put out the fire. Thank God it wasn't the cold frame we built against the house, or we'd be replacing a whole lot more than a few fence slats. Incandescent Christmas lights aren't a safe option in cold frames.

Posted: 2:12 pm on November 28th
PNICH writes: awesome idea imma have to steal that one from you. im knew at this and around here in kentucky that would give me a great jump start
Posted: 5:24 pm on March 6th
woodsyguy81 writes: Another great idea Greg! Thanks for sharing.
Posted: 8:30 am on February 1st
nana5341 writes: What a great (money-saving) idea! I'd be interested where to buy the wireless remote thermometer....
Posted: 8:44 pm on January 31st
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