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Kitchen Gardening

Keep Seedlings Warm Outside With Holiday Lights

Warm your cold-sensitive seedlings with repurposed holiday lights.

  • Photo/Illustration: Greg Holdsworth (All photos)
  • 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?

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!

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