I have no heating element. I spray paint milk jugs black and fill with water (with a bit of bleach added). This works great at keeping night temps up provided there is sunshine the previous day and I put my cover on. The cover is critical for night warmth--combined with the milk jugs. I build the cover out of polystyrene insulation. The cold frame was purchased a few years ago, during the winter it is stored in the basement until early/mid March when it goes on the deck against the house. The inside back and sides are lined with milk jugs. I put stuff like broccoli, lettuce, onions, etc. in there. After those are planted out in mid April, the tomatoes and peppers go out as long as the weather looks not really bad and only milk jugs on the sides (to fit full length flats). Those plants will go through some cool nights into the low 40s--especially when the day or two days before are cloudy and the water doesn't warm up. But it doesn't seem to really bother them, even the peppers. I think keeping them out of the wind and getting natural light is enough good to outweigh the bad. And even cloudy days where the clouds aren't thick, it still warms up considerably in there. And it is auto-venting, I have it auto-vent at ~60F with cool season thing and mid/upper 70s with tomatoes and peppers.
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