Roof heating and pipe heating cable is typically cheaper then soil heating cables. Could I use them to heat a cold frame?
As background: I have to start 850 tomato, eggplant, and peppers seedlings this year. Last year I started a lot of plants, but not that many, and I used strings of Christmas lights to heat the frame. That worked, especially as I put a sheet of insulation over the frame each night. But the Christmas lights had problems shorting out, and on two cold nights I had to run hundreds of pots in through the cold dark before they froze, after the lights refused to work.
So a certain amount of expense would be worth the dependability. 850 plants are quite an investment.