If I place a heating coil and turn it on before there are any crops planted (that it could connect to), all crops planted later will die. Turning it on after planting, in the same circumstances, will work, even on crops planted afterwards.

To make sure it's exactly this causing the problem, I constructed this setup.
I turned on the right coil, a second later planted all the carrots, another second later turned the coil on the left. Results as seen on the screenshot.

I'm running the Homestead modpack.