Windowsills can be cold even if they are sunny. Start something there too soon that doesn’t like colder temps, and it won’t grow.
I don’t know what we might want to do for next year to get around this, as I’ve not yet seen thin heating mats that could go on a windowsill. Still, just because we haven’t had time to look for them specifically doesn’t mean they don’t exist.
[After additional research later: yes, windowsill sized heating mats exist. I found them online. You may want to budget for at least three next year.]
[Note from 2020 Me: Welp, we couldn’t put off replacing the hot water coil in the boiler another year. So no budget for heat mats, and the cats are overly fond of both window sills and radiators. Better luck in future seasons! Maybe you’ll finally build better cold frames or a greenhouse or (heaven forbid!) you somehow manage the walipini, Future Me! I’ve got my fingers crossed for you, because this year ain’t looking good for the kitchen garden.]
