We dig about Oct 15. So no nitrogen for 10 weeks prior to digging.

I plan to overwinter them in the ground.

I am not much of an advocate of overwintering in the ground. Many people in mild climates do so but there are lots of stories of rodents eating the tubers. Good luck and too much nitrogen will cause overwintered dahlias to rot, especially if there is a wet winter. I assume the dahlias will go dormant in the winter and that you will not fertilize for about 6 weeks before they go dormant.