you say that you have clay/loam soil. Madder really likes sandy/sandy loam soil. It's a Mediterranean plant. It needs lots of sun too. So, I would suggest that you look at where you planted it. Maybe add some sand to the soil or try to grow it in a pot. I think it would be fine in zone 7 through the winter. I would think that what's happening is that it's getting too wet.
Good luck!
