A couple hard Winters here have defoliated my Nandina each time. I have not pruned or did anything but water them in warm spells, and they always refresh their stems and leaves in Spring here.
And interestingly, I see more new plants growing beneath them in bad Winters. I think the seeds need the cold.
I am going to be transplanting the new shoots up in the Fall to expand them I really like Nandina!