I have hardy fall mums for many years. I live in zone 6. We have had two successive years of below normal winter temps and much cold wind. I usually cut my mums back to about eight inches every fall. I then let leaves blow into the stems and get stuck there. It causes a loose, protective mulch over winter. Sorry, it may look messy but it works. In spring when I think that the cold is over, usually late March, I will remove the stems and leaves. Underneath are tiny green shoots, even this year after a severe winter. If a cold blast comes, it would be advisable to cover these tiny shoots for a day or two. Trim back the foliage in spring once it reaches 6" or so. Then trim another time or two as necessary to keep shorter. Stop trimming in early July and just wait to enjoy many blooms. Of course fertilize in spring. I like Osmocote because it keeps feeding for months every time you water or it rains. Wishing you success.