How and when should I cut back or prune my Maxamillian Perennial Sunflower...
I would suggest leaving much of the growth from this season standing through the winter as feed for birds and overwintering habitat for beneficial insects. Once temperatures begin to climb in spring, remove growth from last season being careful not to injure the crown of the plant. This will stimulate new growth from buds at or just below the surface.
This fact sheet may be helpful as well. Happy gardening.