How should I cut back the Helenium autumnale at the end of the flowering season? They are 3 feet tall and the flowers are spent. There isn't any green at ground level. Do I cut them back to ground level or just deadhead and leave them tall?
Yes, you will want to cut back your plant not that it is done blooming.
Here is a link with more information.
It is one of the Sneezeweed varieties, and I usually see them in garden centers in late summer, when the plant normally blooms. There are yellow varieties, as well as red, and red and yellow.