Answer: Usually these plants stay green and fresh until a hard frost, then they are cut back to the ground at about maybe four inches tall. Or, if you like the look of the seedheads in winter and have birds that will enjoy eating them, you may opt to wait to cut them back until spring.
If the stems and foliage have already dried, they may either have been water stressed or suffered a disease problem. If this is what happened, remove any dead material and dispose of it in the trash to try to limit the source of reinfection for next year. I hope they are okay.
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