Answer: I think the easiest approach is to take your cues from nature. I would not recommend pruning anything down in the fall. Pruning encourages new growth which would not have time to harden off and therefore would be susceptible to frost damage. If plants are allowed to harden themselves off (which they do gradually as the weather begins to cool) they will suffer less winter damage. So, allow everything to grow, slow, and then sleep during the winter months. In the spring, after new growth begins, you can safely prune things back.
Hydrangeas can wait until spring to be pruned and aside from a protective mulch over the root systems, won't need any special treatment. When you prune, cut the canes back to about 18" above soil level. Ditto for the Monarda. Salvia is not always winter hardy, but after the tops die down, mulch over the soil to protect the roots from winter cold. Astilbe will die back in mid- to late summer all on its own. You can pull or rake away the dead foliage. Mulch over the soil at the end of summer to help protect the roots over winter. Miscanthus tops will die down with first frost. I leave mine alone because I think the dead foliage is attractive in winter. The foliage can be pruned away in early spring. Hemerocallus tops will die down on their own in late fall; treat as you would for Astilbe.
Hope you have better luck with your plants this time!
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