I have all of these you are asking about. You can save the seeds from your hollyhocks or let them reseed themselves. I cut all the headsand stems off my yarrow when done blooming, these too can be saved for seeds. I don't do anything to my sedum unless it's an older one and you can divide it and it will come back even healthier in the spring. I mulch my plants well before winter sets in just as a help to them. But I really don't do anything special. Hope this helps, am sure someone else has ideas also. I live in a cold zone 4 if that helps. Good luck!