My dozen or more Astilbe do well planted in the north & east edges of my garden. They especially need moist soil in order to thrive. That being said, mine have thrived during dry seasons with supplemental water. I haven't ever fertilized my plants; they apparently don't need it. They also thrive in full sun where the soil stays moist.

According to Larry Hodgson's book Perennials for Every Purpose, " Astilbes like things cool and moist. They prefer light to partial shade."

I don't deadhead the flower stalks. In my experience it has no impact on the plants' performance from one year to the next but check the Missouri Botanical Garden to verify their bloom period.

My best advice: relax. Plants will do well or not based on your soil and how you plant them, not on what you do to or for them. They need space to spread their roots in healthy soil. Once established, stand back and let them do what comes naturally.

My garden is also Z6a. I grow perennials from seed via the winter sowing method. My garden beds are now full thanks to seed-grown perennials.

I've never heard or read that you need to deadhead astilbe. Just let them do their thing and enjoy them when they bloom each spring.

Here is a link that might be useful: Astilbe - Missouri Botanical Garden

and you can leave the spent flowers until spring for "winter interest" they usually do not bloom the first year you get them...at least mine did not. also, Dan Hinkley says "Astilbes are disgusting plants"

Dan Hinkley says "Astilbes are disgusting plants":

If they dry out enough so that the foliage gets brown and crispy it has been my experience that no amount of water will rejuvenate them. (Although they will come back just fine the next spring). But in a moist spring and early summer they can have some spectacular blooms.

I'm in Zone 6, southern CT, and all of my Astilbes have been "done" for a while. Here they have one strong blooming season ... spring to early summer. They're not long-blooming perennials.

When the real heat of summer hits, many of my varieties begin to brown up and turn crispy. That's when I cut them down to the ground. I never fertilize them.

My experience with our astilbes is much as the above.

Re chinese astilbe:
although I also have these shaded by taller perennials in summer, I do find they tolerate more sun than other astilbe.

its time to stop fertilizing .... its almost winter ...

and you can run into a lot of trouble.. having perennials and conifers on the same watering system ... they have very different needs ....

you can get IDs on anything.. in the appropriate forum .. including the name that plant forum.. by posting pix of things ...

its near impossible to answer your questions.. w/o really knowing what plants you have .. as i read your facts.. as saying there were more than astilbe

ken