Have several Astilbes in my gardens, some are about 20 years old. I do not cut them at all. I let nature do the watering of my gardens. If there is a dry spell, then will do a general watering over all. In the winter the Astilbes are left alone, never cut down, they are there for winter interest as their left over bloom stocks just stand tall in the snow. And I don't fuss with them at all.