Answer: Astilbe prefers a very rich and generously moist soil and typically turns brown in response to dry soil. This would be the primary reason for browning prior to frost. When you water it, make sure the water soaks down deep into the soil to encourage it to root deep where the soil moisture tends to be more constant than at the surface. (For this reason, a daily light sprinkling is to be avoided.) After watering, dig down and see how effective it has been -- or not. Keep several inches of organic mulch over the root zone of the astilbe to help keep the soil moist and also to help feed the soil as it breaks down.
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