Care of Astilbe
I have 12 astilbe plants and last summer they all looked terrible! I watered and fertilized appropriately. Was it too hot? The plants face the north-west and are protected by shade with a spot or two of sun coming through all afternoon.
Astilbes like partial shade with cool moist soil. Hot afternoon sun can cause the plants to wither, brown, and even die. Consider moving the plants to a shadier spot or one that only receives morning sun. Mulching the soil with shredded leaves or other organic matter will keep the roots cool and moist. And be careful with the fertilizer. Most perennials need very little fertilizer when growing in properly prepared soil. If you do fertilize, use a low nitrogen slow release product in spring to avoid damaging heat stressed plants.