Astilbe japonica 'Peach Blossom' Astilbes have one critical requirement--lots of water. Plants must have consistently moist soil; dryness leads to a quick demise, especially in sun. Increase watering as the foliage emerges and the plumes mature. If the soil dries out, the leaves will develop brown, crispy margins and will begin to drop. Fertilize Astilbes in late spring before flowering starts. Wait to cut back the old foliage until spring; it will help protect the plant from winter damage.