Garden Guides states that Astilbes can be pruned in late summer, mid-winter or early spring. In late summer or early fall, the flower plumes can be removed as they start to wither, brown and dry out as this will prevent the plant from dropping seeds. Begin by removing the plume where the flowers and the stem meet as soon as the flower plume begins to die.
If removing the plume for floral arrangements instead, then remove the plume close to the foliage to give the plume a length of stem. Otherwise, in mid-winter, look for leaves that have begun to brown and wither in the same manner as the flower plumes. Remove these leaves close to the stem using a pruning tool. In spring, the plant can be cut back all the way to the ground. This should only be done when new green growth begins to appear.
In addition to regular pruning maintenance, Astilbes form large clumps as they grow and spread out. The crowns of the root structure break the surface when the clumps grow too large. This routinely occurs every 3 to 4 years. These clumps should be divided at the end of winter, before the spring growing cycle, and replanted separately.