Astilbe is a species of flowering perennial herb that is also commonly known as false goat's beard or false spirea. Certain cultivars can reach 3 feet in height, so you can also use it as a small shrub and plant it en masse. Astilbe is hardy in USDA Zones 4 through 9 and produces tall flower stalks in summer from the current year's growth. According to the University of Vermont perennials like astilbe require little pruning but can benefit from grooming and will tolerate pruning for size to fit the landscape.
Deadhead your spent astilbe flowers in the late spring and summer after they start to shrivel and die back or look untidy. Cut the flower stalk down to the parent branch from which it emanates or down to the crown of the plant and pull it cleanly from the canopy.
Refrain from dead heading flowers if you wish to have seed heads develop for seed collection or to self sow in the soil, creating more astilbe plants in the area.
Cut back the plant for size in the summer, if needed, to prevent crowding other plants. Remove exterior branches down to the crown to reduce the width of the plant. Prune the top of the plant sparingly, and only as needed, to preserve the maximum bloom.
Remove dead plant tissues in the late fall, winter or early spring, depending on your climate. Cut back brown or desiccated branches down to the crown of the plant 1 or 2 inches up from the soil. In cold climates, shear the dead plant foliage to the crown entirely in the late fall or early spring.