Many Thanks to SCMG Cyd Brown for sending along this information from the Master Gardeners of Northern Virginia Website.
Native perennials may benefit from periodic division to improve their vigor and appearance. Indications that a plant requires division are dead sections at the center of the clump, reduced size or abundance of flowers, or sparse foliage. Division can rejuvenate the plant and stimulate new growth. Perennials can also be divided for propagation purposes.
Click on the button below for a table of common perennials indicates the root type of each plant, the best season for division (if appropriate), and specific advice on frequency and manner of division.