Black-Eyed Susans should be divided every three to four years to ensure healthy plants. The best time to divide Black-Eyed Susans is in the spring or the fall. Before dividing your Black-Eyed Susan, trim the foliage to within six inches of the ground. Dig around the plants, and pull the entire clump up from the ground. Dust the soil off the plants, or rinse them with a hose. Separate the plant sections with a sharp spade or knife, and make sure that each section has three or four strong-looking shoots. Replant your divided Black-Eyed Susan immediately, and water in your "new" plants for best results.