September and October are great months to dig up, separate and replant all the perennials you mentioned. Check out the video I made for PlantersPlace on dividing primroses to see how I divide up perennials using a root saw to separate the side plants from the mother plant. If the Maiden grass and Black-eyed Susan are still looking good you can wait until early spring such as March to divide those clumps. Be sure to loosen the soil in the new planting hole and mulch on top of the soil with at least two inches of bark chips or other organic matter so the new plants are not heaved out of the ground in a winter freeze. Fertilize with a slow release plant food like Osmocote. Your flowerbed will be robust and beautiful next summer. Keep growing, Marianne Binetti