From Dennis Patton:
From time to time our garden perennials need to be divided. There is no set rule of thumb about how often this chore needs to be completed, but here are some guidelines to help determine if your clumps need to be tackled. Some perennials are best divided every three to five years while others, such as the peony, may never need to be touched.
To help you determine whether it is time to divide or not, answer these three questions. If you answer yes to any then now is the best time to divide the most common perennials.
1. Have you noticed a reduction in flowering? Overcrowded plants will not flower as nicely. This is true of daylilies. So if you think the plant should have more flower stalks then maybe it is time to divide.
2. Does the plant have an open center or donut look? As crowns mature oftentimes the center dies out. New growth comes from the outer edges of the center crown. Ornamental grasses and some hostas suffer from this problem. Dividing is one way to prevent this problem.
3. Has the plant outgrown its allotted space in the garden? Some plants spread rapidly overtaking their neighbor’s place. If this is occurring, then it’s time to divide. Monarda and Nepeta do this, as do many others. Simply remove the shoots or dig and replant.
A nice size division should contain no more than three to five shoots or growing points. Planting too large of a division will result in a more rapid turnaround and require dividing again. Dividing a perennial can be work, but it is also one of the great joys of gardening to see a plant renewed and ready to perform and create a show in the garden.
One last comment, don’t feel you need to plant every start back in the garden or peddle up and down the street giving them away to everyone. It is okay, and life will continue if you discard the extra plants.