Dividing perennials is a great way to help reduce the size of your plants as well as to maintain overcrowding issues. As perennials grow and mature you may have added too many to begin with or they may have naturally out grown their space which can cause overcrowding. Perennials typically will do fine as they grow together but you may start to see fungi and disease development if air circulation becomes an issue. Dominant plants may just out grow their counterparts and you may lose your desirable plants due to overcrowding.
An easy way to combat these issues is to help your plants maintain their space. Dividing perennials is essentially cutting your plants in half. Most perennials will do fine if divided in the fall. The day time temperatures are much cooler and the ground is usually still moist from residual rain. Spring is also a good time to divide your perennials. Many perennials come up at different times in the spring so you may want to be more cautious to not divide your plants too late.
There are many ways to divide perennials but here are a few easy steps to get started. First, make sure plants are watered. You may want to water the perennials you plan on dividing the night before or just wait to divide after a good rain fall. Use a sharp spade shovel and put in the middle of the plant. Slice through the perennial and dig up half of the plant. Fill in the exposed half of the plant still in the ground with new soil or compost. You can now use the other half of the plant elsewhere in your landscape or donate it to a needy home. If you do plan to save the divided perennial, simply put the plant in a pot and water the roots. Try to re-plant within 24-48 hours.