September

Tip: You should be thinking about dividing your perennials if they are invasively growing into other areas or starting to look ratty.  If you are not sure though, most perennials need to be split every 4 to 5 years - not only is it like getting a free plant for another area of your garden, but this will give them a huge boost the next year. 

Tip: Generally speaking, perennials that bloom in the Fall should be divided in the spring, but perennials that bloom in the spring and summer should be divided right now! 

Tip: While you're splitting and transplanting your perennials remember that overcrowding is not the best thing for your plants as it can weaken them.  Allow for enough space so that each plant at full maturity just touches the others.