Summer-flowering herbaceous perennials can be lifted and divided in September when they’ve finished flowering, but the soil is still warm enough to aid new root development.
In fact, most perennials need to be divided every two to three years, otherwise they get too big for the space they’re in and become congested. Dividing your perennials will, of course, also increase your plant stock for free!
To do it, gently dig out the plant you wish to divide. This is often best done with a fork rather than a spade, as the latter is more likely to sever plant roots.