Irises should be divided after they bloom
Perennials are tough but a few tips will guide you on their care.
Dividing Perennials
To encourage new growth and better bloom, dividing plants is a good idea.
A rule of thumb is perennials should be divided during the season opposite that they flower. So spring and early summer plants should be split in the fall, and summer and fall-bloomers can be split in early spring. The result is that they will have time to get re-established before they need to focus on blooming again. When dividing in the fall, leave the new divisions four to six weeks to settle in before the ground freezes.
Several plants should only be divided just after they bloom such as peonies and irises.
There are two ways to divide: Dig up the entire plant and divide the root ball, or leave the plant in the ground and cut out sections. Be sure your spade is sharp before you start to divide.
Cutting Back Perennials
Perennials can be cut back at the end of the season in late fall or the beginning of the season in early spring. You can make the call based on whether you like a neat looking appearance to your yard throughout the winter or enjoy the aesthetic value of the perennials throughout winter months. Many species (especially grasses) retain interesting seed heads, fruits and foliage that bring a whole new life to your winter garden. Plus, seed heads attract native wildlife to your garden.
Trim the dried stems as low to the base of the plant as possible to allow room for the new growth to emerge.