Cut apart woody rootballs like those of Astilbe or Amsonia with a handsaw
Separate small clumpers like lamb's ears into pieces by hand
Slice fibrous, clumping rootballs like those of hosta into viable pieces with a spade
Cut rhizomes like irises into 2- to 4-inch pieces, making sure each contains buds and roots.
Dividing Perennials: A common maintenance chore in a perennial garden is that of dividing. There is no set rule as to when to divide perennials. Some may need division every 3-5 years, some 8-10 years and some would rather you not bother them at all.
Perennials will send signals to let you know that they would like to be divided. The signals to watch out for include: flowering is reduced with the flowers getting smaller; the growth in the center of the plant dies out leaving a hole with all the growth around the edges; plant loses vigor; plant starts to flop or open up needing staking; or it just may have outgrown its bounds. These are the signs to look for and not a date on the calendar.
If division is indicated, spring is the preferred time to divide. Some fleshy rooted perennials such as poppy, peony, and iris are best divided in the late summer to very early fall.
Division is usually started when growth resumes in the spring. The process starts by digging around the plant and then lifting the entire clump out of the ground. Then, using a spade or sharp knife, start to cut the clump up so that each clump is the size of a quart or gallon sized perennial.
Discard the old, dead center and trim off any damaged roots. The divisions should be kept moist and shaded while you prepare the new planting site. After replanting, water well and protect the divisions from drying out.
Division is no more complicated than this. Some perennials may be more difficult to divide than others because of their very tenacious root system. Division has as its primary goal, the rejuvenation of the perennial planting so it can continue to perform the way it was intended. Many home gardeners have found that the process of division is more traumatic to them, the gardener, than it is to the perennial.