What is a perennial?
Perennials, by definition, are plants that live more than two years. Some perennials, like peonies, live for decades, others like gaura, live 3-5 years. Ballon flowers or Platycodon grandiflorous, live 20 years or more without needing division. Each plant is different, and some, like black-eyed Susan, reseed everywhere in the garden, extending their lifespan for as long as you want.
Perennials are also loosely defined as anything that doesn't have a woody structure, i.e., herbaceous perennials. Though, technically, trees and shrubs are perennials, too.
Why divide perennials?
Many beginning (and not-so-beginning) gardeners think of perennials as a one-and-done. Not so. Perennials provide years of beauty to our gardens but do require some maintenance. Division is a primary task associated with perennials. Why do we divide perennials?
Perennials can get too big for their space, crowding other plants.
Division helps with fungal diseases and pest control as it allows for better air and sunlight circulation.
The center of plants start to "bleed out," forming a donut hole in the middle.
Perennials are producing fewer flowers, or straggly-looking leaves or weak growth. (This could also be a sunlight issue.)
It's a quick and easy way to grow the garden through propagation.
When is the best time to divide perennials?
Now is a good time to divide perennials as they die back. The soil temperatures are warmer than the air temperatures which encourages root growth--a good thing.
Rule of thumb: Divide spring blooming perennials in fall. Divide late-summer, fall perennials in spring.
How to Divide Perennials: Example Shasta Daisy
This clump of Shasta daisies is still healthy, no bleeding out in the middle, but experience tells us that this was it's peak year and dividing now will avoid the hollowing out next year. First, cut back all the dead stems to where the plant growth is.
Use snips or deadheaders.
A favorite garden tool are these red snips. (Here's the link on that--a blog on staff's favorite garden tools) They're perfect for the job of cutting back the dead stems to make dividing easier. Cut back to the plant.
Dig at drip line.
Begin digging at the drip line of the plant. This is the point where the plant ends.This ensures you're digging up the whole root ball. You will severe some roots when you dig and that's okay. Dig around the plant, and at an angle to get underneath the root ball, lifting the plant as you go.
Lift plant out of hole and divide.
If you've dug up the entire plant, the root ball, should look like this. Depending on the size of the clump, it can be quite heavy. Help may be needed. Roll it on it's side for dividing.
First cut: right down the middle.
Begin cutting the plant down the middle. Often the roots will guide you as you make your cuts. Cut where the stronger roots are so that that your new clumps will be productive. The center of daisies are often woody and unproductive, discard any plant material that meets this description. Cut the clump (depending on size and needs in the garden) into 4 clumps or more.
Dig holes about a foot deep and wide, and add amendments: 30% amendment to 70% native soil. Daisies will flop with too much fertilizer or enhanced soil. They prefer native soils. Then back-fill, water, and continue to water until plants have gone completely dormant.