Why Divide Perennials?
There are lots of reasons to divide the perennials in your garden. Among them are:
Keep them healthy. Many perennials grow quickly, forming large clumps. If you don't divide them every three to four years, these clumps can die out in the middle, leaving a bare hole.
Protect plants from fungal diseases and insect infestations.
Keep them beautiful. Overcrowded perennials often have fewer and/or smaller flowers than their well-spaced and divided counterparts. If your perennials are drastically in need of division, they may even appear stunted.
Keep them in bounds. Some perennials (including gooseneck loosestrife, plume poppy, and obedient plant) are especially vigorous or even aggressive. Dividing these plants will help keep them from overwhelming their neighbors.
Make more plants. Dividing perennials leaves you with more plants of the same variety -- perfect for adding to other places in the garden or trading with friends, family, or neighbors.
When to Divide
While you can divide most perennials any time from spring to fall, those two seasons are best.
This is because dividing your perennials can be stressful on the plants -- and they'll recover better from the shock in cool, moist conditions. That said, if you want to divide your favorite perennials in summer, be sure to keep them well watered afterward.
As far as your plants go, wait to divide them until they're large enough that you can make several clumps out of them.
How to Divide Perennials
Step 1: Dig the Clump
Dig up the clump of perennials to be divided by inserting the shovel deep into the soil around the perimeter to loosen roots and isolate the clump.
Here's a hint: Watering the perennial a couple of days before you dig it will soften the soil and save you effort.
Step 2: Remove the Clump
Force your shovel or garden fork under the root ball and lever the ball up and down to loosen and position it on the shovel. Then lift the shovel and root ball. Try to keep the root system as intact as you can.
Once you dig the plant out of the ground, shake, wash, or brush any excess soil from around the root ball. This makes it easier to pull the clump apart.
Step 3: Separate the Crowns and Replant
Pry or cut apart individual crowns. Each clump needs to have sets of leaves and roots in order to grow.
Then replant the divisions promptly so the roots don't dry out. Plant at the same depth as before and water well. Cover the soil with mulch to help conserve moisture while your new divisions become established.
How to Divide Bearded Iris
Bearded iris grow a little differently than most perennials -- they have rhizomes, or fleshy stems that grow along the ground. The best time to divide bearded iris is summer, when the plants are resting.
Step 2: Divide the Rhizomes
If your iris clumps are made of many rhizomes that are growing closely together, you can simply plant them farther apart.
But if you want to make even more, or you have really old plants, you can also divide the rhizomes themselves. To do this, break or cut the rhizome into pieces. Each piece should have at least one fan of leaves and roots growing out of the bottom.
Leave These Be
While most perennials benefit from being divided every few years, there are a few that don't. Avoid dividing these varieties: