It never hurts to divide your perennials. Dividing your perennials will help keep them healthy, beautiful, in bounds, and will make more plants for future plantings.
The reasons for dividing are endless. 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. Overcrowding these perennials can lead to fewer and/or smaller flowers than their well-spaced and divided counterparts—division keeps the blooms bountiful. Dividing vigorous plants (including gooseneck loosestrife, plume poppy, and obedient plant) will help keep them from overwhelming their neighbors. Dividing also 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. Here are all of the ins and outs of dividing your perennials so they last for years to come.
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 being 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 to make several clumps out of them.
Dig up the clump of perennials that will be divided. To do this, insert the shovel deep into the soil around the perimeter to loosen the roots and isolate the clump. You can even use a garden fork or spade to help separate the roots.
Editor's tip: Watering the perennial a couple of days before you dig it up will soften the soil and save you effort.
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.
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.
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 late summer, when the plants are resting. Just make sure to replant them in a sunny spot in well-drained soil—the best growing conditions for them to thrive.
Carefully dig the rhizomes using a shovel or trowel, and wash off the soil to expose the shoots. You should see each rhizome has roots growing out of the bottom and fans of leaves growing out the ends. If you don't have access to a garden fork, a shovel will also get the job done.
If your iris clumps are made of many rhizomes growing closely together, you can simply plant them farther apart. If you want to make even more, or you have some older 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.
If you wish, you can trim the foliage to keep it looking neat and tidy. Cut the fans back to about 3 inches tall. It's also helpful to treat your iris rhizomes with a fungicide to stop mold or other diseases from attacking.
Rule of thumb: Most perennials do best when divided every three to four years. When in doubt, this is the route you should take. These varieties include: