There are really two reasons for dividing your flowering perennials: 1) to promote better plant health and vigor, and 2) to increase their number. Not a bad deal, getting more plants for free and improving the originals!
There are a few exceptions – such as pinks, candytuft, lupine, and oriental poppies – but most perennials that grow in clumps or colonies are easily divided. Sometimes they will die out at the center of the clump, telling you it’s time for division.
Perennials can be divided in early spring or late summer to fall, with the key being to divide and re-plant after blooming is finished and 6 to 8 weeks before severely hot or cold weather arrives. That means dividing summer-blooming perennials in the fall, if possible.
The process involves just a few simple steps:
1. Water the flowerbed the day before you plan to dig, unless it’s already soft enough to easily push your shovel into. It’s better for you and the plants if you do the dividing on a cool, cloudy day.
2. Cut the stems down to a height of 4 to 6 inches.
3. Dig vertically around all sides of the clump with a shovel or spade to create a rootball, then push the shovel or a spading fork under the rootball and lift it out of the ground.
4. Shake or rinse excess dirt off the roots.
5. Use a shovel, spade, or sharp knife to cut the clump into smaller clumps, making sure that the new clumps have both roots and topgrowth. Cut or pull off dead sections and discard them.
6. Plant the new divisions, adding soil amendments to improve the native soil if necessary.
7. Water thoroughly and feed with plant food to promote rapid establishment and root growth. Continue to water regularly for a week or two, until the new plants get established and resume growth.
Perennials typically only need division every few years, so by staggering the division of plants from year to year, you can create a steady supply of new plants to expand and enhance your garden.