There’s lots to remember with the far-reaching topic of gardening. How do you know when it’s the right season to divide perennials? Some are best divided in spring, others in fall. How can you possibly remember which is which?
Luckily there is a very easy-to-remember rule of thumb. If a perennial blooms in spring or very early summer, dig and divide in fall. (Examples are peony, bleeding heart and tulips. All bloom in spring/early summer, so the time to dig and divide is fall, around September.) If a perennial blooms in mid-summer through fall, dig and divide in spring. (Examples are hosta, daylily, tall phlox and chrysanthemum.)
To summarize the rule: Dig and divide perennials during the season opposite their bloom time. The reason? Plants are at their most sensitive and vulnerable time when in full flower. So the peak of bloom is not the time to further stress the plants by digging them up by their roots, tearing them apart, and replanting. Move as far away from the bloom season as possible.
How often do perennials need dividing? If plants have been in place for a number of years and flowering is declining, dividing helps reinvigorate. If the center of the clump is sparse or dead, with the healthy growth around the edges, the clump can be dug and healthy perimeter divisions replanted.
Some perennial typically need dividing more frequently than others. Every 1 to 3 years divide monarda, carnation, coral bells, delphinium, tall phlox, chrysanthemum, tulip and dianthus. Every 3 to 5 years divide astilbe, campanula, coneflower, daylily, lily and iris. Every 5 to 10 years divide hosta. Long-lived perennials like peony, bleeding heart and gasplant can remain in place as long as they are flowering abundantly, and divided as needed.
Some perennials don’t like to be divided at all including baby’s breath, Asclepias butterfly flower, and Russian Sage. They prefer to remain in place, but offshoots, or side shoots can be removed and planted while leaving the mother plant undisturbed.