When to Divide Perennials
Julie Martens Forney
Discover one way to keep your perennial garden in tiptop shape—by dividing plants. Many gardeners only think about dividing plants when they want to multiply their botanical holdings. After all, dividing a plant yields several plants where once you had one. But dividing accomplishes more than providing a low-cost source for plants. For mature perennials, dividing is an important way to maintain plant health.
As perennial gardens mature, individual plant clumps increase in size and flower number. That’s one of the exciting parts of tending perennials: watching the garden change through the years. For many perennials, aging can take a toll on plant health and vigor. For other perennials, maturity brings on such a substantial increase in size that plants begin to jostle and even invade one another. Dividing corrals the chaos of ever-growing clumps and also renews aging plants.
The vast majority of perennials need dividing roughly every three to five years to maintain plant vigor. Some, like garden mums, blanket flower and asters, need division more frequently—every one or two years. Others, like peony, bleeding heart and butterfly weed, rarely demand division and are best left alone.
It’s possible to categorize perennials based on how often they typically need division, but those timelines assume plants are growing in ideal conditions. Keep your own perennial garden in outstanding shape by learning to recognize the signs that a perennial needs divided.
A perennial that stops flowering or produces flowers that are much smaller than normal is a candidate for division. You might witness this situation in coreopsis or bearded iris. For other perennials, the clue that it’s time to divide is that the center of the clump dies out, and new growth occurs only on the outer edge of the clump. Many perennials eventually reach this state if neglected. Others, including yarrow, lamb’s ear and black-eyed Susan, typically develop this growth pattern in just a few years.
Another clue to watch for is bottom leaves becoming fewer and fewer, while growth occurs only at stem tips. In this situation, stems might also be very crowded within the clump. Watch for this in perennials like tall garden phlox or bee balm. For other taller plants, including taller types of sedum and campanula, as clumps become overcrowded, stems may become floppy.
The best time to divide perennials depends on what perennial you’re dividing and where you garden. A general rule of thumb is to divide perennials after they flower. Tackle spring-blooming perennials in fall and fall bloomers in spring. Some perennials, like iris and peony, are best divided in early fall. Most ornamental grasses do best when divided in early spring.
Time fall dividing so new plants are in the ground at least six weeks before your region’s average frost date. In colder zones, many gardeners tackle dividing in early spring, just as shoots are breaking ground. An ideal situation is when soil temperature is warmer than air temperature. This encourages root growth while discouraging fast top growth.
Probably the safest time to divide is when you know you can provide necessary care for newly planted perennials. These young transplants need adequate water until their root systems are established. Try to tackle dividing before your region’s rainy season, if temperatures permit.