Divide and conquer perennials in September

September is a time for performing a labor of love on your hardworking perennial plants.

University of Missouri Extension horticulturist David Trinklein said timing and technique are important when dividing overgrown perennials.

Over time, perennial plants often become overgrown, unproductive “clumps.” They lose vigor and either produce small flowers or quit flowering altogether.

“Keep the clump young,” advised Trinklein. “Dividing a perennial is a good way to accomplish that goal.”

Divide plants after the season of rapid water loss is over. In Missouri, the beginning of September usually marks the time when temperatures moderate and plants tend to use less water.

“Jump in right after that,” Trinklein said. Divide any time in September, but early September is the best time.

Dividing in September allows roots to recover from stress of accidental damage. This gives newly planted divisions time to develop roots while the soil still is relatively warm. Roots are more sensitive to soil temperature than air temperature, Trinklein said.

To divide an overgrown perennial, remove a third to half of the clump.

Use a sharp shovel to dig a semicircular trench around the perimeter of the root clump.

Dig straight down the center of the clump, deep into the soil. Force the shovel or garden fork under the root ball. Sever and loosen roots. Lift the root clump. Fill in the hole with soil.

When relocated, larger divisions generally put on a greater display of color the following spring than do smaller ones, Trinklein said. However, for more propagules (transplantable divisions), separate the roots into smaller groups, leaving at least one growing point per division.

Promptly plant the divisions in well-drained areas, or share the excess with others.

New propagules are at risk. The ability of the plant to take up water is reduced when its roots are damaged. No matter how careful one tries to be, roots receive injury when divided, Trinklein said.

Water the newly planted divisions well. Rainfall will not be adequate in most cases. However, too much water can lead to root rot.

Do not fertilize perennials in the fall, Trinklein says. Fertilize in the spring with a general purpose garden fertilizer such as 5-10-5 to encourage both shoot and additional root growth.

Trinklein said daylilies are among the perennials that “get too big for their britches” fairly quickly, depending on growing conditions. Many of the reblooming varieties benefit from division every other year, or every third year, for best garden performance. Other perennials, such as peony, are slower-growing and do not need to be divided nearly as often. The division interval for perennials depends on the species and growing conditions.

For more information, the MU Extension publication “Flowering Perennials: Characteristics and Culture” (G6650) is available for free download at extension.missouri.edu/p/G6650.