CORVALLIS, Ore. – Autumn is the time for simple garden math: dividing and multiplying. Dividing perennials invigorates overcrowded plants, and it's an inexpensive way to multiply landscape plantings.
"What gardener is not guilty of planting things too closely?" said Barb Fick, home horticulturist with Oregon State University Extension in Benton County. "Or, it's often the case that plants live longer or grow larger than we expect they will."
As herbaceous perennials grow, their roots spread out into large clumps. After a few seasons, the centers may die out and performance declines. The plant needs to be divided.
Divide perennials when they are dormant. Fall is the best season for dividing plants that bloom in spring and early summer. Cooler temperatures and abundant precipitation encourage good root systems to develop before the next bloom season rolls around.
But before you divide, Fick advises, plan where you will multiply. Prepare planting holes that are large enough for division roots to fit without being crowded. Amend the soil at the new location if it needs it.
After the new planting area is prepared, carefully dig around the plant to be divided, leaving as big a soil ball as possible around the roots. Then lift the plant gently from the ground.
Divide plants by pulling them apart at obvious separation points. Select vigorous shoots with both root and crown sections. Discard woody centers and cut off unhealthy roots. Healthy roots are white in the center.
Make large divisions, because small pieces will be slow to reestablish. Plant the new divisions at the same depth as the older plant.
The math is simple, Fick said. "By dividing perennials, you multiply your landscape plantings and delight your gardening friends with plants to share."