Lynne Forbragd, UM Extension Master Gardener in Anoka County, September 25, 2015
Fall is a busy time of year in the garden to get things ready for the upcoming dormant season. This is a great time to take a walk through your garden to see which perennials thrived and which ones didn’t perform as well as you expected.
Look to see if each perennial has outgrown its current spot by reviewing these questions. Is it taking over or crowding out adjacent plants? Does it have a hollowed out center? Does it flop over? Did the plant did not bloom as prolifically as it has in previous years? These are all signs that the perennial will need to be divided.
Late summer and early fall are excellent times to divide perennials and provide ample time for the plant to re-establish its root system. The perennials that can be divided this time of year are those that have bloomed in spring and early summer and have completed their blooming cycles. At this time, the plant is not using its energy towards blooms and can work on developing its roots. Make note of the perennials that are blooming in the fall that need to be divided, but wait until next spring to divide them.
On average, perennials should be divided every three to five years. Some perennials are more aggressive and will need to be divided more frequently, while others should not be moved for 10 years. This is dependent on type of plant, how well the plant has developed, and where the plant was originally located.
When dividing perennials, choose a cooler cloudy day to do your work. Water the plant the day before to help keep it moist for the division.
The method for dividing a plant depends on its root system. For a clumping root, rhizome or tuberous root, a sharp knife works best. For plants with clumping roots, such as hostas and daylilies, dig the plant and gently loosen the root clump. Keep a good root system for each clump divided. For plants with rhizomes, such as irises, discard any diseased or dried up rhizomes. For tuberous roots, leave a bud and part of the stem with each tuber. Plants with spreading roots can be gently pulled and separated by hand. Larger overgrown clumps of roots, such as Siberian Iris or larger grasses, need a shovel or fork and some strength to divide.
When replanting, keep the roots moist so they don’t dry out. Amend the soil by adding compost to the soil and water every week to establish the roots before the ground has frozen. Mulch around the plants after the ground has frozen. Take these steps in dividing your perennials this season so that you can enjoy more blooms and a healthier garden next year.