If your perennials are declining, failing to bloom as often, dying out in the center or taking over your flower bed, it’s time to divide. Division rejuvenates plants, controls size and increases air circulation, which helps fight against disease and pests. Besides, what gardener doesn’t want more plants?
The ideal way to divide a perennial is to dig up the entire plant using the drip line — the point where water drips off the leaves — of the plant for guidance. Most plant roots will reach all the way to the drip line. Once out of the ground, examine root health and keep only those that look nice and healthy.
Replant or share vigorous sections of plants first. The size of the division depends on the species. Hosta, iris and daylily grow pretty quickly after being divided. Other plants such as peonies grow better if divisions are bigger. The smaller the division, the smaller the root system and the longer you’ll wait for new blooms (typically one to two years).
Always add organic matter back into the area where you are working. If you don’t add organic material, the bed will settle and there will be less oxygen and nutrients for the plants and they won’t flourish.
It’s ideal to replant divisions right away, but if you’re stuck with a “holding” period for plants, make sure you keep the roots cool and moist. If time gets away from you and divisions dry out, soak the roots in a bucket of water for about an hour before replanting.
Plant the new divisions at the same depth as the original plant. Make sure you spread out the roots and place them in a hole that’s at least as wide as its roots. Don’t turn root tips up or curl them around the root ball when replanting or you’ll disrupt root growth and it won’t grow vigorously.
How often and when to divide depends on the plant. Fast-growing day lilies, hostas and bee balm can be divided regularly or every three to five years. Some plants don’t want or need to be divided. General rule of thumb for timing is to divide summer and fall blooming perennials in early spring as new growth appears.
Divide spring blooming perennials in late summer or early fall so they can establish root systems before winter. Never divide a perennial when it’s blooming, but if you absolutely have to, make sure to give it extra attention with frequent watering and provide some shade from very hot sun until it becomes established.
Visit Iowa State Extension at extension.iastate.edu/publications/RG319.pdf for a list of general division guidelines for many popular plants. You also can call the Linn County Master Gardener Hotline at (319) 447-0647 with plant division questions.
Lisa Slattery is a Linn County Master Gardener.