Homegrown: Dividing perennials

By Iowa State University, Linn County Extension Master Gardener, Lisa Slattery 

I like to rotate big garden jobs like dividing perennials.  This year I am putting in two more perennial beds one shade and one sun. I'm planting some new plants but am watching my budget so I'm dividing lots of existing perennials – which is hard work but the payoff is great.

There are several reasons to divide perennials besides moving them or sharing with friends.  If your perennials are declining, don't bloom as often, deteriorating in the center, or taking over the flower bed, it's time to divide.  Division rejuvenates plants, controls size and increases air circulation which helps with diseases like powdery mildew.  And every gardener always wants more plants.  

Where to start? The ideal way to divide a perennial is to dig up the entire plant.  Check out the roots for health and destroy any that have signs of insect or disease damage.  Use sharp tools such as shovels, knives, or even a cleaver or axe to separate roots into multiple divisions.  My son and I dug up a HUGE hosta this weekend and we had to use several types of shovels and a saw to divide it!  Iowa State University Extension suggests cutting perennials into sections like a pie since vigorous roots may be at the outer edges of the plant. 

The size of the division depends on the species, your time to tend new divisions and your patience.  Hosta, iris and daylily grow pretty quickly after being divided.  Other plants like peony grow better if the divisions are bigger.  The smaller the division the smaller the root system and the longer you'll wait for new blooms (which typically is 1 to 2 years).

Plant the new divisions at the same depth as the original plant.  Add compost to the planting site and practice good plant maintenance like regular watering and weeding. Mulching is excellent.

How often and when to divide?  This depends a lot on the plant.  Fast growing daylilies, hosta and bee balm can be divided regularly or every 3 to 5 years.  Yet, 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. 

Visit ISU Extension at

http://www.extension.iastate.edu/Publications/RG319.pdf for a terrific publication listing division guidelines for many popular plants.