Homegrown: Dividing perennials

Linn County Master Gardener, Lisa Slattery, provides information on what could be a great weekend project, if the weather holds.

One of the positives to this weirdly warm Spring is that many perennial plants are way ahead of schedule so you can get an early start on dividing perennials.  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 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 of the plant for guidance - most plant root will reach all the way to the drip line.  Look at root health and only keep those that look nice and healthy.  Use 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 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).

Always add organic matter back into the area where you are working.  General rule of thumb is if you take out a wheelbarrow full of perennials make sure you add a wheelbarrow full of compost back into your garden bed.  If you don't add back 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?  This depends 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, but if you absolutely have to, make sure to give it extra attention with frequent waterings and provide some shade from very hot sun until it becomes established.

Visit ISU Extension at

http://www.extension.iastate.edu/Publications/RG319.pdf for good general division guidelines for many popular plants.