From The Ground Up: Divide perennials for healthy plants

Perennial flowers are just that — flowers that bloom every year. Most perennials also will grow, produce more plants and spread. If your perennials are declining, don't bloom as often, are 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 roots 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, so those can be smaller. 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 one to two 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, you'll disrupt root growth and the plant won't grow vigorously.

When to divide 

How often and when to divide? This depends on the plant. Fast growing day lilies, hosta and bee balm can be divided regularly or every three to five years. Yet, some plants don't want or need to be divided. A 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 www.extension.iastate.edu/Publications/RG319.pdf for good general division guidelines for many popular plants. For gardening questions, call the Linn County Extension Master Gardener Hortline at (319) 447-0647.