Dividing Perennials: A Toronto Master Gardeners Guide

Hostas, such as this H. ‘Janet’, should be divided in spring before the leaves begin to unfurl. A sharp, clean knife or spade might be needed to cut the root system apart at the crown. Photo: Helen Battersby

Many, but not all, perennials benefit from division after they have grown for a number of years. This gardening guide describes the common gardening practice of perennial division, including why, when and how to divide your plants. You can replant your divisions, or share or trade them with friends and neighbours, and watch your garden grow!

Why divide

The main reasons to divide perennials are:

  • To get more plants or to keep plants from overcrowding.
  • To keep perennials fresh and young. Many perennials make new growth around the periphery and tend to die out in the centre with age.

When to divide

Perennials can be divided in the spring or the fall. Late bloomers such as chrysanthemums and asters should be divided in spring because there is not enough time to re-establish after bloom in fall. Perennials that bloom in April and May, such as primula, should be left until after they bloom.

Dividing perennials is best done when the weather is cool and the ground moist. The ideal day is overcast so that the roots don’t dry out quickly. Make sure the plant is well watered before you begin. Spring dividing should be done when a couple of inches of growth are showing. Fall dividing should be done early so that the roots have time to establish before winter.

How to divide

To divide a clump-forming plant, lift the whole plant by digging up as much of the root ball as possible. Shake or tease off enough soil from the roots so that you can see the roots and crown. Pull the clump apart into root pieces with several growing points on each section.

For plants that grow by runners or spreading roots, dig up new, extra roots only and move to another location.

Replanting

For clump-forming perennials, choose the strongest parts from the outside of the plant and replant a few pieces to replace the original plant. The extra pieces can be planted in new areas or potted up to give away. Keep newly-potted plants in the shade – they are already stressed enough. (Use plastic bags and wet newspaper around the roots if only holding for a few days.)

When replanting the newly divided perennial, add compost or well-rotted manure to enrich the excavated soil. Plant the divisions at same level they had been growing at in the newly prepared hole. Tamp the soil firmly around the plant. Water immediately and keep watered for the next few weeks to help it survive the shock. Discard or compost the woody centre of the old plant.

Hints for dividing

Hostas, daylilies (hemerocallis) and ornamental grasses are sometimes difficult to separate into pieces. Try working with two garden forks, back to back, and pry apart. If necessary, use a large knife or split with a spade.

Some plants dislike moving or division, such as oriental poppies (a long tap root), peonies, gypsophila (baby’s breath) and euphorbia. But if they are not doing well where they are, you might as well take a chance!

Date revised: January 2012

For printable version, click:  Dividing Perennials – A Toronto Master Gardeners Guide

Produced by the Toronto Master Gardeners, these Gardening Guides provide introductory information on a variety of gardening topics.

Toronto Master Gardeners are part of a large, international volunteer community, all committed to providing the public with horticultural information, education and inspiration.  Our goal is to help Toronto residents use safe, effective, proven and sustainable horticultural practices to create gardens, landscapes and communities that are both vibrant and healthy.

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