The Basics of Dividing Perennials

Janice Hardy

Haliburton County Master Gardener

Many new gardeners wonder not only when but how to divide perennials.  A little bit scary as you never want to kill the plant but would love to have another specimen for a new bed or perhaps the plant is brown in the centre and blooming has tailed off to almost nothing.  While there are several species that have a preference for when to divide, most perennials can be divided at any time - with the right tender loving care. 

So why would you want to divide a perennial?  Many perennials die back in the centre as the roots in this area are so dense they cannot get sufficient moisture and nutrients.  These plants along with plants that you smaller and fewer blooms need to be divided.  Other plants are very fast growers and soon outgrow the designated space in the garden.  Overcrowded flower beds means that plants compete for water and nutrients and reduces air circulation.  Reduced air circulation in the beds may encourage disease.  Perennial division will keep those gardens neat, healthy and in peak bloom.  You may also find that weeds have taken over a perennial or two.  When dividing you can remove many of these weed roots and replant a smaller but healthy plant.  

Generally speaking plants that flower mid-June and later should be divided in spring and plants that flower early to mid-spring can be divided in the fall.  There are always exceptions to any rule of thumb and plant division is no different.  For example peonies are best divided in the fall, irises in July and Iceland poppies in August.  Spring division gives the plant a full growing season in which to recover from division.  

Any time you dig up or disturb the roots of plants they are under stress, reducing their ability to take up water and nutrients.  Watering the plants that are ready for division the day before will help to reduce this stress.  When dividing there are some key steps.

•   Dig up entire clump getting as much root as possible.  Follow at least the drip line and preferably a few inches beyond.  Dig all the way around a plant and then pry out the root.

•   Move to an old tarp or plastic and shake off as much soil as possible.  Sometimes some of the soil can be hosed off.

•   Look for natural breaks in the clump.  If there are none, then using a sharp knife simply divide the plant straight down the middle.  If the clump is large enough then divide each clump again in the same way.  Each clump should be at least 25% of the original clump.

•   Remove any dead roots and plant material.  Also pick out any weeds, including their roots, that are growing in the clump.

•   Add organic matter in the form of compost or composted manure to the planting site in order to renew the soil.  Ensure the hole is large enough to hold all the roots in their natural position.  Replant at the same level as the original clump without curling the roots.  Water well and water on a weekly or more frequent basis if no rain to ensure plants become re-established

•   Plant the divisions or pot up to exchange with neighbours or as a donation to your local Master Gardener or Horticultural Society plant sale.  Plants also make great gifts for friends and neighbours.