Written by Kim Smerek  

Watching your garden mature year after year is one of the great joys of gardening. After a few years though, those once beautiful perennials start producing smaller blooms, grow more sparsely in their centres and have weak outer stems which need staking to keep them from falling over. And more often than not, they start encroaching on other plants' territory.  Time to divide!   

Why do we even need to divide them? When your plants are performing poorly, becoming overcrowded, they have to compete for nutrients and water. Overcrowding eventually leads to disease from restricted airflow.  Dividing your perennials into smaller plants gives them the space they need, stimulating growth and blooming. Division keeps plants that grow more vigorously, under control.  One of my favourite benefits of division is that it will give you more plants for your garden in the most inexpensive way.  

When is the best time to divide your perennials? 

• In fact, the best time to divide is at the end of the season in which it has performed the best. Before it starts to look horrible and starts growing into its neighbours. Generally, most plants are divided every three to five years, or when they have become overcrowded.    

• Rule of thumb says to divide in cooler weather, which means spring or fall, however, some plants can be divided at any time, like day lilies. I find the best time is in the early fall as all gardeners know that spring is already such a busy time. Be sure to allow about four to six weeks before frost to allow the roots to establish themselves. Fall bloomers can wait to be divided in early spring.    

• Choose an overcast day, and be sure the soil is moist to prevent the roots drying out too quickly.  

• Divide when the plant is not flowering so it can focus all of its energy on regenerating root and leaf tissue.

How to divide your plants?  
  • Plant division involves splitting or dividing the crown and root ball. Some plants grow from a taproot and are usually propagated by seed or cuttings, but are not impossible to divide, though more care needs to be taken.    
  • Start by digging at the plant's drip line. This is generally at the outer edge of its leaves to keep most of the plant's roots intact, as the roots usually extend out this far. Dig around the clump, cleanly severing any roots, then cut under the clump at an angle, working your way around the plant until you are able to lift it out of its hole. Larger, heavier plants may require you to slice them in half or quarters with your shovel in order to lift it out in sections. Separate the plant into smaller divisions by any of these methods:                    

• Gently pull or tease the roots apart with your hands;                    

• Cut them with a sharp knife or spade;                    

• Or put two forks in the centre of the clump, back to back, and pull the forks apart.              

• Each division should have three to five vigorous shoots and a healthy supply of roots.  

• If you are not replanting the sections immediately, keep the roots covered, cool and moist until you can get them in the ground. A bucket or container works well to hold the divided plants, keeping them in a shady spot and covering the roots with damp soil or newspaper. If the roots dry out while they are waiting to be planted, be sure to soak them for a bit before putting them in the ground. 

Replanting divided perennials  

• When you take a plant out of the ground, it leaves a hole in the soil. That hole should be filled with compost before putting back any plants. That soil then remains fertile and the plants will have the advantage of the added nutrients.  

• Smaller plant pieces that are about 1/5 to 1/4 of their original clump will grow and bloom more vigorously. Remember that perennials will generally double their size in a season, so dividing a mature plant only in half means you'll be dividing again the next year.  

• Plant only the healthiest pieces. Usually these are the outside sections.  

• Replant your plant divisions in a similar location or another pot.  

• Dig a hole at least as wide as the plant's roots when spread out. Don’t squish the roots into an undersized hole, because you’ll defeat the plant’s natural regrowth mechanisms.  

Guide to root types and division  Thank you to Fine Gardening Magazine for an easy guide to dividing plants with different kinds of roots:  When you dig up a perennial, you will see that it fits into one of five basic root types: roots that form clumps or offsets, surface roots, underground running roots, taproots, or woody roots. How you proceed depends on what root type your plant has.  

   Offsets  To divide a plant whose roots form offsets (small plants growing at the base of a larger one), snap the connection between any of the sections to obtain a piece with ample roots and three or more growing points (or “eyes”). Some denser clumps may have to be cut apart.  Plants that form offsets include Asters, Coneflowers, Hostas and Coreopsis. 

   Surface roots  Some perennials have roots that run on or just below the surface of the soil. They form new crowns and roots when they reach open spaces or make contact with the soil. If you cut between any of the stems as you would cut a piece of sod from a lawn, you will have a division with its own stems and roots.  Plants with surface roots include bee balms (Monarda), black-eyed Susans, Sedums, and creeping speedwells (Veronica).

   Taproots  Plants that have taproots can be divided by using a sharp knife to slice down the length of the root. Every piece that has at least one eye, some of the taproot, and a few side roots is a viable division.  Plants that have taproots include butterfly weeds (Asclepias), Spurges (Euphorbia), and Oriental poppies.  

   Underground running roots  Underground running roots can develop suckers as they grow beyond the shade of the mother clump. These suckers can be cut away from the main plant, or you can dig up the main plant and cut away any piece with an eye or sucker already forming.  Plants with underground running roots include hardy geraniums, Japanese anemones (Pulsatilla) and ostrich fern.  

   Woody roots  Woody perennials often form roots when stems rest on the ground or are buried by gradually accumulating mulch. Make a new plant by simply cutting between the rooted stem and the mother plant.  Plants that have woody roots include candytufts (Iberis), euonymus (Euonymus), lavenders , and sages (Salvia).