Multiply your perennials by division

by Helen Halpenny

Dividing your perennials is an easy way to improve the health of mature plants and increase your stock at the same time. Beginner gardeners often think of perennials as a low maintenance way to have a showy garden- just plant them and forget them! The truth is, all perennials will require some upkeep. One of the most important tasks is to divide and remove the oldest portions of the plant and replant the sections that have vigorous new growth. Although division may seem like drastic surgery, it will, in fact, improve the health of the plant. As a by-product of this job you will likely have more plants to give away or spread around your property.

All new growth in a perennial plant occurs at the perimeter of the plant. As the plant ages, its centre is often well past the vigorous growth stage and the plant starts to decline. Division will increase the vigor and promote better flowering. Overcrowding is another reason to divide.

A plant’s natural rate of growth will determine how frequently it needs division. Some plants like iris, monarda, phlox and artemisia will need to be split in 1-3 years. Others like lilies, campanulas and helenium will be happy and healthy for 4-5 years before they need more space. Still others, such as lady’s mantle, cranesbill geranium, and pulmonaria will probably not need lifting for 6 or more years. Then there are the homebodies that are happy to stay put for 10+ years- peonies, butterfly weed, baptisia and gas plant, etc.

The two seasons to divide plants are spring and fall. Early spring, when growth is just starting is the ideal time for fibrous-rooted perennials. Cool air and abundant moisture at this time of year minimize stress on the plant as it reestablishes. Spring division also allows the plant to grow a vigorous new root system before the next winter. Many fibrous-rooted plants can also be divided in fall with care. Fleshy rooted plants like Siberian iris, lilies, peonies and Oriental poppies prefer September division. The one major perennial that is best divided in summer is bearded iris. At that time of year it is semi-dormant. Perennials that are divided in summer or fall should have their foliage cut back by one-half to minimize water transpiration.

Hostas and daylilies are two tough perennials that will tolerate division any time of the year. There may be some flower loss but they will rapidly re-establish.

The first step in division is to remove the entire plant from the ground. The splitting will be much easier and you can improve the soil before replanting. Use a spading fork to free the plant’s roots. Once the plant is dug, wash the root system and crown with a garden hose. Then you can often tease the roots apart with little damage. Large, old clumps will probably have to be pried apart by the two fork method. Insert one fork into the centre of the plant. Then insert another fork back to back with the first. Push them back and forth until the clump separates into sections. A knife can be used to make smaller portions. Since peonies have very brittle roots, a sharp knife works best and does the least damage.

Replenish the soil before replanting with 2-3 inches of compost. Be sure to keep the new division moist while you prepare the site of replanting. Groom the plant by removing dead foliage and damaged roots. Spread its roots out evenly in the hole. Plant it a little higher than it was prior to disturbance. The newly turned soil will settle in time and then it will be at its proper level. Water well at planting time and keep moist over the next few weeks. A starter solution of fertilizer will help get the plant off to a good start. A mulch of straw or evergreen boughs will be very beneficial over the winter.

Written by Helen Halpenny, who is a member of the Lanark County Master Gardeners.  Want to know more about the Master Gardeners group or ask a gardening question? Visit our website at www.lanarkmg.blogspot.com or contact us at lanarkmg@gmail.com