Herbaceous perennials are plants such as flowers and grasses that bloom for more than one year. For many of these, the growth above the ground dies each winter but the roots remain alive so the plant is renewed the next spring. Therefore most perennials eventually outgrow their allotted space in the garden and need to be divided. Other perennials may require division to rejuvenate, as indicated by reduced bloom. Dead areas in the centre of the clump may be caused by the plant competing with itself for nutrients, light and moisture. Division is also a dependable method of propagation for most plants. The extra plants can be relocated to another area of the garden or shared with friends. While these ‘free’ plants can be a great addition to one’s garden and reminder of friends for years to come, care must be taken as vigorous, invasive varieties are the ones most available.
The majority of perennials may be divided in early spring as the first signs of new growth appear. The weather is generally moist and the divisions will have the entire season to establish before winter. If in doubt, divide fall blooming perennials in early spring and spring blooming perennials in late summer to early autumn, usually while these plants are dormant. Fall weather may be dry, so be sure to water the plants well the day before dividing. Allow enough time after division in autumn for the plants to re-establish before winter. Choosing a few perennials to divide each season will help prevent you from being overwhelmed in an established garden.
Steps in dividing most perennials.
- If you are transplanting the divisions to a new location, prepare the soil well by digging, removing any weeds and adding compost.
- Choose a cool, overcast day. Be sure the soil is moist.
- Trim away any of last year’s growth in the spring or cut back the plant if working in the autumn.
- Start loosening the plant by forcing a spade it’s full length into the ground to form a circle around the crown. Remember the root ball will extend beyond the foliage. With a garden fork dig deeply to remove the entire plant.
- If the clump is placed on a tarp, clean up will be easier.
- Shake off excess dirt and examine the root structure to determine the best method of division. Often small pieces may be pulled off or a clean knife may be used to cut off new sections. If the plant is really stubborn e.g. daylilies, two garden forks may be placed into the clump back to back and then the handles pressed together to give leverage at the tines to pry the clump apart.
- Always discard any diseased or old, woody parts of the plant, usually the centre area. Long or damaged pieces of root can be trimmed off.
- If planting back in the original space, work the soil extensively and add compost.
- Plant the new divisions as soon as possible. Never allow the roots to dry out.
- Several new healthy divisions can be planted in one area. Spacing depends on the plant type. This will produce a drift of colour that is more pleasing in the landscape than several different plants.
- Water the new divisions well and adjust the soil level so the plant is at the same depth as the original parent plant.
- Continue to check daily and water as needed, until the plant is established.
While many perennials such as hostas and daylilies are easy to divide, those with taproots are more difficult, such as monkshood (Aconitum), baby’s breath (Gypsophila) and butterfly weed (Asclepias tuberosa). Delphinium elatum do not like to be disturbed and should not be divided. They normally will not re-establish their anchor root that provides drought and winter protection. Bearded Iris should be divided every few years during July and August. Oriental poppies are also best divided in August. Peonies can be grown for decades and not require dividing. If you choose to divide peonies, however do it after frost in September. Rhubarb is another crop that takes years to establish but is best divided in autumn.
For further information try illustrated reference books, nurseries, horticultural societies or the Master Gardeners’ Hotline at (705) 741- 4905 or www.peterboroughmg.ca Hazel Cook of Blossom Hill Delphiniums and Peonies is a member of the Peterborough Horticultural Society.