Autumn is a good time to divide perennials

Press, 31 March 1989, Page 7

Autumn is a good time to divide perennials

Continued dry weather with warm, sunny days is advancing the autumn colour — for those trees and shrubs which still have their foliage — and the Fothergilla, Aronia, and Epidendrum were a blaze of colour during Easter. The season is also resulting in some odd effects. My “angelica tree” (Aralia sp.), which has been growing at the rate of almost one metre a year and is now well over four metres tall, usually flowers in January and February and ripens its tiny black fruit in March, before the leaves turn golden. Last week the chittering of silvereyes in its upper reaches attracted my attention, and I noticed that a fresh terminal panicle of flowers had just opened (the birds were feeding on the nectar). Lower on the tree were huge clusters of ripening fruit, sharply outlined against the red stalks. Much of the lower foliage had turned bright yellow. I don’t know when I last saw a shrub looking so attractive.

During the week-end I got on with the job of lifting and dividing perennials that were due for the treatment: hostas, early-flowering toad lilies, primroses and violets, mostly, with a few other things thrown in; I thought, for example, that it would be good to have

more clumps of the lovely pale yellow Anemone “Simonii,” so I lifted some bits of rhizome. There are two schools of thought about dividing perennials. One is that all the work should be done in spring, the other that, except for the very precocious flowerers, the work should be done now.

By and large, I favour the latter view, if only because there is more time in autumn; in spring the pressure is on. I think the plants like an autumn change of quarters, too.

‘There is more time

in autumn’

If you tip out an autumn-potted plant in July you will see that already a mass of new, white roots is emerging from the old ones. If the plant is broken up in spring this growth is lost.

New shoots grow strongly in spring, but spring-divided plants inevitably suffer a check lasting several days to several weeks, and sometimes the shoots “get ahead” of the roots. If the summers were wet this wouldn’t matter much because the growth would quickly catch up with itself, as the Irishman said. But in summer-dry areas the few extra days of

spring root growth can be crucial. The principle applies to deciduous trees and shrubs, also; in most of Canterbury these are best planted in May, and certainly before mid-June, and puddled in with several gallons of water if the ground is dry. If you can get the plants in May, that is; growers seem very reluctant to lift them before July. There are exceptions to this: perennials which flower late into autumn — Michaelmas daisies, monkshoods, and their ilk. With these , you get better increase and better plants if you wait for spring. August is the time to divide these. A few are best left a little while, until late September. Into this group come the Michaelmas daisy-like Boltonia asteroides (it hasn’t yet started to open its white flowers); Aster tridentata, the “prairie starflower," which has just begun to flower, and will go on until June if May is mild; and the ornamental grasses, including bamboos. ■

I lost several precious bamboos some years ago by moving them too early in spring, and since then have not touched a bamboo before the third week of September. I cut the old canes hard back when

transplanting — the aim is to stimulate plenty of new canes from dormant underground buds.

Maximum increase with

minimum damage

Old clumps of bamboo are fiendishly difficult to divide (usually, a saw is needed) and if you want to increase your stock it’s a good idea to start with young plants and divide them every second year until you have enough. Lift the whole plant, and wash it free of soil; then you can see how to get maximum increase with minimum damage.

A useful piece of advice that an old nurseryman gave me in my very early gardening days is that to maintain stock of any perennial you need three big clumps: one to divide this year, one to divide next year, and one for the year after. Replant onethird of a clump every year, and you should have a regular supply of plants in perpetuity. Don’t be in too much of a hurry to give away pieces of scarce perenni-

als to acquisitive friends. Make sure you have the equivalent of three large clumps first. Many choice plants have been depleted beyond recovery by continual removal of their young shoots.

Most perennials, if fed from time to time, grow and thrive for years without need of disturbance. As always, though, there

are exceptions. Some plants need fresh soil regularly, Michaelmas daisies among them. These grow and flower prolifically if divided every year, in spring. Throw away the older rhizomes, and keep only the strongest young ones from the outside of the clumps. Small pieces make the best plants.

Have a look at the Michaelmas daisies in your garden. Are they heavily mildewed, with squinny little flowers? I wager that if you divide them as instructed in August and replant small, young pieces in well-manured soil with a dressing of dolomite lime, you will get bigger and brighter flowers and little or no mildew, even in a dry season.

It’s much more satisfactory — cheaper, too — to control disease by managing plants in this way than by spending time and money to spray them with fungicide every fortnight throughout spring and summer.