Wednesday 21 February 2018

Diarmuid Gavin: Divide and conquer

Prepare to propagate herbaceous perennials in October

It's a great time to plan and review what worked best this year in the garden
It's a great time to plan and review what worked best this year in the garden

We've been digging up the garden we created at Dundrum Town Centre (while planning some future excitement in its place) and I've been amazed at the growth of the plants in a couple of seasons... especially the herbaceous perennials.

Most of the planting comprised colourful species, using perennials to add height and interest to the plot. By their nature these plants begging to die back at this time of year. Because they don't produce woody stems, the flower and foliage will wither away while the root system takes a rest over the late autumn and winter.

Herbaceous colour provides most of the interest in contemporary flowering gardens. In my youth, I spent some fantastic years training among wonderful gardeners in Dublin's Botanic Gardens. And one of my most vivid memories from those days was its long herbaceous border. This is a garden feature that overwhelmingly relies on herbaceous-type plants for interest (with occasional bulbs, annuals and biennials adding to the delight). I was enchanted by its seasonal development - almost bare during midwinter and then, just six months later, a huge flourish of colour.

The winter period was when foliage and flowers had died off and the plants needed tending. This could involve digging out the roots of bindweed or couch grass, or feeding with a mulch of well-rotted manure. But my favourite herbaceous job was dividing the overgrown specimens - which we are now beginning to do at the Dundrum plantation. As leaves start to fall and autumn draws in, October makes me prepare for more propagation.

It's a great time to plan and review what worked best this year in the garden. And if there's a herbaceous perennial that did really well and you'd like to see more of it around the garden, the best way to achieve this is by dividing up the plant into smaller plantlets and replanting. This is called 'lifting and division' and it is one of the easiest ways to propagate herbaceous plants. Division is also necessary every few years to keep some herbaceous perennials fresh. Heleniums and Michaelmas daisies tend to grow out in concentric circles, leaving a woody old base at the centre - think of a pebble sending ripples through a pond. The outer ripples are the fresh growth you want to harvest, while discarding the central woody base. Ideally, you'll lift and divide these plants every two to three years. Shasta daisies and phlox will benefit from annual division while hostas, peonies and daylilies will happily clump along, performing well without the need to disturb them.

So how do you go about it? The first rule is about timing: early-flowering perennials are best divided in autumn, late-flowering perennials in spring. So you can leave heleniums, asters, eupatorium, rudbeckias and grasses to keep flowering undisturbed and attack them next spring. But the reality of any timing rule is that you'll probably tackle the job when you have spare time or when the weather is good enough. Some great candidates for division now are primulas (left), brunneras, hardy geraniums and heucheras.

Dig up the plant you want to divide and shake off the loose earth so you can see what you're operating on. Plants with a fibrous root system like hardy geraniums and astilbes will break apart into plantlets quite easily. Others will require a bit more tugging - two garden forks back to back in the centre of the plant is a good way to tease apart thicker clumps like daylilies. The sharp end of a spade is excellent for hacking through tough roots like hostas and agapanthus. Generally, plants with thick fleshy tap roots - Oriental poppies, lupins, acanthus - aren't so easy to divide and are better propagated via root cuttings.

You can plant your divisions in situ, creating drifts of favourite perennials through the borders, but keep them well watered while they establish, and add compost and fertiliser to the planting hole. You can also individually pot up the smaller plants to bulk up before planting out next year.

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