Summer splits

RHS Journals

The Garden
June 2007

Summer splits

Although dividing perennials is usually a job for spring or autumn, recent findings suggest that, for some, summer division is better. Phil Clayton experiments.

Images: Tim Sandall

Moving house is an exciting, often daunting time, especially if you have plants you want to take with you. When moving home myself, last year, I decided cuttings and divisions were the solution, but it was already March and my offer on a house only just accepted. With this in mind, I read with interest an article by propagation expert Peter Thompson in the Hardy Plant Society’s journal on summer division of perennials.

My experience: In truth, as a test for this technique, my experience was a rough-and-ready affair. Weather conditions were terrible for dividing plants and, in the hurry of the move, plants did not receive the care they deserved. Even so, about 90 percent survived. Some were cut into small sections, others kept in large chunks, but as a practical demonstration it proved, given care after divisions are made, that many perennials respond well to summer splitting, performing, in my opinion, at least as well as those made at traditional times. Peter Thompson’s insightful thinking will certainly be welcomed by gardeners for whom time is the most precious of commodities in spring.

Why divide in summer?

Dividing perennials in summer when in full growth is usually frowned upon; received opinion has it that early spring or autumn are best for splitting most herbaceous plants, but Peter maintains summer division is better for many. He has found that dividing and potting up between June and July - while the plant is in growth - results in a higher number of successful individual plants, and that subsequently they are quicker to establish and develop.

It was June by the time I was ready to start, ideal timing by Peter’s method, but too late by conventional means. I had not yet had time to dig new borders, so establishing plants in pots before planting would be useful.

I began nervously digging and potting up favourites from my old garden. The conditions could not have been worse: with the ground bone dry and temperatures topping 32°C (90°F), most plants were stressed even before I moved them. Some were in flower - lovely blue and white Iris ‘Stepping Out’ had to be decapitated (though bearded irises are one of few plants usually moved in summer, albeit after flowering).

Winter-flowering I. lazica, however, seemed a good bet; this, together with I. unguicularis, is often split and sold as divisions when in flower… only to fail. It does not make root growth until summer, so splitting then makes sense.

Spring-flowering Uvularia, Pulmonaria and Dicentra will often defoliate in hot summers, only to re-emerge unaffected the following year. In last year’s Saharan summer these were already retreating underground when I lifted them, so my planting them in moist, fresh multipurpose compost almost seemed kinder than leaving them in situ.

Some easy subjects such as Geranium, Ophiopogon, Sisyrinchium striatum and little pink-flowered Stachys officinalis ‘Hummelo’ (a favourite) I knew were tough. Having plenty, I was happy to split these into smallish pieces (3-5cm/1-2in across), but other subjects seemed much riskier. Hellebores and Epimedium in full leaf, Chamerion angustifolium ‘Album’ (white willowherb) near flowering, leafy Aster divaricatus and, most worryingly, Paeonia mlokosewitschii - a genus fabled to be tricky to move, and in this case a cherished clump grown from seed given by Christopher Lloyd. Initially, all reacted alarmingly - unsurprisingly perhaps - but I avoided splitting them up too small, to chunks 10cm (4in) across, and soon most perked up, although I had to sacrifice the flowers on the white willowherb.

Some plants split without batting a leafy eyelid, including choice Agapanthus ‘Windsor Grey’, Bergenia ‘Admiral’ and various Hemerocallis. They were mostly plants with fleshy roots or, in the case of Bergenia, a rhizome that sustained them once larger leaves were cut off.

In the new garden, having divisions in pots made them easy to care for. I placed them in a shaded corner away from fierce summer heat and kept them watered. Many, including geraniums, Hemerocallis and pulmonarias bulked up welI; others barely grew (such as Libertia peregrinans). Some (including the peony and fern Matteuccia struthiopteris) looked dead. August, thankfully a cooler month with rain, arrived and I dug beds and prepared for autumn planting, although some would not go in until spring.

Encouraging results

On planting, I was pleasantly surprised. The really healthy-looking plants had made good root growth, promising impressive displays the following year. Even those that had disappeared above ground showed signs of life below. In March I planted the last few pots - mostly splits of grasses I had ignored over winter - and sat back to watch what would happen. By now beds were well stocked with store-bought plants and seed-raised material, so I was eager to see how my summer divisions would perform.

Iris lazica flowered and thickened up well, even before the final planting, but the bulk of the spring-flowering plants are, at the time of writing, performing admirably. Pulmonarias and hellebores are in bloom, while hopeful shoots are appearing even on subjects that I feared had died, including my beloved paeony. Libertia has spread modestly, and other subjects such as geraniums, stachys and sisyrinchium have exploded into growth.

How to divide
Splitting perennials in summer

Phil Clayton is Features Development Editor for The Garden

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