Anne Swithinbank’s Masterclass

QWhat is the best way to divide herbaceous perennials, and how big a section is best for replanting? I would like to plant some types in together. Is this a good idea?

Gloria Chadwick, Birmingham

AHerbaceous perennials are longlived, non-woody plants whose growth dies back in winter and returns every spring. Most have specific flowering periods between spring and autumn, and perform well in herbaceous and mixed borders, prairie style plantings, as statements and in containers. I love them for they are a movable feast, rarely objecting when we fork them up out of the ground to plant elsewhere. After a couple of years, new plants have usually expanded sufficiently to be divided into several portions, perfect for swapping or making larger drifts.

Interplanting one sort with another is fun, but don’t expect all experiments to work! In my garden, for instance, yellowflowered crocosmia and white-flowered

Lysimachia clethroides intermingle prettily, blooming together in moist soil. On lighter soils, the pink daisies of echinacea can rise through a blue foam of perovskia. After several years, clumps exhaust themselves, with healthy growth around the outside of scruffy middles; this is easily rectified by lifting, dividing into healthy portions, and replanting.

The best method for division depends on the plants themselves. They originate from temperate regions worldwide and roots vary, from the fibrous masses of symphyotrichums (asters) and heleniums, best divided in spring, to the thick, fleshy roots of peonies, better tackled in autumn. The long, woody anchoring roots of Japanese anemones take well from winter root cuttings or spring divisions.

For most, I favour the old-fashioned method of two garden forks pushed back to back through the middle of a clump, so that when the handles are pulled apart the clump splits. Make smaller divisions or cut through dense, fleshy roots using an old long-bladed kitchen knife.

Aim for a minimum new clump size of 4in (10cm) across to set 12-18in (30-45cm) apart. You can have larger sections, but tiny ones usually die off, unless they are potted up and grown on first.

Interplanting can work wonders, as with this Echinacea purpurea and Perovskia atriplicifolia With quick-growing plants like Aster ‘Little Carlow’, you can buy one plant and a few years later, have enough to make impressive drifts that show up well and attract many insects Anemone hupehensis ‘Hadspen Abundance’ takes well from winter root cuttings or spring divisions