When should perennials be divided, and how do I do it without killing them?

Divide when the clump tells you to: a bare, woody or dead centre with vigorous growth only at the edges, fewer flowers than in previous years, or a plant outgrowing its space. For most herbaceous perennials that is every three to five years.

Timing follows flowering. Spring and early-summer flowering plants are best divided in autumn, after they have finished but while the soil is still warm enough for roots to establish. Late-summer and autumn flowering plants are best divided in spring as growth begins. The general principle is to divide at the opposite end of the year from flowering, so the plant is not trying to do both at once.

The method depends on the root system. Fibrous-rooted clumps such as hosta, geranium and rudbeckia can be lifted and pulled apart by hand or cut with a spade. Dense fleshy crowns such as hemerocallis often need two garden forks inserted back to back and levered apart. Woody-crowned plants such as astilbe may need a knife or a saw.

Discard the old centre. It is the least vigorous part and keeping it is the most common mistake. Replant the outer sections, each with a decent set of roots and at least three or four growth points.

Replant immediately at the same depth, water thoroughly, and keep the divisions watered through their first season — a divided plant has lost root mass and cannot yet reach for moisture.

A few perennials genuinely resent division and are better left alone or propagated another way: peonies, hellebores, oriental poppies and anything with a long taproot such as baptisia or eryngium.