Why Dividing Perennials Matters

A clump of hardy geranium in its third or fourth summer tells you plainly when it has had enough. The middle goes bald, shoots crowd the edges, flowers come smaller and sparser, and stems that once stood smartly through a July shower flop apart at the first rain. None of that means the plant is finished. It means the plant is full — and it needs splitting.

Division does three jobs at once. It restores vigour to an ageing clump, because the outer sections you replant are young, hungry growth with fresh roots and plenty of room. It keeps the border balanced, since healthy perennials swell outwards year on year until one plant bullies its neighbours. And it multiplies your stock for nothing — the best kind of gardening. On an allotment plot, where budget and space both matter, half a morning's work can furnish a whole new row of Michaelmas daisies, heleniums or phlox.

When to Lift and Split

For most herbaceous perennials, autumn is the ideal window. From mid-September to late October the soil is still warm from summer, rain is more reliable, and roots settle in before winter with almost no demand above ground. The plant spends the winter quietly rooting and gives you a decent show the following year. Early spring, roughly March to mid-April, is the second chance for anything you missed, and it suits plants that resent autumn disturbance.

A few perennials prefer spring: Japanese anemones, agapanthus, oriental poppies, kniphofia and most ornamental grasses. Leave those until you see new growth pushing through. Anything that flowers early — pulmonaria, primroses, epimedium — is easiest immediately after flowering, so it has the rest of the season to recover.

• Choose a dull, damp day rather than hot sunshine, so roots never dry out.
• Avoid dividing in the middle of a drought or during a hard frost.
• Divide every three to five years, before the clump becomes a doughnut.

Lifting a Clump Without Damaging It

Water the clump thoroughly the day before if the ground is dry; moist soil clings to roots and comes away from the fork more kindly. Cut back the top growth to about 15cm with shears, which reduces water loss and lets you see what you are doing. Then push a border fork vertically into the soil a good spade's depth from the crown, all the way round, levering gently as you go. Work around the whole circumference before you try to lift. Forcing a fork in one side and heaving rarely ends well.

Slide the fork underneath and ease the clump free. If roots are deep, a second pair of hands with a second fork helps enormously. Shake off loose soil, but never beat the clump against the ground — it bruises roots and does nothing useful.

Splitting the Crown

How you split depends on the plant. Fleshy, fibrous crowns — hardy geranium, phlox, monarda, aster — usually pull apart with two forks placed back to back in the centre of the clump and pushed apart. Thick woody crowns and tough rhizomes need a sharp spade or an old bread knife, and if all else fails, a clean cut down the middle and then into quarters.

• Discard the old, woody centre — it rarely grows away well.
• Keep sections with at least three strong shoots and a good handful of roots.
• Trim ragged roots with sharp secateurs, cutting back into sound tissue.
• Tease roots apart under a running hose if you can — it shows you where the natural divisions lie.

Replanting and the First Year

Plant your divisions at the same depth they were growing, spacing them according to their eventual spread — most border perennials want 30–45cm. Improve the planting hole with a couple of handfuls of garden compost, firm the soil gently around the roots to close air pockets, and water in well even if rain is forecast; a good soak settles soil against roots.

Mulch with well-rotted manure or compost, keeping it clear of the crown, and label each group with the name and the date if you plan to swap plants with neighbours. In the first season, water during dry spells and resist the urge to feed heavily — you want strong roots, not soft top growth that slugs will find first. Many divisions flower a little in their first year; let them. By the second summer they will be as good as the parent ever was.

What Not to Divide

Some plants sulk badly when disturbed, and lifting them can set them back years. Avoid dividing anything with a taproot or a deep, brittle root system unless you have no real choice: lupins, delphiniums, dicentra and oriental poppies all resent it. Plants that spread by runners, such as lamium and ajuga, are easier to propagate from rooted pieces. And if a clump is flowering beautifully and looks content, leave it alone — healthy borders are built on good timing, not on tidiness.