Why Early Spring Is the Ideal Window

There is a fortnight in spring — usually between the middle of March and the middle of April, depending on where you garden — when the soil has warmed enough to work but the plants have not yet thrown themselves into full growth. That is your moment. Lift a clump now and the roots are still relatively dormant, the weather is cool and damp, and the plant has the whole season ahead of it to settle in.

Leave it until May and you will be wrestling with soft, sappy growth that wilts the moment it is out of the ground. Divide in autumn instead and you risk a wet, cold winter rotting freshly disturbed roots, particularly on heavy clay. Early spring gives you the best of both: moist soil, rising temperatures and months of growing time before the first frost.

And the reward is genuine. Most herbaceous perennials are not shy — they simply crowd themselves out. Split them and you get more flowers, healthier foliage and a stock of spare plants to fill gaps or swap with a neighbour.

Spotting a Clump That Needs Dividing

Perennials rarely complain loudly. Instead, they send quiet signals that the centre of the clump has run out of room and energy:

• Flowering declines — fewer blooms each year, or flowers that are noticeably smaller than they used to be.
• The middle goes bald. A ring of healthy growth with a dead, woody or bare centre is the classic sign.
• The clump flops open in the middle of the border, or collapses after rain because the stems are crowded and weak.
• Growth creeps outwards and starts smothering neighbours you would rather keep.
• You can see roots pushing up through the soil surface at the edges.

As a rough guide, most clump-forming perennials benefit from division every three to five years. Vigorous spreaders such as asters, hardy geraniums and goldenrod may want it more often.

Which Plants to Divide Now — and Which to Leave Be

Spring division suits anything that flowers from midsummer onwards. Good candidates include Hemerocallis, Rudbeckia, Echinacea, Achillea, Helenium, Sedum, hardy geraniums, Salvia, Monarda, Astilbe and most ornamental grasses, though grasses are happiest divided just as new growth appears.

Bearded irises are a special case — split them by breaking off the healthy young rhizomes from the outside of the fan and discarding the old, woody centre, replanting so the top of the rhizome sits at or just above soil level.

Leave alone for now:

• Peonies, which resent disturbance and may sulk for two or three years.
• Hellebores, best divided in early autumn if at all.
• Lavender, rosemary and other woody herbs, which rarely regenerate from old wood.
• Plants with taproots such as lupins, delphiniums and oriental poppies — take basal cuttings instead.
• Anything flowering right now. Wait until it has finished.

Lifting and Splitting with Two Forks

Water the clump thoroughly the day before. It makes an enormous difference — moist roots are far more flexible and far less likely to snap.

Push a sturdy garden fork vertically into the soil about 20cm out from the edge of the clump, working your way around it. Do not lever on the first pass; just loosen. Once the clump moves, slide the fork underneath and lift it in one piece onto a tarpaulin or board.

Now the trick that has saved many a gardener's back: two garden forks, placed back to back, pushed straight down through the middle of the clump. Push the handles apart and the whole thing will lever into two halves with a satisfying crack. Repeat as needed until each section is roughly the size of a dinner plate to a small dinner plate and carries a good set of roots and shoots.

For tough, fibrous crowns, a sharp spade or an old bread knife works well. Cut rather than tear. Discard the exhausted centre and any woody, rotten or pest-damaged roots, and shake off as much old soil as you can — it may be harbouring vine weevil grubs or root aphids.

Replanting with Plenty of Compost

This is where the boost in flowering actually comes from. Dig the planting hole wider than the rootball, then fork in two or three generous handfuls of garden compost or well-rotted manure per plant, along with a little blood, fish and bone if your soil is poor.

• Plant at the right depth — the crown should sit at soil level, never buried, or it will rot.
• Space them properly. Roughly 30–45cm apart for medium perennials, allowing room to spread for two or three years.
• Firm gently with your heel, then water well to settle the soil around the roots.
• Mulch with 5cm of compost or bark, keeping it clear of the stems.
• Pot up the spares in multipurpose compost to grow on for planting out later.

Aftercare Through the Growing Season

Newly divided plants need a little attention while they establish. Water them in dry spells for the first few months — a good soak once a week beats a daily sprinkle. Keep the area weed-free so young roots are not competing for moisture and nutrients.

Do not expect fireworks in the first year. Many divisions flower modestly, or not at all, while they put energy into root growth. By the second summer the difference is usually obvious: stronger stems, more flowers, and a border that looks fuller and healthier than before. If you have divided something that matters to you, resist the temptation to feed heavily — compost at planting time is enough, and too much nitrogen gives you lush leaves at the expense of blooms.

Label your spare pots, keep them watered, and by late summer you will have a queue of healthy young plants ready to move into whatever gap you fancy filling next.