Why Divide a Hardy Fern?

A mature clump of Dryopteris or Polystichum is one of the most generous plants in a shady British garden, and dividing it does you two favours at once. The parent plant benefits from being split: old, congested crowns produce smaller fronds each year, and the centre of the clump can eventually die out, leaving a ring of growth around a bare patch. Splitting restores vigour and gives the roots fresh soil to move into. Meanwhile, you gain new plants for free — ideal for filling bare corners under trees, edging a north-facing border, or swapping with neighbours.

Ferns are also wonderfully forgiving subjects for the knife. Unlike many herbaceous perennials, they have tough, fibrous rhizomes that survive rough handling, and divisions settle quickly in cool, damp conditions. If you can dig a hole, you can divide a fern.

Timing Your Lift

Early spring is the moment to strike. Aim for the window between the soil warming slightly and the new fronds unfurling — in most of the UK that means late February to mid-April, with March usually the sweet spot. At this stage the plant is packed with stored energy but has not yet spent it on soft new growth, so divisions re-establish with very little check.

You can also divide deciduous ferns in autumn, from late September into October, provided the ground is still warm and you keep the divisions watered. Evergreen species such as Polystichum setiferum and Blechnum spicant are better left until spring, when they are less likely to sit wet and cold through winter and rot.

• Best: early spring, just as the "fiddleheads" are swelling at the crown
• Acceptable: early autumn for deciduous types, in mild areas with free-draining soil
• Avoid: midsummer, when water loss will scorch the fronds, and hard frost, when new roots cannot work

Lifting the Clump Without Panic

Water the plant thoroughly the day before if the soil is at all dry — it makes the rootball hold together and the digging far easier. Then work a fork around the outside of the clump, at least a spade's width from the crown, easing it gently rather than levering hard. Ferns often root surprisingly deeply, so be patient and follow the roots down.

Once the clump is free, shake off surplus soil so you can see what you are dealing with. You will usually find either a single dense crown or, with running sorts like Matteuccia struthiopteris and Osmunda regalis, a network of thick rhizomes travelling outward. Both are straightforward to divide.

Splitting with a Sharp Knife

For most garden clumps, a sturdy kitchen knife, a pruning saw or an old bread knife will do. Push the blade straight down through the crown and cut cleanly, aiming to leave each piece with at least two or three growing points and a decent share of roots. An established fern will happily come apart into four, six or even eight divisions.

Resist the temptation to tease the clump apart with your hands. Tearing fibrous rhizomes leaves ragged wounds that invite rot; a clean cut heals far better. For very large, woody crowns, two forks placed back to back in the centre and pushed apart will do the job, though you may still need the saw for the toughest sections.

• Small ferns: cut into two or three pieces
• Established clumps: aim for four to eight divisions with visible buds
• Running rhizomes: simply cut the connecting stolons and lift the rooted sections
• Discard: the woody, hollow centre of very old crowns — it rarely regrows well

Replanting and Aftercare

Replant promptly — ferns hate drying out, so have your holes ready before you lift. Set each division at the same depth it was growing before; burying the crown too deep is a common cause of failure. Water in well, even if rain is forecast, and water again every few days for the first month if the weather turns dry.

Ferns in shade gardens appreciate a soil enriched with leafmould and a little garden compost, with a handful of grit worked into heavy clay to keep the crown from sitting wet. A mulch of leafmould around, but not over, the crown will conserve moisture and feed the plant slowly. Slugs and vine weevil are the main pests to watch in that first season — check the new croziers for nibbled tips, and keep an eye on potted divisions.

Spare divisions pot up beautifully in a loam-based compost, kept in a cool, shaded frame or against a north wall. They will be ready to plant out by autumn, or to pass on to a friend who has a gloomy corner crying out for green.

Which Ferns to Divide — and Which to Leave

Most clump-forming hardy ferns divide readily: Dryopteris filix-mas, Athyrium filix-femina, Polystichum setiferum and Polystichum munitum all respond well to being split every three to five years. Matteuccia struthiopteris spreads so freely that digging rooted runners is easier than dividing the parent.

A few are best left undisturbed. Osmunda regalis resents root disturbance and can sulk for a season or two, and Blechnum species are slow to settle. Tree ferns, of course, cannot be divided at all. For these, patience and a generous mulch are better tools than a knife.

Dividing is one of those jobs that feels like a chore in February and feels like pure profit by May, when a refreshed clump throws up a shuttlecock of fresh fronds and your new divisions are quietly rooting in the shade behind the shed.