Why Divide Ferns at All?

A mature clump of soft shield fern or male fern is one of the quiet glories of a shady British border. Give them a few seasons and they bulk up handsomely, filling the ground beneath shrubs with that unmistakable arching frond texture. But there comes a point when the centre of the clump begins to thin out, the fronds crowd one another, and the whole thing looks a little tired. That is your cue to lift and divide.

Dividing does more than tidy things up. It rejuvenates old crowns, gives you free plants to spread through a woodland corner, and keeps vigorous species such as Dryopteris filix-mas from shouldering neighbouring plants aside. The window is short but forgiving: late autumn through to early spring, while the plant is dormant, is ideal. Avoid dividing in high summer, and never work on a fern whose fronds are unfurling, as the new growth is brittle and slow to recover.

Choosing Your Moment and Gathering Kit

Work on a dry-ish day when the soil is moist but not waterlogged. Heavy clay that clags to the fork makes the job twice as hard, and waterlogged roots rot quickly after replanting. If you can, slip a fork into the ground a week before to check conditions.

You will need a sturdy garden fork, a sharp spade, a pruning knife or old bread knife, a tarpaulin, and a bucket of water. A trug of well-rotted leaf mould or garden compost should be waiting nearby for the replanting stage. Gloves are sensible, not because ferns are spiky but because the crown is dense and you will be working close to the soil.

Lifting the Rootball Carefully

Start by cutting back the old fronds to roughly 10cm above the crown. This reduces water loss, lets you see what you are doing, and makes the clump far easier to handle. Do not compost any foliage that looks diseased.

Insert your fork vertically about 20cm out from the crown and work around the entire circumference, levering gently as you go. The aim is to loosen a generous rootball, not to prise the plant out in one heave. Ferns root surprisingly deep, so be patient. Once the clump shifts, slide the fork underneath and lift it onto your tarpaulin.

If the fern is enormous, split it in situ rather than trying to lift the whole thing. Cut straight down through the crown with a sharp spade, then lift each half separately. This is far kinder on your back and on the roots.

Teasing the Roots Apart

You have two options here, and the right one depends on the fern. For clumpers such as Polystichum and Dryopteris, the crown forms distinct growing points that separate cleanly. Look for natural gaps and work two forks back-to-back into the centre, then lever them apart. You will often hear a satisfying crack as the woody rhizome gives.

Where the roots are tightly matted, use a knife to cut through the crown, keeping at least one healthy growing point and a good fistful of roots with each section. Aim for divisions roughly the size of your hand or larger, as tiny scraps take years to establish.

• Shake off surplus soil so you can see the roots clearly.
• Remove any dead, blackened or mushy roots with clean cuts.
• Keep divisions in the shade with damp roots while you work.
• Trim the oldest, woody centre and discard it if it looks spent.

Spreading ferns such as Athyrium filix-femina and Matteuccia struthiopteris behave differently. They creep by rhizome, so simply cut through the runners and lift the younger offsets at the edge of the colony. Leave the parent plant to carry on.

Replanting at the Right Depth

This is where more ferns are lost than at any other stage. Ferns resent being buried, so set each division so the crown sits level with the surrounding soil surface. If anything, err on the side of slightly high, then mulch around it without covering the crown. A buried crown will sulk, rot, or simply fail to produce fronds.

Dig a hole wide enough to spread the roots without cramping them, and fork some leaf mould into the base. Hold the division steady, backfill gently, and firm with your fingers rather than your heel. Water thoroughly even if rain is forecast, and keep watering through the first summer whenever the weather turns dry.

• Space divisions 30–45cm apart so they have room to arch.
• Choose a spot with dappled shade and shelter from drying wind.
• Spread a mulch of leaf mould or bark around, but not over, the crown.
• Label if you are fussy about cultivars, as dormant crowns look alike.

Settling In and Looking After New Divisions

A newly divided fern puts its energy into roots before it puts up fronds, so do not panic if the first season looks sparse. Keep the soil consistently moist, which in most British gardens means a generous mulch in spring and a watchful eye during any dry spells in June and July. Reduce competition by clearing away the debris of neighbouring perennials and pulling weeds before they seed.

If you have divided in autumn, a loose handful of bracken or leaf litter over the crown will protect it through hard frost without smothering it. Remove this in March as new croziers begin to unfurl, those wonderful tight fiddleheads that make ferns such a pleasure in a woodland planting.

Divided clumps usually settle in within a season and can be left undisturbed for five years or more. Divide again before the centre hollows out, and you will always have healthy, vigorous ferns to share with a neighbour or tuck into a bare patch beneath the trees. It is one of the most rewarding jobs in the shade garden, and rarely fails when the roots are treated with a little patience.