Propagating rare ferns by division and spores

Why rare ferns demand a gentle approach

Ferns are far older than flowering plants, and the rarest of them — the oblong woodsia, the Killarney fern, the filmy ferns of our western ravines — survive in Britain on a knife edge. Many cling to a handful of damp crags, old walls and shaded gullies where humidity stays high and disturbance stays low. Growing them in a garden is a genuine privilege, and it comes with responsibility.

In the UK, wild plants are protected by law, and the rarest are listed under Schedule 8 of the Wildlife and Countryside Act. The rule is simple and absolute: never lift a plant, a division or even a frond from the wild. Work only from cultivated stock, or from spores passed to you by another grower. Everything below assumes that starting point.

There are two practical routes to increase your stock: careful division in spring, and the slow, absorbing business of sowing spores. Both reward patience over speed.

Dividing established ferns in spring

Division suits clump-forming ferns — shield ferns, polypodies, wood ferns and most hardy garden species. It does not suit ferns with a single crown or a creeping rhizome that resents being cut, so know your plant first. The window is late March to early May, just as the croziers begin to unfurl but before the plant is in full growth and losing water fast.

• Water the fern thoroughly the day before you lift it.
• Ease a fork under the crown and lift the whole rootball, keeping as much soil as you can.
• Tease the crown apart with your fingers, or with two forks placed back to back and gently levered. Cut only where you must, using a clean, sharp knife.
• Every division needs at least one growing point and a good clutch of roots. Small, rootless fragments rarely survive.
• Replant at exactly the same depth. Burying a fern crown deeper than it sat before is the commonest cause of failure.
• Water in, mulch with leafmould, and shade the new plants for a fortnight.

Divide no more than every three or four years, and only when the plant is vigorous enough to spare the material.

Sowing spores: the long game

Spores are the fern's equivalent of seed, but the process is different in almost every way. Watch a ripe frond for the brown, clustered sporangia on the underside. Harvest when they are brown but not yet split wide open.

• Cut a ripe frond and lay it in a paper envelope. Keep it somewhere dry and cool for a day or two.
• The spores fall as fine dust — darker than you expect, and far finer than any seed. Tap them out gently. Never breathe over an open envelope.
• Sow thinly on the surface of sterile compost. Do not bury them. Spores need light to germinate.
• Water only with cooled, boiled water, ideally from below, so you do not wash the spores into a corner.

Fresh spores germinate best, so sow within a few weeks of collecting. Old spores lose vigour quickly, though some fern spores remain viable for years if kept dry and dark.

Sterile kit and the right environment

This is where most spore-sowing attempts fail. Fern spores germinate slowly, and given half a chance they are overrun by algae, liverworts and moss before they ever get going.

• Use new pots, or scrub old ones with hot soapy water and rinse with boiling water.
• Moisten a fine, peat-free seed compost with boiled, cooled water. Microwave it damp and covered until it steams, then let it cool completely before sowing.
• Seal the pot inside a clear polythene bag or a lidded propagator. The atmosphere should be permanently humid, not wet.
• Place in bright, indirect light at around 18–21°C. Direct sun will cook the sealed container.
• Keep everything off the surface of the compost — no labels plunged in, no fingers, no dripping condensation from a cold lid.

A little green film on the compost after a few weeks is normal. It is the flat, heart-shaped prothalli you are watching for.

Pricking out, hardening off and the first winter

After the prothalli stage, fertilisation requires a film of water, and only then do the first true fronds appear. This can take six to twelve months from sowing, and sometimes longer for the choicest species. Do not rush it.

Prick out small clumps once the fronds reach 2–3cm, lifting them with a fragment of compost attached. Keep them shaded and humid for several weeks. Harden them off by opening the bag a little more each day over two or three weeks — never all at once.

For the first winter, keep young ferns in a cold frame or unheated greenhouse. Protect them from hard frost and, more importantly, from waterlogging. Damp, stagnant compost kills more young ferns than cold ever will.

Growing for conservation, not just the shelf

A rare fern in a garden is only useful if it is looked after and passed on. Keep a simple record of where each plant came from, and share divisions and spores with other growers rather than hoarding them. Hardy fern groups and botanic collections are always glad of well-documented, cultivated-origin material.

Garden the habitat as well as the plant: deep shade, a mulch of leafmould, constant damp but never a swamp, and no slug bait scattered among the crowns. Do that, and a plant that took a year to raise from dust will reward you for decades.