Why grow ferns from spores at all?

Dividing a fern is quick, but it only works for clumping species, and it gives you a clone of the parent. Spores offer something quite different: hundreds of plants from a single frond, for nothing but your patience. It also lets you raise species that rarely turn up in garden centres, and to build up a colony of hart's tongue, soft shield fern or polypody for the price of a few compost-filled pots.

The catch is time. From sowing to a fern you would confidently plant out is usually twelve to eighteen months, sometimes longer. Set against that, a mature hart's tongue can shed millions of spores in a season, and a modest success rate still leaves you with far more ferns than you started with. In a shady British garden, where plants are expensive and choice is limited, that is an excellent bargain.

Collecting ripe spores

Timing matters more than technique. Most hardy ferns ripen their spores between July and October. Look at the underside of the fronds for sori — clusters of dots that turn brown, rusty or golden as they mature. Some ferns hide them, and a few, such as the hard fern, carry fertile spores on separate, narrower, more upright fronds.

Ripe sori look dry and dusty. Tap a frond over a sheet of white paper and a fine powder should fall; hold it to the light and you will see individual grains. Green, plump or moist sori are immature and will not germinate, so leave them for another fortnight and check again.

The reliable method is to cut two or three fronds and lay them flat on paper in a dry room for a day or two. The spores drop on their own. Fold the paper into a packet, write the species and the date on it, and store it somewhere cool and dry. Most spores stay viable for a year or more, but fresh sowing gives the best germination.

Sterile compost and a clean start

Most failures come down to contamination. Algae, moss, liverwort and mould all grow faster than prothalli and will smother them, so begin sterile and stay clean.

• Use a peat-free seed compost, plain coir, or coir mixed with fine vermiculite.
• Sterilise it by microwaving the damp compost in a covered container for two minutes on full power, or by pouring boiling water over it and letting it drain. Cool completely before sowing.
• Use clean pots and trays, and rainwater or filtered water rather than very hard tap water, which encourages algae.

Fill a small pot or a clear sandwich box to within a centimetre of the rim, firm it gently and water from below until the surface is moist but not sodden. There should be no standing water.

Sowing thinly and sealing in humidity

Sprinkle the spores sparingly from the folded paper — a light dusting is ample. Do not bury them or cover them with compost; they need light to germinate. Then close the container with a clear lid or cling film and stand it somewhere bright but out of direct sun: a north-facing windowsill, a shaded greenhouse bench or a cool porch. Around 15–21°C suits most species.

Keep it sealed. Condensation should run down the inside of the lid; if it does not, mist lightly with cooled boiled water and close it again. Opening the lid daily invites contamination, so leave well alone unless mould appears.

Waiting for prothalli

Between four and twelve weeks later, a green film should appear on the surface. These are prothalli — flat, heart-shaped, liverwort-like plants a few millimetres across. They are the gametophyte generation, and they need a continuous film of water for fertilisation to happen. Only then does a true fern begin to grow.

Keep the surface damp at all times; this is the single most important thing you will do. Bright, indirect light and steady warmth keep them growing. If they crowd, thin them with tweezers to improve airflow and reduce mould.

The first true fronds are often small, simple and nothing like the adult leaves. Do not panic, and do not tip the pot out to check.

Growing on, and what to do when things go wrong

Once each little fern has two or three fronds, lift clumps with a teaspoon or a blunt pencil, roots and all, and space them into modules or small pots of peat-free multipurpose compost. Keep them shaded and humid for a fortnight, then harden them off by opening the vents or peeling back the cover a little more each day.

Grow them on in a sheltered, shady spot for a season, potting up as the roots fill the pot. Plant out in spring or early autumn into soil improved with leafmould, setting the crown at the same level it was in the pot, and water through the first summer.

• Green slime on the surface: algae — too wet and too bright. Reduce light and water less.
• White threads or fuzzy patches: mould — remove it, increase airflow, open the lid for an hour a day.
• Moss or liverwort: remove with tweezers before it spreads.
• Nothing after four months: the spores were unripe, or the surface dried out. Some species, including hard fern and many shield ferns, germinate best after a cold spell — leave the sown pot in a cold frame over winter and check again in spring.

Good species to start with include hart's tongue fern, male fern, lady fern and common polypody, all of which germinate readily from fresh spore. Once you have raised your first batch, a shaded border, a north-facing wall or a row of pots under a hedge becomes something you can fill for nothing at all.