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Fern spores are the closest thing in gardening to dust that grows. Flick a ripe frond over a sheet of paper and you get a puff of fine brown powder; sow that powder on damp, sterile compost and a few months later you have a green film, then dozens of tiny fronds. It is slow, it costs almost nothing, and it is one of the most satisfying things you can do on a windowsill. Here is the whole process, from spotting the right moment to potting on your first proper fern.
Division gives you a few plants; spores can give you hundreds. A single mature fern releases enormous numbers of them, so a modest pinch of dust can fill a seed tray. Sporelings are also genetically varied, which means you may end up with a slightly different plant from the parent — interesting if you are growing several species side by side. For tree ferns and many rarities that never produce offsets, spores are the only practical route.
The trade-off is time. Expect several months before you see recognisable ferns and a year or more before they are garden-sized. If you want quick results, buy a plant. If you want a lot of plants, or you simply like watching it happen, read on.
If you want to be thorough, sterilise your own mix by microwaving damp compost until it steams, or by pouring boiling water over it and letting it drain and cool. Either works; both smell worse than you expect.
Ferns do not have flowers. They produce spores in sori — the dots, lines or patches on the underside of mature fronds. Depending on the species these may be brown, orange, black or green. You want them plump and darkening, but not yet split. Tap the frond gently: if a cloud of dust rises, you are too late and the spores have already gone. If the sori are still green and tight, you are too early.
Cut a frond or two and lay it flat between two sheets of paper, or slide it into a paper envelope. Leave it somewhere dry and warm, out of direct sun, for a few days to a week. The sori dry, split and release their spores, which collect as fine dust in the corner of the envelope. It looks remarkably like dirt, so label it immediately.
Only take fronds from plants you own, or with the landowner's permission. Never dig up ferns from the wild, and take no more than a frond or two from any established clump.
Fill your container, firm the compost gently and level the surface. Stand the pot in a tray of water until the surface is damp — watering from below now means you will not disturb anything later. Then sow. Tap the envelope so a thin dusting of spores falls across the surface. Thinner is better than thicker: an amount the size of a match head is plenty for a small pot. Do not cover the spores with compost; they need light to germinate.
Label with the species and the date, then cover the container with cling film, a clear lid or a plastic bag. Keep it at around 15–21°C in bright light but out of direct sun. Sun through plastic will cook the whole tray in an afternoon.
After sowing, water from below rather than above — a heavy spray will wash the spores into clumps. Keep the compost consistently damp but never waterlogged.
Within a few weeks you may see a green film across the surface, easily mistaken for moss or algae. This is the prothallus: a flat, heart-shaped gametophyte only a few millimetres across. It is the stage most people do not know exists, and it is where fertilisation happens. Sperm cells swim through a film of water to reach the eggs, so that thin layer of moisture is not optional — it is the whole point. Keep the humidity high and the surface damp.
Ventilate the container now and then. Warm, stagnant, saturated conditions invite mould, algae and liverwort. If you spot a fuzzy grey growth, open the lid and remove the affected patch.
Germination times vary hugely. Some spleenworts and shield ferns produce prothalli in a few weeks; others take several months, and a few stubborn species take the best part of a year. When fertilisation succeeds, the first true frond pushes up out of the prothallus — a miniature, unmistakable fern frond with a curled tip.
Once the prothalli are crowded and the first fronds are 1–2cm tall, move them on. Lift small clumps — prothallus, roots and all — with a teaspoon and space them across fresh sterile compost. Do not try to separate individual plants; the roots are far too fragile. Keep the new pots in high humidity for a few weeks, in a propagator or a loosely bagged tray, shaded from direct sun.
As new fronds appear and growth steadies, harden them off gradually: open the vents, then leave the lid off for longer each day. When each plant has a few fronds, pot it individually into a small pot of general-purpose compost with added perlite or grit. Most ferns want dappled shade and compost that stays moist but drains freely.
Sow two or three small pots at intervals rather than one big one, so a failure in one does not wipe out the batch. Spring and early summer sowings make the most of good light. Write the species and date on every label, and keep the pots somewhere you will actually look at them. Then leave them alone, mostly, and let the dust do its work. The first frond to unfurl will feel like a small event, and it is.
Photo: Bergadder / Pixabay
