Ferns certainly are very odd plants with a unique life cycle. Unlike most other plants we know, they bear neither flowers nor seeds. Instead, they mainly reproduce by spores.
Easier to Grow Than You’d Think
Few seed companies offer fern spores, probably because they’re convinced that growing ferns from spores is beyond the capacity of the average home gardener. But I beg to differ. I’ve grown all sorts of ferns from spores, starting when I was a child, and I’ve never found them all that difficult to grow.
What is true though is that growing ferns from spores takes a while, largely because the spore gives birth not to a fern, but to a preliminary life stage, the prothallus, and it is only after fecundation that a new fern is produced.
In the wild, it can take up to 4 or 5 years for a fern to reach its full size. However, patient gardeners will discover that it’s possible to grow most garden ferns to a useable size in only a year or two if you start them indoors, as they’ll be able to grow all year long rather than having to stop for a long winter’s rest, plus you’ll be offering them better conditions than in the wild (especially, not competition), so they are essentially able to put on 2- or 3-year’s growth in just one season. In most cases, if you sow fern spores in the spring, you’ll have a small fern ready to plant out the following spring.
The same goes for the tropical ferns grown as houseplants: most will be small but useable plants after only one year.
A Two-Phase Life Cycle
Ferns undergo a two-phase life cycle called the alternation of generations.
Spores are produced on the fern’s fronds (frond is the term used for a fern leaf). Mostly they appear underneath the frond, but some species have separate “fertile fronds” (spore-bearing fronds) that are physically quite different from the others.
What you see under a typical frond are the sori (singular sorus), also called or spore cases, that contain the spores. They usually look like small bumps or lines, usually placed in a symmetrical pattern, and are often green at first, becoming brown or golden at maturity.
Inside the sori are clusters of sporangia (spore-producing cells).
The spores themselves, tiny to the point of being virtually invisible, are very light and usually carried by the wind. They are haploid (they contain half the number of chromosomes of an adult fern).
Harvesting Spores
If you can’t easily find commercial sources of fern spores (although, if you look on the Internet, you ought to be able to find a supplier), it’s easy enough to harvest spores from wild ferns or houseplant ferns. Here’s how:
Sterile Ferns
The vast majority of ferns produce fertile spores… but at least two commonly grown houseplant ferns do not. Both the Boston fern (Nephrolepis exaltata ‘Bostoniensis’) and its many variants, and also the crested elkhorn fern (Polypodium punctatum ‘Grandiceps’ produce deformed spores that aren’t viable, so there is no use harvesting and trying to sow them. Instead you can multiply sterile ferns from division or offsets.
When sori of a fern show their maturity by turning brown, cut a frond or part of a frond and slip it into a white envelope. After 2 or 3 days, you should see brown dust contrasting with the white paper inside the envelope: these are spores. You can sow them immediately (ferns that have green spores, especially, have a short “shelf life” and should be sown without delay) or store them until you are ready to sow them. Most can be stored for up to a year if you keep them cool and dry.
Sowing Spores
Sowing fern spores is not very different from the method used by most gardeners to start fine seeds indoors. There is one difference, though, and that is that fern seedlings are highly sensitive to contaminants (fungi, mold, moss, etc.). It is therefore wise to sterilize everything thoroughly before starting.
To sterilize soil, mix it with water to dampen it thoroughly and place it in a plastic sandwich bag. Put it in the microwave oven without sealing the bag. Set the oven to maximum for 2 to 3 minutes. If no steam starts to condense on the inside of the bag, try a few minutes more. When the bag does steam up, take it out of the oven and seal it. Steam (which is actually the soil sterilization agent) will continue to form for several minutes. Let it cool to room temperature before you use the soil.
While you wait for your potting mix to cool down, sterilize all pots and tools by pouring boiling water over them.
When the potting mix reaches room temperature, spoon it into the pot you chose and smooth out the soil surface.
Broadcast the spores over the surface of the potting mix by holding the envelope so its top is pointing slightly downward, then gently tapping the envelope as you move it back and forth. That should spread the spores out nicely. Don’t cover them with soil: they need light to germinate.
Now place the pot inside a clear plastic bag (it will serve as a mini-greenhouse) and seal it shut. Move it to a warm, modestly well-lit spot, but with no direct sun, perhaps under a fluorescent lamp.
After a few weeks or months, small green growths, generally translucent, heart-shaped and looking a lot like liverworts or mosses, will form on the surface of the potting mix. They’re called prothalli (singular prothallus or prothallium) and are the sexual phase of the fern’s life cycle. They too are haploid, with half the number of chromosomes of an adult fern.
Fern Sex (readers 10 years and younger should be accompanied by an adult!)
Each prothallus produces many haploid male gametes (sperm) and one haploid female gamete (egg). (Okay, a few ferns produce a unisexual prothallus, but that doesn’t change our story much). The prothallus’ male gametes are mobile, but need the presence of a thin film of water to swim to the female gamete of another prothallus, hence the importance of maintaining high humidity inside the bag. When the haploid male gamete finds a haploid female gamete, fecundation takes place, forming a diploid cell, that is, one with a full complement of chromosomes. What was half is now whole and the alternation of generations is complete!
(Possibly there is a small moment of ecstasy as the two cells join together, but I’m not sure that’s been studied.)
After another few weeks, a small diploid sporeling (baby fern) will become visible, first with just one small frond, then another and larger one, then another, etc. The prothallus will soon fade away, leaving you with a small but independent fern!
When the plant reaches this stage, it’s time to acclimate it to outside growing conditions. Do so by opening the plastic bag bit by bit over a period of a week or so the young fern can adapt to the lower humidity outside. Then you can remove the bag entirely.
From Baby to Big
From this point on, you’ll be maintaining your baby ferns (yes, you’re much more likely to have dozens than just one!) like any other seedling.
Offer them good light, decent air humidity, regular moisture (most ferns don’t like to dry out), a bit of fertilizer from time to time and, when the ferns start to crowd together, transplant them into individual pots. Their growth will accelerate considerably at this point and soon they’ll be big enough to plant outdoors (hardy ferns) or to grow as houseplants (tropical ferns).
There you go: growing ferns from spores is not that different from growing plants from seed. It’s even a fascinating project any gardener can handle. Pencil it into your busy gardening schedule for this season!