Growing ferns from spores
Shropshire Flora Group Newsletter 7 - Autumn 1998 - pages 7-10
"Propagation by spores is the most interesting of all means of increasing the stock of plants, and it is very wonderful from first to last" - Birkenhead, 1920.
Growing ferns from spores is remarkably easy. The only really important thing to remember to do is to sterilise the soil prior to sowing. Broad Buckler-fern, Dryopteris dilatata, Male Ferns, D. filix-mas & D. affinis, and Lady Fern, Athyrium filix-femina, will grow with even the most scant of tending. Believe me this is easy. It is also astounding!
The fern life cycle
The life cycle of ferns was first elucidated in the West Indies by a surgeon called John Lindsay in 1794 (Ford 1991). The plants show alternation of generations. The familiar fern shaped things we see and struggle to identify in the field is the sporophyte. A sporophyte (Fig 3) produces spores; the spores germinate to form a threadlike gametophyte which initially looks like green fuzz, without high power magnification. The gametophyte grows to be a small (maximum of 1 cm diameter), prostrate, heart-shaped and almost translucent plant. You can find gametophytes in the field once you know what to look for. Well decayed logs are good places to search. The mature gametophytes (Fig 1) produce male and female sex cells. These fuse to form a further sporophyte, which grows from the underside of the gametophyte. Thus the sporophyte is initially dependent on the gametophyte (Fig 2).
The structures on sporophytes that produce the spores show a bewildering variation in design. In most of the British ferns spores are produced on the backs of fronds in discrete collections of structures called sori. Anybody who has ever tried to identify a fern in the field will no doubt be familiar with these! The sori are composed of a myriad of tiny structures called sporangia, and it is in these that the spores are produced. Frequently a thin flap of tissue, the indusium covers the sporangia.
When the spores are ripe the indusium dries and is pushed up and the sporangia are then open to the air. The mechanism that releases the spores from the sporangia is rather dramatic and worth reading up on, or better still observing with a good x10 lens.
Firstly a legal point. You must have permission from the landowner when collecting spores in the wild. Spores should be collected only when ripe. This is conveniently signified by a change in appearance of the sporangia. Any indusium will lift, hinge, shrivel or otherwise open. The sporangia will change colour, from green to various shades of yellow, brown or black. This is the time to collect the spores. The table gives the sporing times of some of the more common native ferns and the appearance of the sporangia when ripe. Any of the standard fern identification books will tell you what colour to look for in other species. Pinching off a small section of fertile frond and placing your booty in a spore proof packet is the best way of collecting spores. Don't forget to label the packet with the species name. Envelopes make quite reasonable spore-proof packets. The variety that open on the shortest side are best and should be folded to make a seam once closed. The packets should be stored in a warm dry place for a few days then inspected. If you have chosen your pinna wisely a fine dust of spores will have accumulated at the bottom of your packet. If no spores appear to have collected try flicking the packet. This may dislodge spores resting on the fern fragment. If flicking brings no joy then reseal the packet and leave for a few more days.
Some texts recommend taking home an entire frond and leaving it to shed its spores onto a sheet of newspaper. This is wasteful. A single large frond of Scaly Male Fern, Dryopteris affinis, may produce well in excess of 1,000,000 spores and it is not unusual for 90% of them to germinate. It can also be rather messy and inconvenient. A single pinna of any of the bigger ferns is more than adequate. With the Spleenworts, Asplenium spp., half a frond will be enough. Spores collected in this way and stored in dry conditions can be viable for years but for best results should be sown as soon as possible after collecting.
Table: Requirements for cultivating some common species. The appearance of the mature spores, together with the best time to find them, the properties of the growing medium and the maximum light levels under which they will thrive.
Sow the spores thinly onto the surface of rather damp soil. There are a few important points to keep in mind when sowing spores:
1. You must sterilise the growing medium or you will have problems of competition from algae and mosses. This can be done with boiling water or in a microwave. If you use a microwave you need to be really sure there are no stones in the soil as they may explode in your microwave. The growth medium should be fitted to the needs of the species.
2. For all species you need to maintain reasonably high humidity. This is best done by sowing into 5 cm pots and then sealing in plastic bags.
3. Don't sow the spores too thickly. If you do the gametophytes that develop will be slower to grow and will only develop male sex cells.
4. Most species need good but not direct light. I have killed gametophytes with cold, flooding and drought but not with heat (yet!). They grow much faster in the warm (l5º C).
5. In the unlikely event that you get no results from a pot, do not throw it out! There is increasing evidence that like seeds, fern spores have dormancy mechanisms (Sheffield, 1996). Unfortunately almost nothing is known about the extent or nature of these. I have found growth starting in pots 8 months after sowing. Your best bet is to experiment. Move the pots to a different aspect, add more water, let them dry out, just have fun finding out.
6. If you fancy having a go at propagating some exotic fern you have in your house a good guiding rule is that spores should be sown into a slightly damper version of the environment that the parent plant has done so well in.
What will happen next?
The spores will start to germinate within a week. Within 3 weeks the surface of the growing medium will be covered in a green fuzz and look for all the world like a rather small billiards table. From this fuzz the characteristic heart shaped gametophytes will slowly emerge. Depending on the species, time of year and growing temperature, tiny sporophytes will appear within about one year. Some slow-growing species may take two years to produce sporophytes. I have found that the best survival at potting on is achieved if you wait for the sporophytes to be about 3-4 cm high. If you wait any longer the sporophytes in your sowing pot will compete with each other and self-thin. Any sooner and the plants are too fragile to survive handling and root disturbance. If anybody reading this has any good ideas on transplanting gametophytes I would like to hear them... Your fern should be identifiable within a year and a half or so. Do not be surprised if you find strangers in your carefully labelled pots. Spores are minute and very readily transferred from one pot to another during sowing.
Further reading and references
Birkenhead, J. 1920. Ferns and fern culture. 3rd edition. H.B. May & sons, Upper Edmonton.
Ford, M.V. 1991. A brief history of ferns and their cultivation. In Camus, J.M. (ed), The history of British Pteridology, British Pteridological Society, London.
Page, C. 1997. The ferns of Britain and Ireland, 2nd edition, Cambridge University Press, Cambridge.
Sheffield, E. 1996. From pteridophyte spore to sporophyte in the natural environment. In Camus, J.M.. Gibby, M. & Johns, J. (eds.), Pteridology in perspective, Royal Botanical Gardens, Kew.