THE NEWSLETTER OF THE SHADE AND WOODLAND PLANTS GROUP December 2019
Special edition: Ferns
Asplenium scolopendrium (Crispum Group) 'Crispum Bolton's Nobile'
Introduction Joe Sime
This year our special December issue is devoted to a superb article by Marian Goody on the structure, propagation and cultivation of ferns. No shade garden is complete without a good selection of ferns, We have quite a few as Wendy enjoys growing them from spores… much easier than it sounds. I hope that Marian’s article will inspire you to grow more of these beautiful plants. Fronds, Fernlets and Fancies
Marian Goody
I have long had an interest in ferns and have found that an understanding of fern structure helps with growing them, their propagation and identification. If you know what fern you have then you can find out the conditions it prefers and put it in the right place in the garden. Reg Kaye said in the preface to his book in 1968, 'Everyone recognises a fern when they see one', which as a generalisation I think is true, but does everyone know the parts of a fern? They say a picture tells 1000 words, so there are going to be lots of images in this article.
Ferns are grown in their garden for their large leaves - fronds. These are arranged in large shuttlecocks or arise on short rhizomes. The parts of a frond help in the identification of a particular species or variety. It's important to know how many times a frond is divided, the proportion of the frond that has leaf on it, the colour of the scales on the stipe and the shape of the reproductive parts underneath.
If you want to move a fern, it’s best to dig it up about April time, when the roots are starting to actively grow. This means that it has as long a growing season as possible to re-establish in the garden.If you knock out a potted fern during the growing season you should see greenish tips to the roots, indicating active growth. The older parts of the roots are black and often shiny, they aren't dead, just inactive. The roots are a thin and fibrous mass.
If you slice through a fern crown, you'll see how the bases of the stipes are joined together, the older frond bases towards the outside. Between the stipes the fibrous roots emerge. The frond bases, even those of previous years are green. I was fascinated that when the fern to the right was sliced in half, the green quickly faded to a mucky green- brown, presumably by oxidation. So it can be literally seen that previous years frond bases are 'still alive' and in fact are used as a method of propagation particularly for Asplenium scolopendrium cultivars. I have tried to excavate around a fern to find old frond bases, but in practice I think you really need to dig up the plant to find the right bits and then replant!
Frond morphology can indicate what genus and possibly what species and variety you are looking at. But to be absolutely sure of the genus you need to look at the underside of the frond. There are the spore bearing structures. These can be seen in embryonic form as soon as the frond has expanded fully, quite early in the season, and are still more or less visible when the spores have been shed. The exceptions to this are the genera Osmunda and Blechnum. The latter has dimorphic fronds. The fertile fronds are narrower and held erect, the infertile ones are broader and held at an angle, or flat after snow! Osmunda on the other hand has fertile sections of a frond. The British native Osmunda regalis has terminal portions which are fertile - green when the spores are ripe, whilst Osmunda claytoniana bears spores on a section of the mid rachis. Fern spores are the dispersal phase of the fern life cycle, they are tiny and dust like.
They are aggregated into sporangia which are grouped together as a sorus.
The sorus is usually covered by a thin membrane called an indusium.
It is the shape of the indusium that is often diagnostic in identifying what genus you are looking at.
One exception to this in the garden are Polypodiums, which have no indusium. There are still aggregations of sporangia, round or oval in shape and yellow or orange coloured when mature.
Polypodium has oval/circular sori, which are Adiantum bears sori at the edge of yellow or orange and no indusium pinnae, the indusium is attached to the edge of the frond
The genera illustrated above are those most commonly found in the British Isles, it is by no means comprehensive. By understanding the habitats they occupy in the wild, it gives a better idea of where to place them in the garden. Having said that some ferns tolerate a much wider range of growing conditions in the garden.
Blechnum has dimorphic fronds, Osmunda has fertile sections of the frond and these are diagnostic in their identification. Polypodium fronds are once or more pinnate, and are clearly identifiable in winter by the yellow/orange sporangia on the underside. Adiantum fronds are small and always look impossibly fragile as though they will wither, but can be remarkably tough. A marginal indusium confirms its identity. Native members of Asplenium can have entire to twice pinnate fronds, those most commonly found in the garden are entire maybe with a terminal crest. Some of the most common, tolerant and beautiful of the garden ferns are members of Dryopteris and Polystichum. Their fronds are usually twice or thrice pinnate. Until you know them, the general frond shape doesn't give away which is which. Flip over the frond and a circular indusium looking like a doughnut with a central point of attachment makes it a Polystichum. If there is what looks like a line across the doughnut it is a Dryopteris. With practise you can see this with the naked eye, otherwise a hand lens will open up a whole new world.
An understanding of fern morphology indicates propagation opportunities, and here they are outlined in order of ease. At least how easy I find them! Polypodium are some of the easiest, from sections of rhizome. Some Polystichum varieties produce bulbils, generally in autumn and winter. If you find plants of Dryopteris, Polystichum and Athyrium with multiple crowns they can be carefully split to produce new plants. Propagation by spores requires precise preparation and with care is highly successful. In general species come 'true' from spores, Dryopteris and Polystichum varieties close to type, Asplenium scolopendrium and Athyrium filix-femina produce lots of variation. The trickiest method is from frond bases, although those with the knack claim it’s the easiest and produces sizeable plants quickly!
Polypodium in Latin means many feet, and in the garden they creep around slowly by rhizome and eventually can form large clumps. The best time for propagation is whilst they are actively growing, which in the case of Polypodium is September to February. Cut off a few inches of rhizome with a couple of fronds on it. Carefully lift it to see if there are any roots already growing underneath. Pot it up in a gritty compost. One of the most difficult things is to stop the fronds from acting like sails and pulling it over, so pin the rhizome down and stake the fronds to stabilize it. Keep it in a moist shady place and it should root readily. Internodal pieces will also root but they take twice as long to get going.
Polystichums conveniently produce bulbils. From observation it varies from year to year both in timing and generosity. They can appear as late as December. I have successfully propagated the bulbils by detaching them from the frond and placing them on soil and covering in a poly bag.
However the take is quicker and more successful if you detach a frond and place it on a tray of gritty soil. (There is an HPS video by Sally Gregson showing this method). Trim the end of the pinnae if they extend beyond the tray. The trick is to keep the frond in contact with the soil, this can be done either by weighting it down with grit or even better a piece of glass which fits inside the tray. Keep watered in a shady place. After a few months you will see small fronds develop, this doesn't mean that roots have formed. Carefully check that the new plants are well rooted before separating from the rachis and potting up. Another option is to layer the frond. Polystichum x dycei generally produces a few bulbils towards the tip of the frond, which can be pegged down to develop roots.
If you have access to mature multiple crowned ferns, then crown division is possible. Polystichum, Dryopteris and Athyrium with age can lift themselves out of the ground on short trunks which can become unstable. So digging them up, breaking off crowns and replanting at lower depth rejuvenates the plant. If you can't break off a crown then use hand-forks side by side, one protecting the parent, the other to lever a crown off. Peel off the old dead frond bases from the crown and pot up quite low with just the tops of new croziers showing. Remember that roots emerge from between the fronds. Doing it earlier in the year gives a longer season for the new plant to establish, but it can be done between April and October. Keep it well watered in a shady sheltered position.
Earlier in this piece we talked about spores which is the start of sexual reproduction in ferns. The HPS offers spores in its Seed Distribution as does the British Pteridological Society - to its members and they are the source of many ferns that I grow. One of the aims of this article was to introduce you to the anatomy of ferns to help understand how to propagate and grow them in the garden. The next section has the most technical jargon in it. But I don’t want to put you off having a go at propagating from spores.
Sue Dockerill a few years ago wrote an excellent, erudite, jargon free article on ‘Growing ferns from spores, how, when, where and what’. It’s available on the HPS website at: http://www.hardy-plant.org.uk/docs/publications/journal/32b/p25.pdf. It’s so true of gardening that there is often more than one way to achieve the same thing.
If you are gathering spores yourself, keep an eye on the underside of fronds from the end of June onwards. When the indusium starts to take on a blue grey colour and it begins to lift, bulging black sporangia emerge, and it’s time to take a pinna of a frond and put it in a paper bag. As it dries the 'spine' or annulus of the sporangia dries and contracts, rupturing explosively. Spores are then flung out. It’s great fun to watch with a hand lens. If you are unsure as to when the spores are ripe then take a pinna every few days until you see the brownish dust of spores in the bag. I visited a friends’ garden and they very kindly said I could take a frond of a special Athyrium which had started to shed spores, it was next to Dryopteris cycadina and little did I know that I was sowing spores of both. 'Contamination' can occur when there are many ferns grown in together in a garden. For Polypodium the time spores mature is in December/January and the spores are yellow or orange.
The next thing is to prepare a 7cm pot of compost. Sometimes I put some pieces of perlite on the top. Fold up a paper towel so that it fits inside the top of the pot covering all the soil. Then pour boiling water onto the paper towel until really hot water is coming out of the bottom of the pot. This is to sterilise the soil. Let the pot cool and whilst you are waiting make up the labels. Ferns spores are tiny and need to be sown as thinly as you can. If you get spores from a seed/spore distribution scheme, they are likely to come in a piece of folded aluminium foil about an inch square. Open it up very carefully and shake most of them off onto a piece of paper! Take the paper towel off the compost, turn the foil over so that the surface with spores on it is face down, close to the compost and tap smartly once. This will release spores into the air, so try and sow different species and varieties at different times or different places. I then place the labelled pot in an unheated, lidded plastic propagator which has capillary matting soaked in boiled water.
Put it in a cool shady place, an unheated bedroom, or cold frame. From a couple of months onwards, what looks at first like a green haze will start to develop, which should turn scaly, these are fern prothalli. The reason that you have gone to all the trouble of sterilising the soil and keeping it in an enclosed propagator is that mosses and liverworts will also start to grow and outcompete the young prothalli if they are not killed by the sterilisation.
The prothalli are 1 cell thick and have no cuticle, so are very prone to drying out, hence they must be kept moist all the time in an enclosed propagator. On the underside of the prothalli the male and female organs are borne and the male antheridia swim on a film of water to fertilise the female organ. There is evidence that if the prothalli are too close together, then only male organs are produced, hence the need to sow as thinly as possible.
In the photo the spores were sown onto perlite. It does make it easy to patch out – by picking up a piece of perlite and putting it onto a pot of freshly sterilised compost in the same propagator. This is a way of spacing the prothalli out. The young leafy sporophytes, the result of fertilisation, which grow into the garden generation are just starting in the pot at the bottom left. When big enough and before pricking out, the young leafy ferns need ‘hardening off’ from the moist environment they have enjoyed. Prop up the lid of the propagator by an inch for a few days, then raise it further over a couple of weeks till the lid is off and the ferns are holding their own. Then they can be pricked out. Prick them out into pots larger than you usually would, or several into a large pot, it provides a more constant environment, less prone to over or under watering, and the young ferns take off better. It can take a couple of seasons before they are big enough to plant out in the garden. Ferns are vulnerable to vine weevil in pots. I apply nematodes in September to all my pots to control them.
The final method of propagation is by frond base, I haven't been successful with this method, I hope you fare better. The photo of the crown sliced through, near the beginning, shows what look to be old dead frond bases are actually green inside. I have tried to find suitable material by carefully excavating around a mature fern, but don't usually succeed. The other option is to dig up the plant and replant afterwards. The frond bases can be taken at any time of year, from the point of view of the donor plant it’s probably best done in spring. Gently pull downwards at the old bases to remove them. Cut off the brown wispy bits where it was attached, and trim the top part back until it shows green. Lay them on wet sand, some authors recommend upside down, the latest recipe I have it’s to lie them down so that both ends touch the sand. Label and put a poly bag over them, keep them in a warm moderately lit place. After a few months growths appear which develop into a tiny frond and single root. Using a needle slide them off and pot into a pot of sterilised compost, keep a poly bag over the top for moisture until growing well.
I've talked a lot about fern morphology and its application in propagation and the garden. I'd like to finish off with some photos of fern fronds showing the beauty and variation that they can add to shady places in the garden. I hope they enthuse you to seek them out to grow in your garden. The individual fronds are not to scale, so please check their heights and requirements before planting them.
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SHADE MONTHLY is compiled by Joe Sime and this web-friendly version was produced by Tony Bays.