Special edition: Ferns 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.

THE NEWSLETTER OF THE SHADE AND WOODLAND PLANTS GROUP December 2019

Asplenium scolopendrium (Crispum Group)

'Crispum Bolton's Nobile'

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 yellow or orange and no indusium

Adiantum bears sori at the edge of 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.