Working out the ideal temperature and environmental conditions for a fern collection can be confusing. This is my journey as a ‘fern newbie’ trying to make sense of the terminology surrounding temperature and greenhouses.
It was clear I would need to consider other possibilities. You see, my interest in keeping tropical ferns had swelled to the point where I could no longer keep them all inside the house. There were too many. Ferns everywhere! I came to the realisation I would need to start reclaiming some real estate from the garden and invest in some new fern-holding structures.
I had so many questions—I was never wired particularly well to cope with a ‘trial and error’ approach that so many gardeners seem blessed with. Looking in the obvious places, I struggled to find a comprehensive ‘how to’ on growing tropical ferns that could push me in the right direction. In houseplant books from the 1970s and 1980s, there was the odd page on bird’s nests or maidenhairs alongside explanations on how to build a moss pole or bottle terrarium. But I was thinking bigger. I had grand notions of
something approaching Kew’s glasshouses in my back garden… fast forward a year or two and I had acquired a 180 cm × 60 cm wooden-framed greenhouse. Progress!
Tim Penrose, of Bowden’s, put me in touch with Dick Hayward—this was way before I had discovered the BPS. Dick kindly wrote to me about how he constructed and maintained his own greenhouse. His practical advice and positive encouragement inspired me further on my tropical and tender fern journey. My metamorphosis into a ‘fernaholic’ had truly begun.
My search for information led me to a few antique fern books. These books
seemed very exotic, they listed so many ferns with names I hadn’t heard—
many of which I have since learnt were synonyms of more familiar species.
I was discovering the quite formidable history of tropical fern horticulture.
I’d heard of pteridomania, fern-fever and the Victorian fern craze. I’d even
heard of Wardian cases on Tim Harford’s BBC World Service series 50
Things That Made The Modern Economy. I realised my own endeavours
and ferny dreams were following on from this rich history.
Indoor and Tropical Fern Group Winter Newsletter 2021 13 December 2021
My first greenhouse
that I keep as a 'hot
cupboard’.
14
British Pteridological Society A. J. Macself
One of these books I had acquired was Ferns for Garden and
Greenhouse (1952) by A. J. Macself. In the half of the book
devoted to non-hardy ferns, Macself neatly arranges his
chapters by the environments in which we can keep ferns. He
goes through unheated greenhouses, Wardian cases, keeping
ferns in indoor rooms, heated greenhouses and hothouses.
Macself favours a less is more approach when it comes to
heating greenhouses.
“Among ferns which cannot survive actual freezing … there are
sufficient of those which grow happily in moderately heated
greenhouse to render it unnecessary for the average amateur to
attempt to grow those which must have hothouse accommodation
or ‘stove’ houses as they’re formerly termed. I repeat that
experience has taught us that very high temperatures maintained by
excessive firing are not equivalent to natural heat, and many ferns as well as
other plants have been proved to retain more robust health in cool
greenhouse conditions than they did in the very high temperature, but
oppressively stuffy atmosphere, of a
hothouse.”
The chapter on Wardian cases
reminds me that they are the
forerunners to what we would now
refer to as terrariums. As a listener to
BBC Radio 4’s Gardeners’ Question
Time, it seems rarely a week goes by
without references to terrariums as a
creative approach to indoor
gardening. Macself considers how the
temperature of the room in which the
Wardian case is placed will affect the
ferns that can be grown. He also
predicts:
“My belief is that the availability of cases will be the means of inducing great
numbers of amateur gardeners to start cultivating filmy ferns and other
suitable plants and that the demand for cases will grow in consequence. In
years past it was not unusual for the enthusiast who started with a single
Wardian case to add periodically to the number until he possessed half
dozen or more.”
Hands up. Guilty. My kitchen windowsill currently looks like some weird
sci-fi laboratory with my collection of mini terrariums housing my more
filmier ferns.
Indoor and Tropical Fern Group Winter Newsletter 2021 13 December 2021
AJ Macself, Ferns For
Garden And
Greenhouse (1952)
My kitchen windowsill
and a collection of
terrariums containing
filmy and aquatic fern
species.
15
British Pteridological Society Macself gives us a chapter on growing ferns inside the
house, aimed chiefly at those who do not own a
greenhouse. He identifies the fact that maidenhairs are
generally speaking not good ‘room’ plants, but identifies
Nephrolepis exaltata, Asplenium bulbiferum and
Davallia canariensis as being ideal candidates.
Macself defines ‘tender’ to differentiate between ‘ferns
which cannot survive in frozen conditions and those
which can endure and live through the frosts of the
average British winter.’ Tender ferns then divide into two
cultivation groups: ‘greenhouse’ and ‘hothouse’ ferns. In
the greenhouse sufficient heat is required to maintain the
temperature safely above freezing. In the hothouse
considerably more heat would be assumed.
Under the banner of greenhouse ferns, Macself gives an
extensive list of ferns, including many well-known varieties
of Adiantum, Davallia, Doodia, Nephrolepis and Pellaea, tree ferns such
as Alsophila and Dicksonia, and now less familiar genera such as
Arthropteris and Gleichenia.
Cultivation Temperatures
In 1952, ‘hothouse’ was the modern word for ‘stove’, which Macself
informs us was the general term used by gardeners of the last century.
John Birkenhead in his little book Ferns and Fern Culture (1892) devotes
a chapter to ‘Selections of Stove Ferns for Pots, Baskets, Blocks of Cork,
Rockwork, Walls, Cutting’ and another for ‘Stove Selaginellas’. Macself
considers a hothouse to be a greenhouse maintained at a temperature of
18 ºC (65 ºF). He also says one maintained at 10 ºC (50 ºF) would be
termed a warm greenhouse and that a cool greenhouse would go no lower
than 4–7 ºC (40–45 ºF). In my opinion this is quite a neat terminology.
13 December 2021
My recently divided
Nephrolepis exalata
‘Green Lady’. It lives
on a sunny windowsill
in a bathroom, 21 ºC
and has been in this
position for 4 years.
GREENHOUSE TEMPERATURE CATEGORIES
Unheated Greenhouse no artificial heat
Cool Greenhouse no lower than 4 ºC (40 ºF)
Warm Greenhouse no lower than 10 ºC (50 ºF)
Hothouse no lower than 18 ºC (65 ºF)
Philip Swindles, in his identically titled book Ferns For Garden and
Greenhouse (1971), also uses the cool and warm greenhouse distinction.
He again identifies a cool greenhouse as one in which artificial heat is not
normally given except in the depths of winter to ‘keep the frost at bay’. His
warm greenhouse descriptor says a a temperature no lower than 7 ºC
(45 ºF), a little lower than Macself writes in 1952. Swindles doesn’t
Indoor and Tropical Fern Group Winter Newsletter 2021 16
British Pteridological Society mention hothouses, but does list several ferns as candidates for the warm
greenhouse that are commonly considered to require higher temperatures.
John Birkenhead uses the terms stove, warm greenhouse, cool greenhouse
and cold greenhouse. He provides more elaborate seasonal descriptors for
temperature ranges. He gives the minimum nighttime temperature for the
stove ferns to be 16 ºC (60 ºF) during the winter. Warm greenhouse as
7 ºC (45 ºF). Cool greenhouse 4 ºC (40 ºF) but states 2 ºC (35 ºF) would
‘not do any harm’. Cold greenhouse was his term for unheated greenhouse.
Macself lists relatively few hothouse ferns. His list includes more
Adiantum, then a selection of a few other ferns including Asplenium and
Davallia. He singles out Platycerium grande as preferring hothouse
conditions. His post-war diligence, is perhaps the leading factor in the lack
of ferns he lists in this category. His comments resonate with us today in
light of soaring energy prices and climate change.
“We have in fact learned, to a
great extent through
compulsory reduction in fuel
consumption, that many plants
thrive equally well and maintain
even more vigour and
robustness when grown in
moderately heated structures
than was formerly the custom.”
My Set-Up
In addition to the ferns left in
the house and being kept in
mini terrariums on windowsills,
my own set-up includes my
wooden-framed greenhouse
(180 cm × 60cm), that I
maintain at 18 ºC as a hothouse
—more of a hot cupboard. I
initially tried keeping it at 10 ºC
but, as winter set in, very
quickly the majority of the ferns
began to suffer, so I opted for a higher temperature using a 2 kW electric
fan heater and a thermostatic controller. I still find it very confusing
navigating what is winter dormancy and deciduousness against genuine
cold stress. I also have an unheated greenhouse (300 cm × 180 cm), which
is bubbled-wrapped. I’m trying to maintain it as a cool greenhouse and
praying the nights don’t get too cold—I’m hoping to keep it above freezing
without artificial heat. The humidity in the hothouse and unheated
greenhouse is always above 80% RH. In the hothouse this is aided by a
13 December 2021
Bubble-wrapped for
winter. My cool
greenhouse
containing tender
species that have
spent the summer
outdoors.
Indoor and Tropical Fern Group Winter Newsletter 2021 17
British Pteridological Society small ultrasonic humidifier and the plants stand on trays
with expanded clay balls which are kept wet.
My home keeps a fairly stable 21 ºC across the year, but
the humidity is rarely above 45% RH. Attempts at
artificially raising the humidity in the house were pretty
futile. The house is largely open plan, and the 6 l tank
that came with the humidifier needed to be filled almost
daily and soon there was a white chalky deposit on every
surface across the house—including the plants. So the
humidifier had to be aborted. Nevertheless, my
Nephrolepis and Davallia grow happily across the year in
these conditions. My living room also has seven huge
IKEA plastic tubs functioning as ‘temporary’ closed
terrariums containing a collection of more unusual
tropicals.
My fern list has quadrupled since Covid began and things
are currently too crowded. My set-up is neither practical nor elegant. I’ve
also struggled to specialise in one particularly genus or group up to this
point. So I’ve taken the plunge and have invested in a new, doubled-glazed,
lean-to hothouse and on completion I will move things around and keep
the current hothouse as a warm greenhouse. Progress is slower than
anticipated, due to Covid and Brexit supply issues, but fingers crossed it
will be completed in January before the worst of the winter weather.
Hopefully I can share the new hothouse with you in the summer edition of
this newsletter.
Tropicals Outside
Due to the space limitations in my greenhouses, I am often trying ferns
outside that would normally be considered tropicals. I’ve good results
keeping a few Lepisorus species (L. microphyllus, syn. Lemmaphyllum
microphyllum, and L. ovatus, syn. Neolepisorus) outside this year from
May. I am keeping them in the cool greenhouse over the winter to see how
they fare. These are two ferns that Dick is keeping indoors in his
terrariums. Similarly several of my Nephrolepis have been outside all
summer and I have only just brought them back into the house now. My
Aglaomorpha coronans has faired less well outside over the summer, but
is hanging on in there. I know of someone keeping their A. coronans
outside with only minimal protection over winter, but they are further
south than me and in a milder climate. Mine will overwinter in the cool
greenhouse.
The BPS on-going fern hardiness survey is revealing fascinating insights
into, and also questions about, some fern species, once-thought tender,
that some of us are managing to maintain outside in the UK. If you are a
BPS member and have not contributed to the survey, you can find the
Indoor and Tropical Fern Group Winter Newsletter 2021 13 December 2021
IKEA tubs serving as
temporary fern
nurseries whilst the
new hothouse is being
completed. Also some
Nephrolepis brought
indoors for the winter.
18
British Pteridological Society information on the BPS website.
From a tropical and indoor fern
perspective, we need to start
collecting data on the temperature
(and relative humidity and light
levels) in which we are keeping our
collections. Perhaps breaking this
down further with summer and
winter figures. Collecting and
collating our tried and tested
temperature and humidity
parameters tropical and tender
species would certainly be a
resource to develop and a future
project for our group.
Final Thoughts
Very little of what I’ve written here will be new to the more experienced
growers and collectors in our group, but hopefully it is a reminder that, for
a fledgeling fern grower, it can be very hard to find practical advice on
growing indoor ferns and creating a greenhouse to keep tropical ferns.
Indoor ferns have developed a reputation for being tricky, possibly due the
lack of information or the generalised guidance out there. Finding
information on the horticulture of rarer ferns is even harder to piece
together.
The tropical and indoor fern group is looking forward to working with the
BPS to develop its resources for those interested in growing tropical and
tender species. Gaining data on greenhouse temperatures, humidity and
even light levels across the seasons and collating details of the species that
thrive in these conditions would help us all.
MATTHEW REEVE
Matthew is an amateur
pteridomaniac with a passion
of tropical, tender and aquatic
ferns. He works in theatre as
a musical director and
composer. He recently joined
the BPS committee and co-
leads the Tropical and Indoor
Fern Group with Peter Blake.
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