Elizabeth McCorquodale explains an age old method of heating up the
soil to give your plants a great start in life, instructions on how to
make your own hotbed from scratch and how to get great results
throughout the year.
See also:
Straw Bale Gardening
How to Protect Crops over Winter
HOTBEDS OF HISTORY
Manure-fuelled hotbeds have been around since the Egyptians (who used
them to incubate eggs as well as to grow plants), and they have never
really gone out of fashion. Mad Roman emperor Tiberius grew cucumbers in
wheeled hotbeds, and the heat generating ability of manure was well
understood by the ancient Greeks. The Moors of Southern Europe raised
seedlings in small boxes filled with donkey manure, and by the Dark Ages
the practice had spread to the monastery gardens of England. The heyday
of hotbeds, however, really began in the 18th century with the need to
cosset the precious new seeds and cuttings that came flooding in from
the New World, and culminated in the Victorian era, when whole sections
of walled kitchen gardens were set aside for hotbeds tended by armies of
gardeners dedicated to supplying out-of-season vegetables for the
kitchen of the big house.
Traditionally, hotbeds were heated by fermenting waste – usually
horse manure – but in the 18th century, oak bark, which was left over
from the process of tanning leather, was ground up and then piled into
frames. The heat generated from this tanners’ bark lasted for a good six
months. With the technological leap of the Industrial Revolution, steam
and hot-water pipes came into vogue in the grand gardens, then in the
early 1900s electricity took over, and greenhouse hotbeds and frames
were heated by cables laid snake-like in the soil.
Properly made, a manure-powered hotbed will supply free, consistent
warmth for two months, and then provide a rich, moist bed for cucumbers,
courgettes, squash or melons for the remainder of the summer. For
starting seedlings off in spring, for striking cuttings, and for
furnishing a warm bed for tender plants, a hotbed is the perfect
ecological solution.
THE FUNDAMENTALS OF DESIGN
The basic design of any hotbed is a shallow growing frame placed on
top of a fermenting pile of straw and fresh manure. As anybody who has
ever turned a compost heap knows, fermenting waste generates heat.
Manure is a storehouse of heat energy, and the trick isn’t just in the
generation, but in the maintenance of heat; anybody can build a hotbed,
but the knack is knowing how to keep it cooking.
Manure on its own will burn out in a couple of weeks, but
if you mix in an equal amount of straw, by volume, the release of the
stored energy will be eked out over two months rather than two weeks,
providing a regular, gentle heat that will continue until the warmth of
the spring sun takes over.
Hotbeds can be built above or below ground, and can be sited either
in a sheltered spot in the garden, or inside your greenhouse or
polytunnel. A sunken pit takes advantage of the extra insulation
provided by the soil by minimising radiant heat loss, but the risk of
waterlogging in the pit makes the above-ground model a better option in
many cases. A sheltered spot against a south-facing wall or in the lee
of a fence or hedge will help to regulate the temperature of outdoor
frames, and provide protection from the worst of the wind and the
weather. More satisfactory still – if space permits – is to site the
frame in your greenhouse, where you will gain the extra advantage of any
lost heat from your hotbed raising the ambient temperature in the
house.
Hotbeds, like compost heaps, do not come in one particular shape or
size, and, like compost heaps, the fundamentals must be understood for
the whole thing to work. The real essential to grasp is that the whole
project hinges on how you treat your manure.
BUILDING THE FRAME AND THE BASE
Building a hotbed can be broken down into three parts: building the
frame and the base, preparing the manure, and finally, assembling the
hotbed.
To the cost-conscious home farmer the overall size of the hotbed will
probably be determined by the size of available timber, glazing
materials, and space. The front of the frame should be several inches
lower than the back to shed the rain and make best use of the available
sunlight, and the lid should be hinged at the back of the frame to allow
easy access. The simplest lid is made from old wooden windows, but a
more practical, though less picturesque solution, is polycarbonate
roofing sheets fitted into a lightweight frame.
The manure and straw mixture can be contained within a timber or
brick support, or left free-standing. If left free-standing, the sides
should be gently sloped and covered with something to prevent heat loss,
erosion in heavy rain, or damage by being kicked while you are tending
your plants. Inverted grass turves are the ideal solution, but sacking
or old feed bags would do the job at a pinch. The minimum depth for the
finished pile should be 60cm (2ft), but the optimum is 90cm (3ft), and
the minimum length and width should be 60cm x 90cm (2ft x 3ft) in order
to maintain an even temperature. Traditionally, the manure pile, whether
free-standing or encased in a base of brick or timber, was 30–46cm
(12–18in) larger all round than the frame which sat on top of it, in
order to ensure an even temperature within the frame. However, in a
small greenhouse or garden this would be a luxury, so build it a scant
15cm (6in) wider, and keep an eye on the plants that are growing in the
cooler regions around the edges. Just keep in mind that you will need to
reach all of your plants, so don’t make the frame too large.
THE IMPORTANT BIT: PREPARING THE MANURE
This part of the process takes nine days and will result in a mixture
that is ready to put into the base or make into a free-standing pile.
To begin with, assemble an adequate quantity of old straw and fresh
horse manure, the fresher the better, and using a garden fork mix the
two together well, breaking up any clumps and incorporating as much air
into the mix as you can. Sprinkle the mix with water, fork it into a
neat pile, and leave it for three days to begin fermentation.
On the third day turn the heap, again incorporating as much air as
possible, and if it is dry, sprinkle it with water. To achieve an even
heat it is important that the straw and manure is evenly distributed and
any clumps removed. Again, leave it in a pile to cook for three days,
and on the sixth day, turn the heap again.
Perfectly cooked straw and manure.
On the ninth day, repeat the process and then fork the fermenting
mixture into its final position, adding it in 15cm (6in) layers and
settling it firmly into place. As air is a key ingredient in the
fermentation process, the pile must be firm, but shouldn’t be overly
compacted. Once the base has been filled (or the free-standing pile has
been fashioned), leave it to heat up for three or four days. Insert a
stick into the centre of the pile and leave it there to act as an
improvised thermometer. To begin with, the temperature will rise
sharply, but it will then fall and level out to provide a gentle,
consistent heat. Don’t be tempted to finish your hotbed before this
heating and cooling process is complete, as the very high temperature
will damage the chemistry of the soil if it is added too early. Check
the temperature by extracting the stick and holding it: your bed is
ready for use when the stick can be held comfortably in your hand.
Now is the time to lift the frame into place and fill it with soil.
Keep an eye on the hotbed for the first few days to see if steam is
still escaping from the pile. If it is, leave the lid open to allow it
to escape and wait until all the steam has dissipated before planting.
For the first year, the soil you use can be any good peat-free cutting
or seedling mix laid on top of the fresh manure to a depth of 15cm
(6in). After that, the soil can be made up of the nutritious remains of
the previous year’s manure, well mixed with the same quantity of garden
loam and riddled to a fine tilth.
Put your plants to bed by closing the lid and laying an old curtain
or sacking across the glazing to stop all the precious heat from
radiating out each night; if tacked to a piece of lath, the curtain can
be rolled up easily each morning. During the day, regulate the
temperature in the frame by opening and closing the lid, just as you
would a cold frame.
CHOOSING YOUR PLANTS
Long-season vegetables like pumpkins, squash and melons will
appreciate the extra growing time if they are planted early and allowed
to establish a good root system before being transplanted out into the
garden. Warm-climate vegetables like sweet potatoes will repay the extra
warmth and early start with a healthy, heavy crop. Strawberries, too,
are good hotbed crops, as they grow quick and sweet in the heat.
Hotbeds are terrific for striking cuttings, as the bottom heat is
exactly what they need to quickly grow roots and be ready for planting
out in a seedbed in late spring to grow on over the summer. Gooseberry,
raspberry and other bush fruit and herb cuttings will appreciate the
addition of sharp sand mixed into the soil to increase drainage.
Many flower and vegetable seeds fancy a bit of warmth to get them
going, and the gentle heat of a hotbed is ideal. The seeds of all the
cucurbits will appreciate a warm bed, as will tomatoes, peppers and
impatiens. Beware though, as some cold-weather plants such as lettuce,
carrots, delphiniums and geraniums will demonstrate very poor
germination if planted in soil which is too warm.
Summer-built outdoor hotbeds can be used to extend the growing season
for any quick-growing plants that are day-neutral. Baby beets, pak
choi, fennel, kohlrabi, dumpy carrots, rocket, turnips, lamb’s lettuce
and land cress can all be started in late August, September, or even
October, and will keep on cropping in the heat and protection of a
hotbed. Cool-weather crops may bolt or grow woody if they get too hot,
so be ready to harvest crops like radishes and spinach while they are
young.
Within the extra protection of a polytunnel or large greenhouse,
bumper crops of sweet potatoes and melons can be grown in pit hotbeds
glazed with polythene sheeting stretched across a frame, rather than
with glass or rigid plastic lids.
Your hotbed can also be used to grow plants in pots. Rather than
planting directly into the soil, it is possible to sink planted pots in
up to their rims, giving you the advantage of being able to lift plants
without disturbing their roots. The exotic, highly esteemed pineapple
was grown in this way in Victorian hothouses so that they could be
lifted and the manure or tanners’ bark replaced and revitalised several
times over their long growing season.
The result of the simple chemistry of straw and manure mixed and
piled into a heap is a magical addition to the armoury of any home
farmer. It gives us some small control over the weather at seed planting
time, and extends the range of vegetables, fruit, and even flowers that
we are able to grow. It is easy to see why hotbeds have stood the test
of time.
https://homefarmer.co.uk/how-to-make-your-own-hotbed
