The Cold Greenhouse: Heating a Greenhouse
In
climates where frost is severe or prolonged, heating is inevitable
unless the greenhouse is to remain empty through the winter. Where
winters are not severe however, it is worth looking at the limitations
of a cold greenhouse. From about mid-spring to late autumn, all the
plants that need cold greenhouse conditions thrive happily. From late
spring to early mid-autumn, even warm greenhouse plants will grow, so we
are left with that period of the year where prolonged frosts are
expected, that is from late autumn to early spring. At this time warm
greenhouse plants must be kept near to their minimum temperature
requirements or they will deteriorate rapidly and eventually die. Many
cool greenhouse plants will stand temperatures down to freezing or even
below for short periods, providing they are kept almost dry, but once
the roots start to freeze, then severe damage or death results. Plants
growing in this respect and in areas which regularly suffer cold
winters, the less hardy perennials, shrubs and climbers are best grown
in this way. Large pots and tubs can be lagged with glass fibre
(fibreglass), wood wool, dried bracken or any other insulating material
that can easily be secured with wire, plastic netting, hessian etc. This
is not of course practicable with smaller pots but these can be plunged
in peat or sand.
During an average winter in maritime areas of
the temperature zone, frosts occur mainly at night, the temperature
rising at least just above freezing by day. Periods of prolonged day and
night frost without daytime sunshine which will heat the greenhouse,
are fairly infrequent. For night frosts, particularly the radiation
kind, a surprising amount of protection can be given by covering pots
with paper or cloth. Plastic sheeting can be used but this encourages
unwanted condensation around and on the foliage. Two or three
thicknesses of newspaper or one or two sheets of thicker brown paper are
easiest to deal with. Cover the plants completely, paying particular
attention to the area between pots and the glass. For even better
insulation, the polystyrene (plastic foam) sheeting used as a lining for
wallpaper can be used, preferably over a sheet of newspaper to prevent
too much condensation. Remove each morning once the temperature has
risen above freezing and replace it in the afternoon before freezing
starts again. Although one can bring a surprising number of the not too
tender plants through the winter in this way, it is a chancy business
owing to the vagaries of the winter climate. To get the best of winter
sun and wind protection for a cold house, a lean-to against the south
wall of a house or free standing wall is ideal. There is also a partial
compromise in growing a few, favourite tender plants in one of the
commercial heated propagation cases.
Minimum Temperatures
Even
in the completely unheated greenhouse it is possible to have winter
interest by growing hardy plants. Several garden annuals and perennials
can be either specially grown or lifted from outside in autumn and
brought in to continue flowering after those outside have succumbed to
rough weather, winds and frosts. Evergreen foliage plants can also be
used and practically all hardy bulbs will flower ahead of their normal
times without heat.
After considering the limitations of the cold
greenhouse and seriously deciding to install heating, the following
should be considered. A wide and satisfying range of plants can be grown
if minimum of 7c can be maintained. However, it costs twice as much to
maintain 13c than 7c, and three times as much to keep the temperature at
16c. These temperature examples above are not arbitrary, but equal the
three grades of heating generally recognized today: 7c represents the
so-called cool greenhouse, 13c the intermediate and 16c the warm
greenhouse. Formerly greenhouses kept at minimum temperatures of 18c or
above were known as stoves. However, as the majority of tropical plants
we cultivate survive at 16c or even down to 13c, the expense of running a
stove house is no longer justified.
Conserving Heat
A
careful look at the greenhouse should be made to see that warmth will
not be wasted once the heater is installed. In maintaining a chosen
minimum temperature, the problem is supplying enough heat to allow for
losses through and round the glass, or other glazing material, and
whatever building is used. These heat losses are obviously greater when
it is cold outside, but particularly when it is cold and windy. The
wind, as it blows around the greenhouse, can suck away as much as 50% of
the heat, because the speed of the air moving across the glass is a
major factor.
It is obviously important to choose a site protected
as far as possible from the prevailing coldest winds of winter. If this
is not possible or the greenhouse is already in position, a windbreak
can be considered as long as it does not exclude any light or sun. A
hedge or open latticed fence is best and if sited at least three to four
times the height of the greenhouse away from it, the shading factor
will be minimized. Another reason for not putting the windbreak too
close to the greenhouse is the problem of turbulence. This is most
severe when the screen is a solid one such as is provided by a wall or a
close boarded fence. If there is a good wall, it is best to use it as
the back of a lean-to type, especially if one is lucky enough to have it
on the north side facing south. The lean-to structure is the easiest to
heat, as the wall will absorb and hold the sun's heat and continue to
release it slowly after dark.
The way a greenhouse is glazed can
also affect heat loss. Glazing can be dry, the glass merely sliding into
grooves as in the Dutch light structures or it can be secured in a more
airtight fashion with putty or a sealing compound or strip. Although
there is more heat loss around the edges of non-sealed glass, it is not
too significant unless it fits very badly. There is much to be said for
this method as it allows more air exchange with the outside and prevents
extreme stagnation of the atmosphere even when there is no other
ventilation. Double glazing reduces heat loss and is fairly easy to
carry out, either with double-paned glass, or using clear polythene
sheeting with single glass panes. This sheeting is used to line the
greenhouse inside, creating an air gap between the plastic and the
glass. There are snags, however. The polythene sheeting cuts out as much
as 15% of available sunlight, even when it is clean and it also acts as
a condensation collector and the water droplets cut out even more
light. During the colder months of the year, the polythene will probably
be continuously damp, providing ideal conditions for the formation of
green films of algae which will disastrously reduce available light. The
sheeting also reduces air exchange through glass overlaps and other
crevices and as a result the atmosphere will become more humid. At
temperatures less than 7c this can do more harm than good, if only by
encouraging the growth of grey mould. For these reasons it is now
recommended that only the side exposed to winds, but away from the sun,
should be lined. A special insulating curtaining composed of two sheets
of polythene sheeting sandwiching a layer of air bubbles is on the
market. This is used to cover part or the entire greenhouse.
Fuels
Fuel
sources for heating are the same as those used domestically and the
equipment is similar though tailored to greenhouse requirements;
household heaters should not however, be used in the greenhouse as they
are not insulated against the damper conditions. The main fuels are:
coal and other variations of solid fuel, oil, gas and electricity. Solid
fuels are used in boilers to heat water. The hot water is then piped
around the inner perimeter of the greenhouse close to ground level. The
water circulates through the pipes and boiler by gravity using the
principle whereby water lightens and rises when hot and sinks again when
it is cold. The pipes must be installed correctly otherwise there may
be circulatory problems, though small pumps can be fitted to get over
this. There is no doubt that the time-honoured practice of using piped
hot water gives superior heat, but it is more expensive to install. The
old hand-fed solid fuel boilers were messy to use and the heat difficult
to regulate, but more modern, semi-automatic, hopper-fed solid fuel or
oil-fired boilers provide a good method of heating. If such boilers are
already in use for domestic heating it is always worth consulting a
heating engineer or plumber to see if it is feasible to link the
greenhouse to the existing system. Gas and electricity can also be used
to heat water, but are best used in a more direct way.
Natural gas
and propane can be burned in special heaters which are very efficient.
In some parts of the world natural gas is also very cheap. Oil in the
form of paraffin has also long been used in this way. Formerly, heaters
gave off harmful fumes unless kept immaculately clean, but modern
designs have eliminated this fault and they can now be controlled with
greater accuracy. Even if not chosen as the main source of heat, one of
these heaters should always be held in reserve in the event of a
breakdown of the main method. Both oil and gas give off water vapour
which greatly increases humidity unless the greenhouse is well
ventilated. If maximum temperatures are kept above 7c this extra air
moisture is not harmful and can in fact be beneficial as long as cacti
and succulents are not the main plants being grown.
Electricity
has now come into its own for powering various items of greenhouse
equipment including heating. Used in tubular heaters, it provides warmth
similar in coverage to that of hot water pipes. Although the most
expensive way of heating the greenhouse, with the aid of a thermostat it
gives instant heat at the turn of a dial or touch of a switch and will
run for months without attention. The vigorous air movement also
prevents stagnation within the house and dries up any surplus water
quickly. This is very valuable where the temperature is being maintained
at or below 7c and definitely reduces the incidence of grey mould.
Heating
costs can be reduced if the greenhouse is a lean-to or a sun room with
access to the house, or is linked for heating to the household system.
By merely leaving the door into the house open, warmth will flow in and
frost can be kept at bay expect during severe spells when the heat loss
from the house will be excessive.
Heating conditions have changed
much during the last 50 years and continue to do so. Many of the larger
commercial greenhouses are heated with warm air ducted, sometimes with
the aid of fans, to all parts from a central heater, using polythene
sleeve. So far though, this system has not proved easy to adapt for the
smaller structure. In the USA, solar heaters which store and use the
sun's heat are finding favour for heating both greenhouses and dwelling
houses, and already have a strong hold in Britain.
Methods and
designs of solar heating vary. Those in current use or in the developing
state have heat collectors on the greenhouse roof and use water or air
and stones to absorb and store the sun's warmth. Designs using water
require a grid or series of black metal pipes or hollow plates. Water
moves through the plates or pipes, is warmed and passes into very
efficiently insulated tank. From the tank, water can then be circulated
to the greenhouse as required. Methods using air work from a heat
collector rather like a large, flat, glass box. Air enters the bottom of
the box (usually assisted by a blower), is heated by the sun and passes
into an insulated container full of stones. The stones absorb and hold
the heat and are surrounded by the warm air which can be drawn off to
heat the greenhouse. It is recommended that a supplementary, traditional
heat source is coupled to both air and water systems in the event of
long sunless periods or extra-cold- spells. In addition, insulation
ideally in the form of blinds which can cover the roof and walls at
night or during cold spells will help to conserve heat. Naturally
enough, these solar heating methods require a climate with a fairly high
sunshine average in winter to make their installation worthwhile.
Soil/Bench Heating
This
discussion of heating has been so far entirely concerned with
artificially raising the air temperature to a level essential for warmth
demanding plants. Where for one reason or another, this sort of heating
is not contemplated, electric soil or bench warming is beneficial. This
sort of warming is done with cables of known resistance either buried
in ground level borders or placed in deep benches. The cables are
arranged in horizontal loops 10-15cm apart. Those on benches are covered
with an inch or two of stony, washed sand or grit kept moist. Those in
the soil are buried 20-25cm deep. The cables have different resistances
and electrical loadings for benches or soil and when purchasing make
sure that they are right for the purpose intended. In an otherwise
unheated greenhouse, soil or bench warming not only prevents freezing of
the roots media during prolonged cold spells, but stimulates root
growth at all levels. Indeed it is invaluable under a propagating case
or bench to aid the rooting of cuttings. It is essential under a mist
propagating bench. A temperature of 18-21c is normal for such warming
cables, but under a mist unit it must be 24c as the rooting medium is
continually wet.
