Choosing a greenhouse
When choosing a greenhouse, always buy the biggest you can afford
and accommodate. You will always be able to fill the space, and
a small one will soon make you frustrated with its limitations in
busy times, such as the spring. To economise on fuel costs, partition
off a section for heating in the winter leaving the rest of the
greenhouse to provide welcome shelter for the wide range of plants
that benefit from winter protection, especially if it is insulated.
Any greenhouse is better than
no greenhouse, and a small one, even a mini-greenhouse, will increase
the range of plants that you can raise and grow.
The main shapes and their advantages
are set out below, but there are some general points to be considered.
MATERIALS
Aluminium alloy is the most popular choice for the framework,
combining strength and low weight with a narrow profile (which
allows more light to penetrate). Erection is simple – usually
the components just bolt together, and the glass is held in position
with special clips.
Timber is still frequently chosen
on aesthetic grounds. However wood requires more maintenance and
regular painting is essential to keep the greenhouse looking good
and to extend its life. This applies especially to the less durable
timbers. Cedar is a popular choice of wood because it has a natural
resistance to rot and does not require regular painting, but it
should be treated every year or two with a cedar preparation to
help retain its colour.
Galvanised steel is strong, but seldom
used. Exposed areas must be painted to prevent rust.
For the glazing, glass is still much
the best all-round material and is more commonly used than anything
else. The light transmission is good and it does not scratch or
discolour with age, unlike plastic. Plastics are useful for a
curved profile, or where safety is important.
POSITION
When most greenhouses were made of timber, with thick glazing
bars that could cut down the light, much thought was given to
positioning to give the best light penetration. The principle
was to orientate the greenhouse with its long axis running north
to south if it was used mainly in the summer, but east to west
if good light in spring (for raising seedlings for example) was
the most important factor.
In most small modern gardens, there
often is not much choice of position, but most modern greenhouses
are made of aluminium alloy which has a small profile and the
panes of glass are usually large, so light loss is marginal.
A more important consideration, if
you are heating the greenhouse in winter, is to avoid a windy,
exposed position because wind lowers the temperature and thus
increases the fuel bills. Try to position the door on the leeward
side of the prevailing wind. Try, also, to avoid shade from trees
and tall hedges.
The greenhouse will also work better
if it is positioned where you can provide a power source for heating,
lighting and propagators. Supplementary lighting on dark days
can then make up for any slight loss of light penetration.
VENTILATION
Adequate ventilation is essential. When choosing a greenhouse,
always consider the number and type of opening windows, or ‘lights’,
that it has – many are sold with an inadequate number of
ventilators. You can usually buy additional ones as optional extras,
and it is easier to install these when you erect the greenhouse.
Install plenty of louvre vents in the sides and low down at the
end, and if possible have hinged ventilators along half the length
of the roof. This combination creates a better movement and exchange
of air, which will help to keep the greenhouse atmosphere healthy
and easier to cool on hot days.
Automatic vent openers work on the
principle of metal or wax expanding or contracting according to
temperature changes, and it is well worth investing in at least
one or two. The temperature can sometimes rise rapidly early in
the morning, and a couple of vents which open automatically will
prevent excessive temperatures building up before you can open
the rest of them manually.
If you have a power supply, a ventilation
or extractor fan is a useful accessory.
INSULATION
This should be your first priority. You could recoup the cost
of installing insulation within a few seasons if you attempt to
keep the greenhouse more than just frost-free over winter.
Double-glazing is one of the most
efficient methods of insulation, but adds considerably to the
cost, and is normally only used where the initial cost is not
a significant consideration.
Bubble plastic or even just plastic
sheeting is inexpensive enough to replace after a few seasons,
and makes a significant reduction to heating costs; for most small
greenhouses it is the most efficient form of insulation. It is
quite easy to fit although it does reduce the amount of light
available during the winter.
There are also various proprietary
insulation systems available on the market, such as thermal screens,
but these are more appropriate to commercial than domestic use.
HEATING
There is no ideal heating system for everyone, as much depends
on the temperature you want to achieve, whether the greenhouse
is well insulated, and the cost and availability of fuel.
Electricity is the most convenient
way to heat your greenhouse, and if close to the house it may
be easy to provide a power supply without much expense. If it
is a long way away, you will need to have a supply installed and
connected by a qualified electrician. This could make installation
expensive.
With thermostatic control and good
insulation, running costs of an electric heater may not be any
higher than other forms of heating. And there is the big bonus
of convenience.
Electric tubular heaters are usually
positioned at the sides of a greenhouse, just above the floor,
but fan heaters have many advantages. They distribute the heat
quickly and efficiently, and you can use them as cooling fans
with the heating element switched off in warm weather.
Paraffin heaters are relatively inexpensive
to buy and install, but running costs can be high if you need
to maintain a high temperature. You also have to carry and replenish
the paraffin. They may produce toxic fumes and excessive condensation
if care is not taken to ventilate the greenhouse adequately.
Gas systems can sometimes be run
off the mains supply, but usually the position of the greenhouse
means using bottled gas. Propane gas releases fumes and water
vapour as it burns so, as with paraffin, you must ensure that
ventilation is adequate. Bottled gas is useful for achieving a
high temperature if mains electricity is not available, but running
costs can be high, and you will have to manhandle heavy gas cylinders
regularly. You must have at least two bottles connected by an
automatic switching device in case one runs out.
FLOORING & STAGING
A central path of some kind is essential for easy access to the
plants, but it will take up valuable growing space. Path space,
greenhouse width and the type of plants that you wish to grow
have a bearing on each other.
Preformed wooden paving panels can
look pleasing but are more likely to harbour debris and pests,
so you should be prepared to remove and clean them occasionally.
Bed them on gravel to ensure free drainage.
If you are tall, and the ridge of
the greenhouse is low, consider sinking the path by 15 cm (6 in)
or so – you can build a low retaining wall of bricks to contain
the border soil.
In all but the tiniest greenhouse,
staging (slatted or solid shelving) will provide a home for plants
in pots and a surface on which to work.
Slatted and mesh staging is particularly
useful for plants that like fairly dry air and compost and good
ventilation, such as cacti and alpines. For most pot plants, solid
benching is often more practical and can be covered with a layer
of sand or capillary matting if extra humidity is required. If
you are using a sand system a lipped edge is helpful, but a flush
surface is suitable for capillary matting. Some systems can be
reversed to suit the particular need.
If storing dormant plants below staging,
slats can provide extra light, but beware of drips from above.
For many gardeners, fixed staging
along one side supplemented by free-standing staging along the
other, offers flexibility to suit various needs throughout the
year.
MAKING THE MOST OF SPACE
Think about the way in which you will use your greenhouse.
If you are mainly going to grow pot
plants on staging, then a greenhouse wider than 1.8 m (6 ft) may
be a waste. Most proprietary staging for a small amateur greenhouse
is about 60 cm (2 ft) deep (anything wider is difficult to reach
across) and a central path 60 cm (2 ft) wide is usually adequate.
If the greenhouse is to be used mainly
for border crops, such as tomatoes, chrysanthemums or winter lettuces,
then a greenhouse 2.5 m (8 ft) wide will increase the growing
area by about 50 per cent – at much less than a 50 per cent
increase in cost.
Rather than running a path the entire
length of the greenhouse, leave 60 cm (2 ft) at the end for more
staging or for growing crops in the border. This gives you the
maximum growing space with the minimum area taken up by the path.
In a very large or wide greenhouse,
you can run the path around the outside edge with a large bench
in the centre. This is more appropriate for ornamental displays
of pot plants than for a mixed-use amateur greenhouse.
To make maximum use of space, dormant
plants can ‘rest’ below the staging.
SHAPES
TRADITIONAL SPAN
Most greenhouses are this shape, and the design generally offers
a large amount of usable growing space for relatively modest cost,
and is suitable for many crops.
The smaller sizes may have limited
headroom. Look for a good height to the eaves if you are likely
to want to grow tall pot plants on staging, and if you are tall
yourself, check that the ridge is high enough so that you can
work comfortably standing up.
If you plan to grow mainly pot plants
and seedlings, choose a design with a solid base up to staging
level (the insulation is usually better). Choose a design with
glass to the base if you plan to grow crops in the greenhouse
border, or to stand tall plants (such as tomatoes and chrysanthemums)
in pots or growing bags on the ground.
The Alpine House is a useful addition
to a rock garden. This differs from the conventional greenhouse
in that there must be continuous ventilation along the eaves of
the house and vents at bench level, with, ideally, a door at both
ends. Here alpine plants that require controlled conditions to
achieve their full potential, such as those which will not tolerate
winter wet, can be cultivated, although some of the commoner species
can be grown in here too. No heat is necessary, although some
growers keep their houses at a minimum of 1°C (34°F).
DUTCH LIGHT
This design is superficially similar to a normal span greenhouse,
but the sides slope, and the panes of glass are much larger. The
sloping sides help to make the most use of available light, and
the glass-to-ground design makes it well suited for tall crops
in the border.
Bear in mind that, if there is an
accident, the large panes of glass will be more expensive to replace
than the traditional, smaller ones.
MANSARD (curvilinear)
The curved shape, created by angling the rows of glass, is intended
to make the maximum use of available light. The additional light
penetration may be minimal in most small gardens unless the greenhouse
can be sited in an open, sunny position, but some people find
this type of design visually more pleasing than the traditional
span greenhouse.
LEAN-TO
A lean-to is effectively half a greenhouse with the ridge supported
against a wall. The distinction between some of the larger modern
ones and a conservatory or garden room is blurred. Some can be
used primarily for people or for plants, but they are obviously
less ornate than a proper conservatory. The true lean-to greenhouse
will also have its door at the end, and is not intended for positioning
against patio doors.
A lean-to has many advantages. It
is usually comparatively easy to run heating and light from the
house, the back wall is ideal for training tender climbers against,
and with suitable staging the entire height from floor to ridge
can be used for an ornamental display. Warmth is generally retained
by the bricks and a lean-to should not lose heat as rapidly as
an equivalent size glazed all round.
Light is more likely to be a problem
than with a free-standing greenhouse. If the wall is south-facing
it may be difficult to keep the greenhouse sufficiently cool on
hot days, so an effective shading and ventilating system is essential.
If positioned on a north wall, the lack of direct sunlight may
limit the types of plants that can be grown successfully.
A variation on the normal lean-to
is the three-quarter span, which has a normal ridge and a short
extra span of roof sloping down towards the wall against which
it is fixed. Light penetration may be marginally better, but this
design is now very uncommon.
The mini greenhouse is usually a
small lean-to, but a range of small, free-standing pyramidal shapes
is now available. All the space is intended for plants, and you
must work from outside. Mini-greenhouses have low running costs
and take up little space.
It is important to choose a design
with adequate ventilation and to site your mini-greenhouse so
that it receives good light but is not in direct sunlight for
most of the day.
DOME
Dome-shaped greenhouses have multi-angled panes that create a
shape with very little wind resistance (good for an exposed site),
and they can look very stylish. Headroom and growing height may
be limited around the edges, however, and you will probably have
to use the manufacturer’s staging and fittings.
POLYGONAL
Octagons and other polygonal shapes usually provide only limited
growing space, and ventilation can sometimes be a problem, but
this type of greenhouse is usually chosen for its visual appeal.
A greenhouse of this type can look appropriate in a small garden
or even on a patio, where a traditional shape may appear out of
place.
CONSERVATION GREENHOUSE
The few greenhouses with this label are generally much more expensive
for a given growing area. Their advantage lies in reduced running
costs and low energy consumption for heating. Plants may also
benefit from high light levels created within the greenhouse by
the angled roof and side panels and reflecting mirrored surfaces
inside. Double-glazing and a high level of built-in insulation
are features of this type of greenhouse.
Taken from Reader's Digest New Gardening
Year - a month-by-month guide to success in the garden.
Feature: Karen O'Grady, Homebase
