Introduction

There is no mystery in the management of a greenhouse. Regular attention and common sense are the basis of all success under glass, while the application of intelligence, ingenuity or expenditure can overcome most problems even when time or common sense are in short supply! The one essential is a will to grow plants, not just look at them. The key questions are 'Do you want a greenhouse?' and 'Why do you want it?'

The greenhouse must give pleasure and fulfil a role, even if this is one that is not recognised by the neighbours or indeed by the family, let alone by any horticultura! expert. A greenhouse is a very personal thing and difficult to share, even if its maintenance does sometimes demand the cooperation of others.

Today the conservatory has become fashionable as well as useful and is another option. A lean-to greenhouse may be used as a conservatory and is often the cheapest way of extending the home.

When it comes to growing plants one cannot ignore personal prejudices. Those who cannot throw anything away will have constant trouble with pots of ailing worn out plants cluttering up the staging. Those who believe that everything they touch will die are probably suffering more from the stresses of an artistic temperament or a disorganised iife-style than a lack of horticultural knowledge.

To have a small greenhouse is not one of life's major decisions. It is a fringe benefit that may cost more than some wish to spend on a hobby, but it should not cause any more difficulties than buying a freezer or a washing machine.

The fact that every healthy plant grows bigger every day is an everlasting problem. It is sometimes soived by using the greenhouse as a production unit, or as a staging post for plants that will decorate the garden or the home when full grown. There is always an element of this. Some people are only attracted by rare plants whereas others are equally devoted.to familiar flowers. It will be

Opposite, above: Lapagerio rosea, a beautiful evergreen climbing plant suitable for the cool greenhouse [see page 55). Opposite, below: Tigridia pavonia, an almost hardy bulbous plant suitable for a frost-free greenhouse (see page 54).

found in time that the idea of continuous bloom, so attractive to the beginner, is ultimately as boring as living in a sweet shop. Anticipation is probably the keenest pleasure in growing plants, and the increasing skill and knowledge that can be gathered through life is perhaps the greatest satisfaction. Seed sowing and propagation are essential to a lasting interest in a greenhouse and make this Form of gardening a good and inexpensive bobby in experienced old age.

As far as pot plants are concerned the greenhouse can be a nursery, a forcing house, a display centre or, alas, a casualty ward and convalescent home. The more knowledgeable and enthusiastic one becomes, the harder it is to limit and rationalise the use of glass; and this often leads to there being not one small greenhouse but two or three, when one large one would have been so much easier to manage.

It is a sound idea to start with a mixed collection. In fact it is almost unavoidable; there are many plants so prodigal of seed or easily rooted cuttings that they pass from hand to hand and turn up in most collections.

Although it may seem less than friendly to look a gift-horse in the mouth, it is madness not to examine a gift plant extremely closely before adding it to your collection. A hand lens is helpful for identifying pests, and the tender growing points and the backs of the leaves are the places to look. In any case two weeks isola-lion is a wise precaution with spraying and fumigation as possible alternatives. In a mixed greenhouse or conservatory one hopes to avoid a serious build-up of the pests that are attracted by any one plant family. There is also more scope for a continuous rather than seasonal display. Much depends on the artistic arrangement and this is an incredibly neglected subject.

The true plantsman who sees each plant individually may not be concerned with the general display at all. The only consideration will be the choice of position of light, shade, warmth and air to suit each treasure best. For there is quite a divergence of microclimates even in small greenhouses. Unhappy plants are worth moving and there should also be no hesitation in examining the roots. It does no harm to knock a plant cleanly out of its pot and replace it, and much can be learnt.

In the very limited space of the small greenhouse or conservatory there is no room to grow a large number of plants perfectly. Indeed half a dozen exhibition plants might fill it. There are ways of overcoming this difficulty to suit different temperaments. One is the use of hanging baskets, pots and shelves to create layers of growth.

Another is to concentrate on the 'mini' plant. These have greatly multiplied in recent times, as the need to transport flowering plants by road in cardboard containers hastened the development of compact strains of the most popular plants. Calceolarias, cinerarias, cyclamen, chrysanthemums, gloxinias, saintpaulias and pelargoniums are just some of those that have been induced to concentrate their charms, and together they cover every season. Other plants are naturally small or propagated yearly from small pieces. The enormous and enduring popularity of fuchsias, chrysanthemums and pelargoniums is partly due to the fact that, as they become unmanageably large, the old plants are replaced by young cuttings taking up much less room.

Another way of saving space is to confine everything to pots too small for full development. With careful feeding this can be remarkably successful but it does not suit either every plant or every person. Some of us wish to grow the best and discard [hose that do not meet our high standards.

All greenhouses are much the same in summer, depending only on the management of ventilation, humidity and shading for their differing climates. The key questions for year-long interest are what plants can be over-wintered and how early in the year active growth starts. Many plants have a natural rest at a much lower temperature than for their normal growth, while some, like fuchsias, will either rest or grow according to the temperature. Others will grow in winter and rest in summer if given suitable conditions. Many greenhouse plants come from the southern hemisphere and some can be persuaded that a dry rest in our Winter corresponds to the summer drought of their homeland,

The final choice of what to grow will depend on the temperature that is maintained in winter. Without any heat at ail the scope for an all-the-year round display is limited by the fact that n plant that is not frost-resistant cannot be guaranteed to survive. All the same a lean-to against a south wall can be very rewarding, and spring comes much earlier under any form of glass.

The so-called 'cold' greenhouse sometimes means one with sufficient heating to exclude frost at all times. This creates something like a good Mediterranean climate, depending on the skill of the cultivator. Many tender plants can be grown if kept rather dry in winter.

If a minimum winter temperature between 4 °C [40° F) and I0°C (50°F) is assured, the greenhouse is usually called 'cool'. In any case this is the most popular form of heated greenhouse, even though some prefer the word 'temperate' and set their thermostats around 7°C (45°C). Ail such greenhouses become easier to manage in winter with every extra degree of heal, although the virtual doubling of the cost of fuel with every extra five degrees (F) is a powerful deterrent.

Traditionally the cool greenhouse became 'intermediate' when the minimum temperature maintained was 13 °C (55° F), but today 'warm' seems a more suitable term and the only question is the definition of warmth. There is a tendency for warmth to begin at 10°C (50°F) in rather the same spirit that life begins at 40! In other words hope springs eternal and some tropical plants will survive although they are happier at 13°C (55°F), This last temperature is the highest that amateurs generally aim at in the free-standing greenhouse. Where there is a conservatory against a house wall connected with the central heating system higher temperatures may prevail and the tropical plants we call house plants can be freely grown.

For those who prefer to specialise, a wide range oFbulbous and alpine plants can be grown in the coo! greenhouse.

For those who prefer to specialise, a wide range oFbulbous and alpine plants can be grown in the coo! greenhouse.

Top left: Streptocarpuscome in a wide range of colours; this is the hybrid 'Tina' (see page 57).

Top right: African violets (Sain tpouiio) are amongst the most popular of houseplants, and are easily grown from cuttings. Below left: Gloxinias (Simiingia) are some of the besl known of the gesneriad family (see page 57}.

Below right: Hippeaslrum 'Apple Blossom', a spectacular bulbous plant suitable for the warm greenhouse (see page 54).

Site and structure

An amateur greenhouse of up to 1000 cubic feet does not usually interest either the planning or rating authorities, unless it is on a boundary or attached to a building. The local building inspector will have to approve any structure being added to a house and there are regulations about size and use so that it is advisable to check the position locally. If you are a tenant, remember that once fixed to a permanent concrete or brick base, the greenhouse is no longer a tenant's fixture.

Some greenhouses are erected directly on the ground while others have portable base plates of concrete, wood or metal. A really solid base does add to the life of the structure and it is obvious that, if in time the building sags, the glass is likely to crack and leaks will develop. The glass needs to be set in some soli material (putty is no longer used) to avoid leaks and loss of heat in winter.

The materials of which greenhouses are made vary in popularity as their cost and ease of maintenance increase or decrease. It is never difficult to find a possible scientific advantage for either a new or cheaper material. Vet everyone has a preference for wood or metal, regardless of its intrinsic merit, and it is a pity to spend one's leisure in a building one instinctively dislikes.

In some settings the traditional white painted softwood is so much more visually satisfying that it must be preferred for a conservatory. Nevertheless the cost of frequent painting needs to be faced. The glazing bars of even a wooden conservatory will probably be of aluminium today.

The most popular wood, known as western red cedar, is a rot-resisting softwood from North America. This wood from the tree Thuya plicaia, is usually stained with a solution to make it waterproof and retain the natural red colour. Another method of keeping the wood in good condition is a yearly treatment with thick penetrating oil.

The aluminium alloy greenhouses so popular today need less maintenance than wood, but vary greatly in quality both in design and strength. They can also be had in anodised colour which can have a bronzed appearance or white vinyl covered aluminium.

On a windy site one needs to choose a greenhouse of sound construction fixed to a solid base.

Although metal is colder than wood this is not of serious consequence in small lightly constructed buildings. Wood is more convenient when it comes to fixing up shading material or a plastic lining. Maximum light in winter is vital, but this is affected more by the position of the greenhouse than its construction nowadays. An east-west orientation is best for winter light but in a very small building this does not make much difference. Galvanised steel is another possible construction material. This is more rigid than aluminium alloy, but will need regular painting as galvanizing is not a permanent protection.

A lean-to built against a wall saves a great deal of fuel in winter; but it receives less light and is more difficult to ventilate efficiently than the ordinary span-roofed type. A good choice in certain circumstances is a three-quarter span. This over-tops a wall and has two-way ridge ventilation but is an expensive option as it has to be custom-built. A south wall is regarded as the best position, though very hot in summer. Nevertheless, a lean-to on any wall is possible and often a joy, if the plants are chosen for the light available. Sometimes the layout or lack of space makes a round or hexagonal greenhouse the best choice.

If a greenhouse is on a solid base, it is less suitable for growing plants in the ground. Brick walls up to the staging save fuel, but are now rare because of the cost of the brickwork. A substantial wooden base is some substitute, but plastic panels do not save fuel unless thick insulating material is fitted inside them. As many additional plants can be grown under the staging when there is glass to the ground, a panelled base can be a false economy.

It is not possible to find all the virtues in one ready-made structure, but it is well worth comparing one with another. The Chelsea and Southport flower shows are good places to examine a variety of greenhouses and conservatories. As the available site may affect the choice this needs to be considered first. The distance from a source of electricity is important even if it is not going to be used for heating.

Lighting, fans, seed-raisers, mist propagation and power drills are just some of the many extras using electricity. If natural gas is to be used for heating or if an outside oil tank or fuel store will be needed, this is the time to decide where they will be. Mains water is also helpful, even though rain water is often preferred to tap water. Automatic watering may need mains water pressure or a water tank at least 3 feet (90cm) above the staging.

If plants are to be grown in the ground, the soil must be reasonably well drained and this is more easily attended to before the greenhouse is put up.

Above: A small cedar greenhouse in one of the mode) gardens at RHS Garden, Wisley.

Below; A small lean-to greenhouse/conservatory.

Interior of a small mixed greenhouse, showing some of the range of plants that can be easily grown by the amateur.

A position that is not overshadowed by trees or buildings and open to winter sunlight is the aim, and one does not want a hedge or fence too close although shelter from wind is important. There is some conflict of opinion as to which way to align the greenhouse, though this may be determined by the site. I favour an east-west alignment which is good for winter light and gives a sunnier and shadier side.

Here are some points to look for when choosing a small greenhouse or conservatory:

  1. Method and amount of ventilation and whether it can be automated. Extra ventilation is almost always essential as when the ventilators are open they should equal at least one fifth of the floor area.
  2. General stability and likely wind resistance if the site is exposed.
  3. Foundations required and method of construction.
  4. Strength and height of staging provided and whether it is suitable for your purposes. Much gravel, clay pots or sand capillary benches can be heavy. One needs an absolutely flat surface for all capillary watering.
  5. Arrangements for disposing of water from condensation inside the greenhouse.
A newly constructed aluminium greenhouse showing various ventilation systems.
  1. Width of entrance.
  2. Check terms if manufacturers deliver and erect, and look at 'Do it yourself' building instructions.
  3. Make sure glass is included in the price and discover if it will be delivered with the greenhouse.

If a greenhouse is put up on grass, the turf should be removed with an inch of top soil and stacked to make potting soil. However, the busy gardener may feel more inclined to cover the grass with black plastic sheeting until it is dead and then cultivate the soil or remove it for use elsewhere. If plants are grown in the ground, the soil is managed much as in the open garden, bearing in mind that it does not have the benefit of frost or rain. A friable loam containing plenty of organic matter is the aim.

When ground is not cultivated, the floor inside the greenhouse can be treated in various ways. On a wet site concrete is probably the best. For retaining moisture in summer, when the floor is sprayed to increase the humidity, rammed soil or ash with a duckboard path has been the traditional working arrangement. A central path of paving blocks with gravel on the soil under the benches is more attractive. A conservatory often looks best if the same paving as the terrace continues inside or alternatively if the flooring in the house is carried through to the conservatory. This is not always practical. Wall to wall carpeting cannot be 'damped down' and vinyl tiles can be death traps when wet.

The actual foundations of a free-standing greenhouse should rest on firm sub-soil. When laying concrete or brick foundations, leave entry points for electric cables and water pipes. Pieces of hose do well for this, although I once found that a pygmy vole took up residence as a result!

Heating and ventilation

The cost of heating has multiplied so many times in recent years that we hesitate before deciding to heat a greenhouse at all. At the same time the savings made by raising, preserving and propagating our own plants have also greatly increased.

An amateur's collection of plants can be irreplaceable, costly to replace, or almost valueless; so the cost of installing and running truly adequate heating for a severe winter may be either well worth while or quite unacceptable. Every ten years or so there is a winter when low temperatures persist for several weeks and plants will certainly be lost unless the realities are faced.

It is possible to grow only hardy plants or those that become hardy with shelter from our winter rains, but this is not usually the aim. Most of us have a mixture of tender plants and want some to be decorative at all seasons. This is even more true of a conservatory attached to the house. Few plants mind a brief drop in temperature so long as they do not freeze, and daytime power failures rarely cause serious harm.

The cost of heating will vary with the region of the country and the exposure, as well as the elevation of the site and the degree of double-glazing or other insulation. Although many tender plants may be brought safely through mild winters in an unhealed glasshouse against a south wall in the warmer parts of the country, one is always at the mercy of the elements without artificial heat. Many people try to nurse tender plants through the winter by lighting a heater on cold nights only. This adds human error to the dangers and can be quite expensive, as it is often done without thermostatic control,

Greenhouses can be heated by natural gas, solid fuel, paraffin, bottled gas or electricity. Their cost also progresses in that order, with electricity the most efficient and expensive unless you are able to benefit from the 'night rate' on a two-rate tariff. This can halve the cost of the night hours.

The major decision about heating is what minimum temperature to maintain in winter. Unfortunately every extra degree of heat tends to make the control of the environment easier and the results more rewarding, A temperature of 4 °C (40°F) will exclude frost and is often chosen for a cactus collection, which rests in winter, as well as by many of us on grounds of cost. What is known as the 'cool' greenhouse should not fall below 7°C (45 °F)

A group of cacti showing some of the wide range available {see page 57).

and this is the lowest satisfactory temperature for plants that are actually growing in winter. One can preserve them and expand into a splurge of growth in spring so long as frost is excluded, but 1 think year round happiness and comfort for alt concerned are greatly increased at 7°C. To raise the temperature to 10°C [50°F} will double the fuel bill, while the 'intermediate' house at 13°C (55 "F) will double it again, The warm greenhouse at 18°C [around 65°F) could cost five times as much as the cool greenhouse.

The daytime temperatures will always be higher than the minimum set, except in very bad weather. Very little artificial heat is used in daylight at the lowest temperatures, but a warm greenhouse needs continuous heating for at least half the year. It is possible to have a thermostat with different day and night temperatures and this is the ideal.

All greenhouse heating systems are rated at a maximum heat output expressed in either kilowatts or British Thermal Units per hour (lkW = 3412 Btu/h). This will be found printed on the equipment or in the instructions and is essential information. One can only make a realistic assessment of the heating needed to maintain a given temperature in a particular greenhouse by making a calculation of the heat-loss of the structure. This is simpler than it looks as the thermal factors of the various materials are known. These are expressed in Watts per square metre per degree Centigrade (W/m2 deg C) in the following table and are known as 'U1 values.

HEATING AND VENTILATION Thermal trans mission co-ef/icienis ('U' valuesj o/materials

W/m2deg C

Horticultural glass

5.7

500 gauge polythene film

5.7

Wood, 2.5cm

2.6

Wood, 3.8cm

2.4

Brickwork (unplastered) 11.4cm

3.4

Brickwork (unplastered) 22,8cm

2.7

Brickwork (unplastered) 34.2cm

2.1

Concrete, 10cm

4.0

Concrete, 15.2cm

3.6

Earth floor

1,9

Concrete floor

0,7

The following example will show how the calculation is done:

Area of glass ends (0.9m x 2.4m) + (0.6m x 1.2m) x 2 = 5.8m2

Total area of glass = 22,4m2 Heat loss from glass @ 5,7W/m2 deg C: 22.4 x 5.7 = 127,7W/deg C Area of side walls (brick) (3.6m x 0.6m)x2 = 4,3m2 Area of end walls (brick) (2.4m x 0.6m) x 2 = 2.9m2 Total area of brick walls = 7.2m2

Heat loss from walls @ 3.4W/m- deg C: 7.2x 3.4 = 24.5W/deg C Area of floor (soil) 2.4m x 3.6m = 8.6m2

Heat loss through floor @ 1.9W/m2 deg C: 8.6 x 1.9 = 16.3W/deg C Total standing heat loss from greenhouse = 168.5 W/deg C (i.e. sum of total heat losses).

Add l for air leakage from the structure through joints and cracks etc.

168.5 x 1.3 = 219.1 W/degC To achieve a 14°C temperature life and 7°C minimum winter temperature for this greenhouse a loading of 3067W or 3.067kW is indicated (i.e. 219.1 x 14W or 219xl4kW)

1000

For a 20°C lift to maintain, say, 13°C the figures would be 4382W or 4.382kW. Rounding up or down to the nearest 500W one could use 3kW or 4.5kW heaters respectively. In a very cold area one might feel it wise to instal the capacity for a greater temperature lift in severe weather although aiming only at a low minimum temperature.

The running costs of all heating systems depend on the set minimum temperature, and effective thermostatic control is absolutely vital if much electric heating is to be used. The newer electronic thermostats are more accurate and reliable than the mechanical rod type, and both need checking yearly against a mercury thermometer. Thermostats need careful placing so as to reflect conditions among the plants; they should not be in the rooF or on the floor. A shady spot away from the door is best. Aspirated thermostats can help with both heating and ventilation control in difficult conditions.

Electricity in your Garden published by the Electricity Council gives guidance on electricity consumption in various parts of the country, and the local electricity board can advise on the most suitable tariff.

The most primitive form of greenhouse heating is the small paraffin burner that depends on frequent filling and wick trimming. This is the favourite emergency standby. Although in good hands this kind of heater has been made to work wonders for decades, it is not suitable for maintaining a reasonable temperature in cold weather. It is sometimes difficult to assess what heating can be given by paraffin heaters, hut the makers of the more effective ones do give the number of British thermal units produced per hour by their heaters and also the number of hours the heater will run on one filling. The burning of one gallon of paraffin will produce about 72,000 Btu's and so it is possible to make a rough assessment of the actual temperature one of these heaters can give. One Btu per hour is the equivalent of 0.293 Watts; so one may be trying to heat a greenhouse with something little better than an electric light bulb.

Burning paraffin without an outside flue discharges a considerable amount of water vapour into the atmosphere. Some damaging fumes build up unless a chink of ventilation is always left open. The fact that oil heaters burn on regardless of the temperature has been partly overcome by non-e]ectric thermostats, which reduce the rate of oil consumption as the temperature rises. These are fitted to some of the larger heaters. It is also possible to have a flued paraffin boiler to heat water pipes in a greenhouse.

Natural gas is a relatively new and reliable source of heat that can be used in glasshouses without a flue. The local gas board has to approve the type of heater and how the supply pipe is laid. They may charge very reasonably to do the work but have done little to encourage this use of gas. The gas heaters have a non-electric thermostat and are designed for small greenhouses, so that more than one may be needed. There must be ventilation at all times when gas heaters are burning and, like paraffin, they add moisture and carbon dioxide to the atmosphere. A possible hazard with natural gas is that, if ventilation is inadequate, some plants may be affected by leaf or bud drop or some other side-effect from the substances in the gas, which are similar to the natural ripening agents of those plants.

For a greenhouse needing BkW or more heating one can have a natural gas heater with an outside flue and electrical controls. This blows warm air either freely or through ducting. Some flue-less gas heaters are designed for use with either natural gas or bottled gas. A bottled gas heater is a useful emergency heater when other fuels fail, but it is expensive and gas bottles do not last for long. Propane is the gas to use as it does not freeze.

In the past the traditional method of heating glasshouses was by means of 4-inch cast-iron water pipes heated by a solid fuel boiler. Today solid fuel is more attractive in the colder north of the country. Fuel is restricted in smokeless zones, but a variety of fuels can be used. In an emergency the only certain fuel is the one stored on the premises. Obviously all boilers need regular attention and maintenance, although the stoking and cleaning have been simplified in recent years. Modern piping can be assembled by the handy man and the system adjusted to any size of greenhouse, Unless the boiler is protected from the weather, it loses much of its efficiency and a fuel store is also needed. However, for the higher temperatures solid fuel is worth considering.

Electricity is undoubtedly the most convenient and adaptable fuel for the small greenhouse. Unfortunately cost may prevent its use, but it should always be considered for frost protection. A fan heater distributes warmth well and avoids installation costs, but a thermostat incorporated in it will not reflect the temperature very accurately where the plants are growing. One always needs a maximum and minimum thermometer to check what is actually happening in various positions. There are always cool and warm spots that one can use to advantage.

Electric tubular heaters distribute the heat well and can he fitted to warm any size and shape of greenhouse. They are banked round the sides of the structure and should be 4 in. (10cm] from the outer walls and at least 8 in. (20cmJ above the floor. Space should be allowed for the warm air to rise between the glass walls and the benches on which the plants are growing.

Before leaving electric heating I should mention soii-warming as a most economical way of using electricity in horticulture. Soil-warming cables are laid evenly in parallel lines with gentle curves at the ends, so that the wires never come close to each other and overheat. The most even temperature over the whole surface is achieved when the spaces between the cables are equal to, or less than, the depth at which they are buried. If they are not deeper than the width between them, there will be greater warmth directly over the wires. Screened mains cable controlled by a thermostat is used to achieve a soil temperature between 21 °C and 24°C {70°F-75°F] for rooting cuttings and raising seeds. Where only a few plants are wanted a small electric propagator is invaluable.

Electricity in a greenhouse is a tremendous heip for so many purposes, but any installation needs, at the very least, checking by a competent electrician in such a damp and potentially dangerous situation. Any exposed cable must be of a suitable kind to withstand sunlight and ail installations need looking over regularly.

Whatever form of heating is decided upon the area to be heated needs to be kept as small as possible and some form of insulation used in winter. If the warmth is to be held in, some loss of light is inevitable and there is always a shortage of light in winter. It depends on the plants grown, but with tender shade-loving plants I would have no hesitation in lining the rof as well as the sides of the greenhouse. However, access to the ventilators should always be left. Lining materials tend to attract condensation droplets which obscure the light still further, and the humidity of the greenhouse atmosphere is also increased. Bubble plastics give rather more protection than simple sheeting. It is not impossible to put up a materia] to give shade in summer and some insulation in winter, but it will not be the best for both purposes. There are meshed materials which shade but give little insulation and double-walled plastics which give more protection from cold but are opaque. Most materials have a limited life in the greenhouse and both they and the glass become obscured if they are left up permanently. Even with the most complete and careful insulation it is unwise to count on more than a 30 to 35 per cent saving of fuel. If either internal or external blinds are used for shading some warmth is conserved by lowering them at night in winter.

It is now possible to have double-glazed glasshouses and this is worth very serious consideration, particularly for conservatories and when a high temperature is going to be maintained.

The key to success under glass is the control of climate (of which heating is only a part) in a way that adjusts to the vagaries of the weather. The aim is always a steady temperature without violent fluctuations and preferably only moderately cooler at night than during the day. It is one of the great virtues of glass that the sun shining on it raises the temperature inside much more quickly than the trapped heat can escape. This saves a great deal of fuel even in the coldest weather, though it does mean that ventilation must respond fairly quickly to weather changes, particularly in the smallest greenhouses where temperature fluctuations are the most rapid. March, April and October are perhaps the most difficult months, as the sunshine can be warm though the outside temperature may be too low for free ventilation when the sun disappears.

For a few people with leisure, who are tied to their homes, it may be a pleasure to keep a constant eye on the weather and wind direction, so that greenhouse ventilation can be adjusted to the best advantage. To most of us this is either impossible or unthinkable, Indeed it can be the reason for rejecting the very idea of a greenhouse.

Although I am not a great believer in extractor fans, they are perfect for saving a closed greenhouse from frizzling up in unexpected sunshine. Often a greenhouse door is left open in warm weather, but must be locked if the owner is out or away. Then, too, a fan can save the plants. Although it is sometimes recommended, I do not regard an extractor fan as a satisfactory substitute for all other forms of ventilation. Proper greenhouse fans have louvres to prevent wind blowing in and should be controlled by thermostat.

Ventilation is measured by air changes per hour, and 30 changes per hour are considered enough for amateurs. As a rough guide on this basis a 228mm fan is said to be sufficient for a volume of up to 14 cubic metres and a 300mm fan up to 28 cubic metres. Professionals are always advised to have at least 60 changes of air per hour - from which you can draw your own conclusions. One of the controlling factors is that in a small greenhouse it is difficult to find a place to fit a large fan. All extractor fans need their thermostatic control set at least 2°C above the temperature of the heating thermostat.

It is believed that greenhouse ventilators should open up an area of not less than one fifth of the floor area. In practice it is usually a good thing to have as much ventilation as can be fitted in small greenhouses. Plants need fresh air for healthy growth, and in summer good ventilation is necessary to keep down the temperature to reasonable levels when the sun is shining. In general all temperatures above 24°C (75°F) are undesirable, though not always avoidable. In winter stagnant cold damp ah encourages every sort of mould and rot amongst the plants that are barely growing. Then, ventilation is still needed and artificial heal adds to the occasions when ventilation can safely be given.

At the time of writing, the popular automatic ventilation controls that do not require electricity are activated by a cylinder filled with a composition which expands and contracts according to the temperature. These ingenious devices come in various forms and are adjustable within limits. They are fairly satisfactory in most weather conditions but I like to have at least one adjusted by hand. They are not supposed to operate centrally pivotted ventilators, but with counterweights and ingenuity they can be made to do more than their makers claim. There is also one for louvred ventilators.

Shade is another way of lowering summer temperatures under glass, and if there was perfect ventilation very little would be needed. Shade-loving plants and delicate seedlings may need shading in March, and a high proportion of greenhouse plants benefit from some shading from April until the end of September, It is, of course, only actually wanted when the sun is shining, hut it is usual to compromise and create a filtered light that is not too dark on dull days.

It is possible to have automated wooden slatted blinds fitted outside the structure but at great cost. This gives both shade and the maximum cooling effect. Internal roller blinds do not cool the structure in the same way, but are much used and could be automated. Another possibility is internal slatted blinds, the angle of which can be adjusted according to the weather. The simplest arrangement is to use one of the temporary shading materials, which are diluted with water and lightly painted on to the outside of the glass. This can be done progressively as the sun strengthens, starting with a light coating on the south side to protect vulnerable plants in March. These materials can be wiped off at the end of the season. Fine plastic netting designed for greenhouse shading is also effective and has a life of several years.

The other means of combating excessive heat in summer is by increasing the humidity, and this is discussed in the next chapter.

Building Your Own Greenhouse

Building Your Own Greenhouse

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