Plants for Greenhouses and Conservatories

GREENHOUSES AND CONSERVATORIES

The word ‘greenhouse’ describes a building used for a wide variety of horticultural purposes, including propagation, raising bedding plants, producing food crops, protecting tender plants over winter, and cultivating ornamentals. The word ‘conservatory’ on the other hand defines a place where decorative plants are displayed to please the eye.

Most conservatories are designed to be attractive structurally, both inside and out, and to be pleasant places where people can sit and relax. Ideally, they should be attached to a dwelling with a communicating door for easy access. They generally take the form of a lean-to. However, an ordinary free-standing greenhouse can be used instead in many cases.

Both greenhouses and conservatories can give enormous pleasure, and a greenhouse can also be a sound financial investment, soon paying back its cost in the form of produce. The weather protection it provides enables a vast range of plants from all over die world to be grown. Greenhouse gardening is a fascinating hobby that can be enjoyed all year round by both young and old.

TEMPERATURES

Much can be done without any artificial heating whatsoever. The protection from weather extremes that an unheated greenhouse gives is adequate for many plants and a glass structure traps enough free solar radiation to be warmer than outside for most of the winter. Providing enough heat to keep conditions frost free greatly widens your scope, and a minimum of about 4-7 C (40^15 F), ‘cool house’ temperatures, enables many subtropical plants to be grown. Temperatures above this can become rather expensive to maintain and are best provided for only small compartments, such as propagators, when required. Most of the plants dealt with in this section do not need high temperatures and are within the growing ability of most people.

SIZE AND STRUCTURE

Many people prefer to buy a prefabricated structure rather than build their own. Unless you are reasonably skilled in construction, this is by far the wisest procedure. There are now many types, sizes, and shapes available in a selection of materials. So it should not be difficult to suit individual requirements. The most popular size for a practical home greenhouse is about 3m x 2.4m (10ft x 8ft). However, if you can meet the expense, it may be worth buying something a bit larger, particularly if you do not plan to heat it; many almost hardy plants can become very large and will soon cry out for space. If you do plan to heat it, estimate the cost of fuel before buying. Some greenhouse designs can be extended by simply adding extra units as required. This is a feature to look out for when buying. Partitions with communicating doors can also be fitted to some. This means you can, for example, heat only a small section for greater economy, or provide a special environment for a particular group of plants.

Always buy from a reputable and, preferably, long-established firm. There are plenty of cheap greenhouses on the market, but they are likely to be a very poor investment. It is advisable to consult your local authority regarding planning permission before buying. This is particularly important if die structure is to be attached to a dwelling; there may be building regulations to comply with and often a low rate will be demanded. For small greenhouses, especially if they can be described as portable – that is, easily taken down and re-erected – there is rarely any regu-lation or rate requirement. Also check whether buildings erected on ground that is not your freehold become the legal property of the landlord.

GREENHOUSE DESIGN

Greenhouses come in a great many shapes and sizes, all with their own advantages and disadvantages. If you are thinking about buying one, it is a good idea to become familiar with the main types available, so that you end up with one that meets your requirements.

TRADITIONAL

This is the ‘barn’ or ‘tent’ shape. It forms a square or rectangle, with vertical sides and has a span roof with a central ridge. This design is still the most practical and allows the maximum utilization of available space. In some examples the ridge can be set off-centre to meet site requirements. This is usually known as three-quarter span. Nowadays it is not a common feature.

The roof should always have a good slope to shed any collected dirt, dead leaves and snow, for example. This applies to all greenhouse designs.

TUNNEL

This design is technically called ‘curvilinear’ and, with the exception of those made of plastic, is built of six or more panes of glass set in a continuous curve to give a ‘tunnel’ shape. For reasons de-scribed under Dutch light, this model allows excellent light entry, but suffers from the same disadvantages regarding lack of headroom and poor use of space.

CIRCULAR

These structures have at least six sides arranged to form a circle. They are by no means new – the shape was a favourite in Victorian times for conservatories. They were usually highly ornamental and there are models available today that will create the same atmosphere. A small circular greenhouse could be of special benefit to a disabled person, since most of it is easily accessible without much physical movement.

DUTCH LIGHT

This design has sloping sides. The idea is to set the glass as near as possible at right angles to the sun’s rays so that light has a minimum thickness of glass to travel through and there is consequently minimum absorption. A pane of glass can absorb up to 20 per cent of light passing through it, even though it appears perfectly transparent to the eye. The Dutch light shape is of particular use to those growing winter crops, as the slightly increased light levels may give earlier maturation. However, sloping sides can be a nuisance: they make working close to the sides inconvenient and reduce headroom, and it may not be so easy to fit in taller plants or make as much use of the floor area as in traditional houses. Certainly, designs with sides set at a great angle should be viewed very critically.

Slanting sides can give greater stability to a structure, and this design should be given special consideration where the site is very windy or exposed to gales.

To the professional grower, the term ‘Dutch light’ also means a framed glass unit of specific size, used to make up frames or other structures. They are rarely employed by home gardeners.

DOME

The dome shape is a fairly recent introduction. It is made up of many triangular panes of glass supported by a strong metal framework. The effect is quite striking and futuristic. Light entry is excellent and the same advantages and disadvantages described under Dutch light apply. Better headroom around the sides can be obtained by mounting a dome on a low base wall. Domes make delightful conservatories and sun traps, but it’s doubtful whether there’s much point in using them for down-to-earth plant growing.

HIGH SOUTH WALL

This is another new introduction. It consists of two slanting sides of different heights, with a sloping roof between them. The taller of the sides is intended to face south or the direction with most light. Whether this has any advantage over conventional shapes is a matter of opinion.

LEAN-TO

This is a popular design with numerous special applications, and it’s a favourite for conservatories, garden rooms, and sun rooms. It’s also particularly suited to the cultivation of crops such as grapes and other fruits which can be trained against the rear wall. Lean-tos are usually set against brick walls at the side of a house or garage, but in some cases a tall weather-proof boarded fence would suffice. There is also a model which is half greenhouse and half timber garden shed. When set against a substantial wall, the greenhouse stays warmer overnight. Lean-tos are always the cheapest design to heat artificially.

Lean-tos can incorporate Dutch light sloping sides and such features as curved shapes and base-walls. The type of environment offered by a lean-to, regarding light and warmth, depends on orientation. Lean-to home extensions are rarely suitable as conservatories.

BASE WALL OR GLASS TO GROUND

Some greenhouse designs have timber-board or brick bases. Various forms of composition sheeting are also used, but asbestos, which was once much used, is no longer for safety reasons. Base walls help to conserve warmth when a greenhouse is heated, but they obstruct sunlight and cut off a free supply of solar heat. A base-walled greenhouse is known as a ‘plant house’ and is usually fitted with staging to about the height of the wall for convenient working with potted plants. Space under the staging can be employed for those plants demanding less light. There are also designs with only partial base walling, the side admitting most light being left glass to ground. Some manufacturers produce models with removable base panels. These can be clipped on in the winter to conserve heat and removed in the summer months. Generally, a glass-to-ground house is the most versatile because of the amount of light admitted. It is easy to provide shade when wanted but no simple matter artificially to match the intensity of daylight. When a framework is designed to fit on to a brick or concrete base wall, it is best to have the base built by a professional bricklayer unless you are reasonably skilled.

SPECIAL DESIGNS

For some groups of plants, and for certain growing purposes, special designs of greenhouse are available. Some of the designs already mentioned would be suitable for some of these purposes. A glass-to-ground house is ideal for cacti and other light lovers, such as tomato plants and carnations. For perpetual flowering carnations, special carnation houses are made which give plenty of light, adequate height, and efficient ventilation. Base-wall greenhouses can be kept warmer and are hence suited to all particularly tender subjects, such as orchids and tropical pot plants. They are also useful for propagation needing moderate warmth. Designs with a base wall fitted with staging on one side, and glass-to-ground on the other, to accommodate tall light lovers, such as tomatoes, allow a wide range of plants to be grown happily in the same greenhouse. For alpines there are special greenhouses with plenty of ventilation and high staging to bring the dainty plants nearer the eye. These examples emphasize the importance of deciding what to grow before choosing a greenhouse. However, a purpose-designed greenhouse is not usually necessary. For example, various orchids, alpines, and other specialist plants will normally be quite happy in a mixed greenhouse if treated with reasonable care.

CONSERVATORY DESIGN

Many of the basic comments made about greenhouse design also apply to conservatories. It has been pointed out that the lean-to is the favourite shape. If it can be sited where there is a direct communicating door giving easy access, a conservatory becomes almost an extension of the house. A design to blend architecturally with the dwelling is usually desirable. There are firms that can make reproduction ornamental structures if this would be more fitting.

Many ordinary ‘greenhouse’ lean-tos can be used as conservatories. Some people try to convert garden-room or home-extension buildings to conservatory use. But many of these have flattish roofs made of plastic which tend to collect dirt, dead leaves and the like, and need constant cleaning. They also exclude too much light, which limits the range of plants you can grow successfully.

If your house is a bungalow, be careful to check that any prefabricated structure you propose to buy is not too tall. The choice is often limited by the height of the house.

Lean-tos, designed for conservatory use, often have extra-wide sliding doors, allowing easy access to the garden. This is useful if you intend bringing in garden furniture or using the space as a patio extension. Some designs have partial or complete double glazing, others have curved glass pane eaves, an attractive feature which is now-becoming popular. Structures with toughened glass are also available. This would be a wise choice where there are small children or elderly or infirm people. Where a conservatory is to be used for sitting out, as a place of relaxation, a design with a base wall may be preferred as it gives more privacy. The best conservatory design for a particular site is also, to some extent, governed by the direction in which it is to face.

CHOICE OF SITE

GREENHOUSE ORIENTATION

A square or rectangular greenhouse is best sited with its ridge, or length, running as near as possible from east to west. This gives maximum entry of sunlight in winter, when the sun is low in the sky, and hence maximum solar heat. In summer it is only necessary to shade the south side.

LEAN-TO AND CONSERVATORY ORIENTATION

The environment in a lean-to is greatly influenced by the direction it faces. A southern aspect gives the warmest and brightest conditions. In summer, shade must be given, or the greenhouse used for only warmth- and sun-loving plants. A north-facing lean-to tends to be chilly in winter, and is generally shady, but provides perfect conditions for most popular decorative plants, which prefer cool conditions. An eastern aspect warms up rapidly as the sun rises. A westerly facing lean-to gives warm conditions overnight. How a lean-to is used, therefore, obviously depends on orientation, and common sense should be applied in making the best use of the environment provided.

FOUNDATIONS

Suppliers of greenhouses usually recommend the most suitable form of foundation. When special kerbing is offered, this is a wise purchase. The ground must always be firm and level. Some modern alloy structures use a ground-anchor system. The frame is erected on firm level ground, and holes about the size of a bucket are dug at intervals around die base of the frame. Alloy anchors are then bolted on so that they drop down into the holes, which are then filled with concrete. This is the method most people find easiest to carry out. Another simple way to make a foundation for a small greenhouse, is to dig a trench about 150mm (6in) deep and 150mm (6in) wide to match the base size, and then fill it with a rather liquid concrete mix, which will find its own level.

It is worth taking trouble over constructing a sound foundation. If there is movement or subsidence after erection the glass may crack and the entire structure may have to be re-erected.

THE SITE

In a small garden there may be little choice of site for the greenhouse. For the convenience of running services, such as water, electricity or gas, a site near the dwelling house is desirable. An open, bright position should be chosen whenever possible. Avoid close proximity to large trees, especially evergreens. However, a wall, fence, or hedge, on the side of the prevailing wind and distant enough not to cast shade, makes a useful windbreak. Avoid wet and waterlogged ground, and places where frost is known to form.

STRUCTURAL MATERIALS

GLASS VERSUS PLASTIC

There is no doubt that, at present, glass is the best choice if a permanent long-lasting greenhouse is wanted. Glass has excellent solar heat trapping properties. You do not need to know much about the technicalities to appreciate this. Go into a glass greenhouse on a bright day when it’s freezing outside and you will immediately feel the difference. In plastic structures the temperature can fluctuate widely with changes in weather conditions. With glass it’s usually possible to maintain a steadier environment, and any artificial warmth provided is more easily retained.

Although some of the more rigid plastics are fairly long lived, none is as permanent as glass. Plastics have a tendency to become brittle with age, warp or crack, and some lose transparency or become tinted. Many plastics also look less aesthetically pleasing- they do not have that sparkling clarity. Plastics are also very soft compared with glass and soon scratch or become abraded by wind blown grit. Once dirt becomes ingrained, cleaning is difficult or impossible. There are a number of different types of plastic now used as glass substitutes. The best are, however, quite expensive.

A problem that arises with plastic is condensation. Water collects in droplets on a plastic surface and does not form a transparent film as it does on glass. The droplets may cut down light entry and raise humidity excessively. They can coalesce, producing annoying drips. Corrugated plastic roofs are particularly troublesome in this way. Despite this criticism, plastics do have their place and when used with common sense can be a better choice than glass in some cases.

An important property of plastic is safety. It may be a wise choice where there are small children or elderly people around or where structures are at risk from vandals. Plastics can also be used to partially ‘glaze’ a structure. They should be fitted where glass is most likely to break.

Plastic’s lightness makes it easy to move and it is therefore useful in places such as a vegetable plot, where the entire structure can be moved as part of an annual crop rotation. Polythene houses are particularly useful in this respect. They are also convenient for crops that need little more than protection from the weather, or temporary accommodation.

Polythene and other plastics used for growing, must be of the ultra-violet light inhibited kind. This ensures maximum life when the plastic is exposed to sunlight. The letters ‘U.V.I.’ are what to look for. Even so, polythene is relatively short lived and must be replaced every two to three years. The best models are designed so that a new polythene cover can easily be slipped over a sturdy metal or timber frame.

TIMBER FRAMEWORK

Timber is the traditional material and has been used for many years, but it has numerous disadvantages, such as its liability to rot or warp in a warm, damp greenhouse environment. However, timber has attractions: it’s easy to work with, it’s easily adapted and has a pleasing appearance. Its durability and the maintenance it needs depend on the quality and type of wood.

So-called western red cedar is reasonably priced, pleasing to the eye and remarkably resistant to rot, although HOI immune, as is sometimes suggested. It’s advisable to have the framework chemically treated by the supplier before purchase, either with an anti-rot preparation or water repellant. All timber frames will need periodic painting or treating with preservative to keep them in good condition. For large greenhouses this can be inconvenient, expensive and time consuming. Be careful to use preservatives that are harmless to plants. Never use creosote.

Most traditional timber frames need linseed oil putty for glazing, although some special glazing mastics, which never set really hard, are to be preferred (see below). In some models the glazing bars are slotted and the glass merely slides in with no need of putty. This system produces a surprisingly watertight fit.

METAL FRAMEWORK

Metal greenhouses are often said to be ‘cold’ when compared with timber. It is true that metal is a much better heat conductor than timber but, since it’s stronger, much less of it is necessary and more solar energy is therefore admitted. In fact, there is usually little thermal difference between timber and metal constructions except in houses with base walls, where there are large areas of untagged metal. This should be strictly avoided. Large metal surfaces conduct away much valuable heat and become ‘refrigerating’ plates in frosty weather.

Although galvanized steel is sometimes employed, especially for the frames of polythene houses, the preference these days is for aluminium alloy. This material is extremely strong, and produces a framework that is light and easily transported. Erection is quick and simple, and dismantling, should the structure require removal at any time, is also easy. Aluminium alloy will not corrode, rust, rot, warp, or suffer attack by boring insects. It needs the minimum of maintenance and should retain its strength and quality for more than a lifetime.

If you don’t care for the metallic appearance of alloy, you could buy a model in one of the newer white, green or bronze finishes. These look particularly good in conservatories. Unadorned metal will eventually form a dull grey coating that will protect the metal from further decay.

GLAZING

Glass should be of good quality and clear. For most general purpose glazing, it should be about 3mm thick (24oz/sq ft), but for some purposes it should be thicker or toughened, particularly where there is a greater need for safety, such as in conservatories. Where curved eaves are fitted, they should preferably be made from glass rather than plastic, although this is more expensive.

With a metal framework, linseed oil putty should not be used. A mastic that never sets hard, and hence allows expansion and contraction with temperature changes, should be employed, otherwise the glass is likely to crack. Many metal-framework houses now have glazing systems involving clips, strips, and plastic cushioning.

When glazing with putty or mastic, you need only bed the glass – do not put putty or mastic on top of the glazing bars as is done in domestic glazing. Also, you only need about 10mm (0.4in) of overlap between the panes. An excessive overlap collects dirt and algae.

FEATURES TO LOOK FOR WHEN CHOOSING A GREENHOUSE OR CONSERVATORY

General strength and rigidity of frame

Strong ridge bar

Plenty of ventilators

Built-in guttering

Sliding doors

Good foundations – either kerbing or the ground-anchor system

Provision for the addition of an extension or partition

Well-fitting doors and vents – no gaps that might admit draughts

The roof, especially that of a lean-to, should have a moderately steep slope Lean-tos designed for bungalows should be of the correct height

EQUIPPING A GREENHOUSE

Before a greenhouse is ready for use, a certain amount of basic equipment, such as ventilators and staging, has to be fitted. It is also worth thinking about automatic watering, and, if the greenhouse is to be used for raising tender plants, some form of heating.

VENTILATORS

The provision of ventilators is a matter that should be checked at the time of buying a greenhouse or conservatory; some suppliers consider them as extras. Most alloy greenhouses can be fitted with as many ventilators as you like. The basic number is usually included in the price. Always make sure that you have enough to ventilate the interior freely. It is better to have more than necessary; you do not have to open them all together, and a good distribution around the structure allows you to use them according to wind direction. The average 2.5 x 3m (8 x 10ft) greenhouse has at least two ventilators, one on the roof and one at the side; the number should be increased in proportion.

The conventional hinged ventilator with a stay bar is still used in most timber houses, but some designs incorporate sliding vents. Alloy structures are now being increasingly fitted with louvred vents but make sure these are tight fitting. Ideally, the greenhouse ventilation system should be installed so that the roof ventilator is positioned as high up as possible, and the side ventilators are at ground level. This allows warm air to flow out freely at the top while cool air is drawn in from below. However, when there is staging or base walls, the vents are normally positioned just above them.

DOORS

Sliding doors are now often fitted to both timber and alloy structures, but the design of some leaves much to be desired. Metal doors can freeze up in winter and some have gaps which let in draughts. Doorways should be wide enough to allow free access for wheelbarrows or, in the case of conservatories, to admit plants in large pots or even garden furniture. Larger structures often have double doors. Sliding doors can be used as extra ventilators, since they are more easily adjusted than the hinged type. Where there are children, doors should be lockable for their safety.

GUTTERING

This is a highly desirable feature, since the constant shedding of rainwater around the perimeter of a greenhouse or conservatory can, in some cases, cause subsidence of the foundations because of the solvent action on the soil. The water can also seep inside, causing cold damp conditions. Many alloy structures are now equipped with built-in guttering, but, if not, plastic guttering is easy to fit to any type. The rainwater should be led away to a proper soakaway at a convenient distance from the greenhouse or conservatory or collected in a butt. This water cannot be used for greenhouse irrigation. Today, we grow our plants in special composts which are clean and relatively sterile and to use rainwater collected from roofs is utter folly. It’s a ‘soup’ of weed seeds, slimes, algae, pests, and diseases. Roofs can also collect weedkiller drift, which, even in trace amounts, can ruin greenhouse crops.

STAGING AND SHELVING

To make the most of space, some staging (benching) and shelving is needed. Sometimes this is at least partly supplied with a greenhouse and is included in the price, but it may be considered as an ‘extra’. Timber greenhouses are often equipped with conventional slatted wooden staging. Alloy structures have metal supports topped with a variety of materials, including stout wire mesh on which pot plants are stood. In both cases shelving is generally of similar construction. Check that staging is strong enough to take a reasonable weight and that the greenhouse framework is similarly strong enough to support shelving laden with pots.

Staging should be covered with a layer of fine shingle or similar material to retain moisture during the summer months. This helps to maintain the right level of atmospheric humidity. Slatted or mesh-topped staging will consequently have to be covered with polythene or some other suitable material to prevent the shingle falling through. Instead of shingle, capillary matting is often substituted. This holds moisture well and can be managed much more easily. In winter, it’s desirable that any moisture-holding covering be removed to give dryer air conditions and encourage good air circulation. Slatted or mesh-topped staging can at this time be particularly beneficial.

Often it is useful to set staging along just one of the sides. A popular position is along the north side, leaving the south side free for tall plants. In many cases, modern staging is designed to be portable, and may also be adjustable for height and adaptable into different shapes and configurations. It is also available in tiered or stepped forms, for displaying plants in a conservatory, for example.

FRAMES

These are very useful adjuncts to the greenhouse, where they can relieve space for growing on the shorter plants. They can be used for the more hardy plants and for hardening off bedding plants. Frames can be economically warmed with electric warming cables and can be used inside the greenhouse for raising the temperature of a limited area. They make excellent large propagators treated in this fashion. Whether they are used outdoors or in the greenhouse they are best situated with a northerly aspect, where they get little direct sunshine.

SERVICES

Electricity, water, and natural gas are all very useful in the greenhouse. In all cases the work of installation should be done in consultation with the authorities concerned and by professional contractors. Electricity is very useful for operating gadgets as well as for lighting and may be used for heating. Special fittings designed for safety in damp greenhouse conditions are available. A nearby supply of clean mains water, which can be used for automatic watering if desired, is also worthwhile. Natural gas is now becoming an important fuel for greenhouse heating.

CONSERVATION AND COST

The cost of raising the temperature of an average-sized greenhouse or conservatory to somewhere between frost free and 10 C (50 degrees F) should not be prohibitively expensive. There are many ways in which heating costs can be kept to a minimum. A bright site will ensure maximum free solar heat. Shelter from excessive wind also cuts heat loss. It’s essential to eliminate all sources of draught, such as gaps in the structure, ill fitting doors, vents, and glass. Except in some conservatory designs, double glazing is either very expensive or impractical for certain technical reasons. Lining with polythene is a simple and inexpensive way to achieve insulation that is almost as good. Clear polythene film, or preferably ‘bubble’ plastic which is sold especially for the purpose, will prevent at least 40 to 50 per cent of heat loss. It must be put up so that it encloses about 20 to 30mm (0.75 to 1.2in) of static air between the glass and the plastic. Special devices are available for fitting the plastic to metal framework, but on timber houses drawing pins can be used.

ASSESSING HEAT REQUIREMENTS

When installing heating equipment bear in mind that it must have a heat output to match heat loss when outside conditions are coldest. This can be roughly calculated from the size of the structure, the type and surface area of construction materials used, the minimum interior temperature desired, and the lowest outside temperature expected. Given these figures, suppliers of greenhouse heaters will recommend equipment with the most suitable rating in terms of British thermal units per hour, or wattage, in the case of electricity. It is most unwise to buy heaters without making this check first.

ELECTRICAL HEATING

This used to be considered expensive, but it must be remembered that there is virtually no waste and it involves the minimum of attention, and there is absolutely no contamination of the greenhouse atmosphere, which means ventilation, and therefore loss of heat, can be reduced considerably. Non-flued paraffin- and gas-heaters sometimes produce fumes, and there is always condensation and excessive humidity owing to the water vapour produced on the combustion of fuel. With these heaters, some ventilation must be constantly provided, to keep the air fresh and admit oxygen for the fuel’s combustion. This means some waste of heat is inevitable, and therefore the fuel does not work out as cheap as might be supposed.

Electric fan heaters are very popular and give excellent distribution of warmed air. However, be sure to install a model in which the fan and the heater are both controlled by the thermostat. Models in which the fan runs all the time and only the heater is thermostatically controlled are available, but in these the air continues to circulate after it has been warmed and consequently cools down more quickly. These systems are more expensive to run, but lining the greenhouse as described will reduce heat loss.

Tubular heaters are also favourites. They are best distributed evenly around the greenhouse and not banked all in one place. They are better positioned near the central pathway on both sides, than close to the greenhouse sides.

Convector heaters give moderate circulation of warmed air in the same way as fan heaters. Surprisingly, they are not often installed, but are nevertheless a good buy.

Warming cables have many uses. They can be used in beds or on limited areas of the staging to provide economical localized warmth. They are invaluable for gentle forcing of winter vegetable crops, and they can be used in frames or cases to warm small areas to a higher temperature than the greenhouse generally. This is especially useful for propagation. For the home greenhouse, special warming cables are sold. These should be used strictly according to the manufacturer’s recommendations.

Only electrical heaters especially designed for use in the greenhouse should be used. Domestic equipment can be extremely dangerous in damp conditions and must not be used.

GAS HEATING

Piped natural gas is probably the cheapest fuel at present, and bottled gas the most expensive. Both can be used as fuel in the special greenhouse gas heaters that are now available. A high degree of thermostatic control is attainable with gas appliances, thus reducing heat waste. However, constant ventilation is essential. When the heater is not in operation and if weather permits the greenhouse should be freely ventilated.

PARAFFIN HEATING

The wick-type paraffin heater was at one time widely employed, but it is not easily controlled thermostatically and the fuel is expensive. Considerable waste is inevitable unless one is constantly at hand to adjust the wick according to weather conditions. Even so, this form is so reliable that it is a good idea to have an appliance at hand in case of an emergency, such as a breakdown in the main heating system or an exceptionally cold spell of weather.

If used as the main source of heat, the remarks made for gas regarding ventilation apply. Condensation is often excessive. Make sure you buy a properly designed greenhouse heater with adequate heat output. Check also that the BthU/hr rating is satisfactory. This is most important, since heaters are often purchased with outputs that are far too low. Gadgets for automatically topping up oil reservoirs are available, and these cut the chore of filling. The best type of greenhouse paraffin heater is a blue-flame type with a circular wick. It gives efficient combustion and is less liable to produce smells and fumes, provided you learn how to light the burner properly and obey the maker’s instructions.

PIPED HOT WATER AND CONSERVATORY HEATING

Hot water pipes were often used at one time. They operate more efficiently where higher temperatures are required. They can be fuelled by solid fuel, oil, or gas. In conservatories or lean-tos, hot water heating can sometimes be run in from a domestic central heating system. It is essential, however, to consult a heating engineer beforehand to make certain the system can be extended. Pipe heating is not now commonly employed for the average small home greenhouse. Even so, a modern boiler and equipment are much easier to install and operate than earlier types.

AUTOMATION

Thermostatic heating control is an important feature and should always be looked for, when buying equipment; the more accurate the thermostat the better.

There is now much automatic watering equipment on the market. One of the best systems employs special capillary matting which is kept constantly moist. This is spread over the staging and the plant pots are pressed on to it, uncrocked, so that moisture can pass into the potting compost from the matting. Various trickle irrigation, overhead misting and spraying systems are also available but have more restricted, and sometimes specialized, applications. Generally, automatic watering is well worth considering, especially if you have to leave the greenhouse or conservatory unattended for long periods.

Ventilation can also be efficiently controlled by special greenhouse extractor fans operated by a thermostat. They should be of the type designed to prevent back draught. The size of fan, and the volume of air moved, depends on greenhouse size. Consult the supplier before buying and installing. Because fan-ventilated greenhouses are liable to dry out very quickly, some form of automatic watering or humidity control is a wise addition.

For the ordinary home greenhouse, automatic ventilator operators, controlled by temperature change, can be simply fitted. They need no electricity and require little attention. They are now very popular.

Automatic shading can be achieved using motor-operated blinds controlled by photoelectric cells. Unfortunately, this system is extremely expensive and is only used for special purposes.

GROUND SOIL, BEDS, FLOORS AND PATHS

SOIL SICKNESS

Crop rotation and the reasons for it are familiar to most schoolchildren. It is a practice followed by most outdoor gardeners – yet it is often promptly forgotten in the greenhouse. For the average small non-commercial greenhouse it is best not to use the ground soil for growing. Reasonably good results may be obtained for about two years or so, but after that what is known as soil sickness nearly always sets in. This condition produces serious plant deterioration due to a build up of excess unbalanced fertilizers, waste biochemical products from plant roots, and possibly pests and diseases. Under the cover of a greenhouse, where the soil is unexposed to weathering, ‘sickness’ soon becomes a problem, even if some crop rotation is carried out. Flooding the soil each year to wash out salts, sterilizing, or changing the soil, can help to overcome the trouble. But these measures are inconvenient, laborious, and rarely entirely satisfactory.

It is wise, therefore, to ignore the ground soil and grow everything in a suitable compost.

GREENHOUSE FLOORS AND PATHS

When the ground soil is not used for growing, a greenhouse can be erected almost anywhere on a sound flat surface. Concrete, asphalt, paving slabs, and the like make suitable floors. However, if there is no drainage, water may collect in puddles when the greenhouse is damped down. Where a greenhouse is erected on soil, as is usually the case in the average garden, a simple effective floor can be made by levelling and firming the ground, and then strewing it with clean shingle or gravel, as used on driveways. This holds plenty of moisture during summer and will maintain atmospheric humidity without puddling. It is cheap and reasonably attractive.

Most small greenhouses are given a central path but, in houses more than 3m (10ft) wide, there is usually room for an additional central run of staging with a path on both sides.

CONSERVATORY FLOORS

Most people prefer a conservatory floor to be decorative or at least to be in keeping with the surroundings. Whatever is used should be un-affected by water, although, in conservatories, damping down is done with more care and discretion. Vinyl flooring, as used in domestic kitchens (but not cushion backed) lasts well provided it’s put down on a perfectly smooth surface. Special self-levelling surface mixes for application to concrete bases are available. Ceramic floor tiles can be used, but plastic flooring tiles are liable to lift if water gets under them.

BEDS

Where beds or borders are wanted in greenhouses or conservatories, dig trenches of the required size and depth and remove the soil. Line the trenches with polythene sheeting. Make slits at intervals for drainage and fill with a suitable compost. Raised beds or borders can be made similarly by draping polythene over a frame of boards of the required height. The polythene isolates the compost from the ground soil so that it cannot become contaminated, and the more restricted volume of compost is easier to replace. It is also easier to maintain a better fertilizer balance. Electric warming cables can be used in the beds to provide localized warmth. Beds and borders such as these can be filled with peat for die display of ornamentals. The plants are left in their pots and plunged to just over their rims. This also allows plants to be easily changed about according to the season. You should always try to arrange them in an attractive way, especially when using the beds and borders for display. Raised beds could be of special benefit to some disabled people.

SHADING

Nearly all greenhouses need shading at one time or another. An unshaded structure in a bright position can become so hot in summer that all the plants will be severely damaged or even killed. Blinds and special paints for application to the exterior glass are employed. The former can be expensive but are usually preferred for conservatories. For this purpose, slatted exterior blinds are best. Flimsy blinds of plastic or textile are liable to blow away in a strong wind. Interior blinds may give protection from direct sun scorch, but do little to keep down temperature.

Shading paints can be difficult to apply and to remove. However, a recently introduced electrostatic type, sold under the name Coolglass, is much easier to handle. It is a powder and mixes instantly with water. It can be sprayed or brushed on and is quite fast, even in torrential rain; yet it wipes off easily with a dry duster.

THERMOMETER

To help maintain the correct temperature in a greenhouse, a maximum and minimum thermometer should be installed. This type of thermometer has indicators which record the lowest and highest temperatures reached since die thermometer was last set. This is useful to show the temperature range in your absence. There are various designs. It is a small item – but a vital piece of equipment for proper management of greenhouse temperature. If you have separate sections, each needs its own thermometer.

23. August 2013 by admin
Categories: Featured, Garden Management, Top Tips | Comments Off on Plants for Greenhouses and Conservatories

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