Light is the most important factor in growing crops under glass, and glasshouses must be designed to give maximum light transmission at all times of the year.
There are two types of light:
1. DIRECT SUNLIGHT and
2. DIFFUSED SUNLIGHT (sunlight scattered and reflected by clouds and dust).
In one year, about half the light reaching the earth is DIRECT SUNLIGHT, and half is DIFFUSED LIGHT.
Light ravs travel in straight lines. The proportion of light passing through glass is dependent on the angle between the glass and the light rays. This angle is known as the ANGLE OF INCIDENCE. If this angle is less than 40 degrees, then most of the light passes through the glass. On the other hand, if this angle is more than 40 degrees, then a high proportion is reflected by the glass rather than passed through.
Direct sunlight. In the period autumn to spring, all the sunlight comes from a southerly direction and the average angle of the sun above the horizon from mid-October to mid-February is 12 degrees. It is therefore important that the orientation of the glasshouse takes advantage of winter light; it is vital that maximum use is made of winter sunlight to optimise plant growth and development in the protected environment. More of this later.
FACTORS AFFECTING THE AMOUNT OF LIGHT ENTERING A GLASSHOUSE
1. Orientation of glasshouse. An ‘EAST-WEST’ house lets in more WINTER SUNLIGHT through its south side due to the favourable angle of incidence.
2. Width of house. For an EAST-WEST house, the width should be approximately 4 times the height of the eaves to let in the maximum of winter sunlight.
3. Eaves height. The higher the eaves, in an east/west house, the more sunlight is admitted through the south side.
4. Distance between houses. Optimum space between east-west houses is 4 times the height of the ridge, minus half the width of a span.
5. 5. External factors. Buildings and trees cast long shadows in the winter.
6. Glass size. The larger the sheets of glass, the fewer glazing bars are needed, and the fewer dirty overlaps.
7. 7. Opaque building materials. Glazing bars and the structural members of the glasshouse need to be kept to a minimum so that there is little reduction of the amount of light getting into the glasshouse. Of course, any reduction in the number of structural members needs to be consistent with adequate strength.
8. Clean glass. A 10% loss of light in winter means 10% loss of plant growth rate.
9. Cleaning glass. Materials available; none of the following are really satisfactory.
10. I) Hydrofluoric acid. Cannot be recommended because of health hazards and
11. because it etches the glass.
12. ii) Oxalic acid. 3%. With hard water, these form a deposit of calcium oxalite, which further reduces light transmission.
13. iii) TeepoL 2%. This is a detergent/wetting agent, and is better than plain water.
14. Reflecting surfaces. Aluminium or white painted wood is better than bare wood. Alloy houses have improved light reflection from the structural members which do not need painting.
15. GLAZING MATERIALS
16. Glass is the most common material used. Horticultural glass is 3mm thick. Glass transmits about 90% of visible light. It only transmits 1% infra-red radiation. (It is this long wave radiation which is the heat loss from soil/plants/, and this heat is trapped by the glass so much better than by single polythene sheets.
17. Polythene transmits 85-90% of visible light when new; last 2 to 3 seasons before breaks down and splits. Transmits 77% infra-red.
18. P.V.C. (Polv-vinyl chloride). 85-90% visible light transmission, when new and clean. Lasts about 3 seasons. Only transmits 12% infra-red.
A Mansard house – uneven span
Glasshouse glazing bars cast a shadow. If the glasshouse is orientated east to west, this shadow is less serious than in the north/south house where with the low winter sun angles the light lost may be nearly 100% at times.
Vz Bubble shaped houses are probably the ideal for light admission. Tunnel houses also have uses in addition to those directly associated with plants I.e. as protection for equipment, stores and people.
Glasshouses were always designed with the intention of allowing as much light as possible inside, and very little consideration was ever given to heat retention. For a long time growers of A. Y.R. (all-year-round)saved fuel by using their black-out sheets over the crop at night. This practice led to the design and use of thermal screens. The higher the winter temperature setting, the more fuel was used, and the greater the justification for thermal screens.
The system relies on providing a completely enclosed skin over the crop and heating system during the hours of darkness. Many different materials have been used for this purpose and all have given on average a saving of 20% to 25% on fuel.
If left on all day, a thermal screen can result in light loss of 10% to 14%, but on very cold days in the winter months this will be acceptable in relation to the fuel savings made. Thermal screens are drawn over the crop at night to act as a barrier against heat loss. As they enclose the crop and heating system, the natural air exchange which occurs through leaks in the glasshouse structure are reduced andis increased. The savings made will depend on several factors:
(a) the physical characteristics of the screening material,.
(a) reduction in heat loss surface area,
(c) external wind speed over the glasshouse, and
(d) the completeness of the seal.
There are several materials on the market available for the use as thermal screens, some are for night time use only, others are for night time and day time shading. There is also a choice. of materials which are permeable or impermeable. The permeable types allow a lower humidity level below because they permit a proportion of warm air to pass through, however they are obviously not as good at total heat saving as the impermeable screens.
The permeability of the screen material is important because an excessive increase in humidity levels can cause more fungal diseases in a crop.
Glasshouses may be made in many shapes but the fundamental aim is usually the same – to provide a warmer, sheltered environment. The problem usually occurs in the winter – and for some growers this is not a problem as they may only grow summer crops, but for the plants inand growing in the winter the key provision which is hardest to come by is winter light.
Protection for plants is also available from the traditional lath house – a lath was a thin strip of poplar used with plaster to construct walls. Used without the plaster the laths were nailed on to batten wood to provide shade and shelter for plants. A lam is no longer available off the shelf at the builders’ merchants but a similar effect creating a shade hall, or protection from wind is possible with Paraweb and some other products. There are other products of a similar nature made of polythene-like plastics in the Netlon range. The protection given relates to wind speed reduction, lower light levels, a reduced rate of water loss and some protection from extremes of temperature.
A lath-house can be used for shade in order to create improved conditions during hot summers and are ideal for particular crops which suffer from-scorch easily or that will grow better in cooler conditions. In particular, many herbaceous plants, hardy ferns and Astilbes. These type of lath-houses have been used for plant collections in large public gardens and especially as houses in garden centres.
Ground hugging films like the ‘fleece’ system are floating film covers for plants which provide the plant with a reduced windspeed, higher temperatures and some opportunity to grow. Market gardeners may produce very early bunched carrots and early outdoor lettuce on favoured sites by such techniques.
This type of film has also been used to encourage faster and earlier growth on re-sown sports turf areas to obtain a thick sward prior to winter on September renovated cricket wickets. The film/fleece is very permeable as it allows rain to pass through and oxygen exchange. Being very lightweight there is no problem with disease outbreaks beneath this type of cover because the air movement prevents stagnant conditions developing.
Some heat is lost by the warm air heating the glass which in turn warms the outside air, but this rate of heat loss is much slower than the heat transfer by short wave radiation.
Double glazing with glass has never proved very popular for glasshouses because of its high cost. There is the cost of the extra glass, and also the cost of a stronger construction to bear the greater weight of glass. Consequently more shade-casting structural members are required.
However some of the advantages of double glazing are achieved by using a polythene lining since this traps a volume of air close to the glass and the rate of heat transfer from inside the glasshouse to outside is much reduced. Draught may also be reduced and this improves heat retention.
Polythene by itself is a popular cladding – it is cheap and does not break like glass, but it does become brittle, discoloured and tears after a time depending on its thickness and situation (1 to 3 years seems to be about the life of a 600 gauge polytunnel cover). Polythene does not trap longwave radiation so the warmth inside quickly radiates out: however, if the polythene is wet inside due to condensation there is a much improved heat retention.
Double skinned polythenes come as bubble polythene – useful but expensive systems of glasshouse inner linings – and as twin-skinned polythene tunnels where a very modest fan inflates the space between two sheets of polythene. This may be one of the best systems because the outer skin is usually lifted clear of most of the points at which it could overheat or chafe on the metal support hoops and so may last longer. The inner lining also may last longer than it would have on its own. These tunnels are worth heating and their light structural members mean very good winter light penetration.
Rigid plastics like the twin and treble walled polycarbonates have a future because of their heat retention, light weight and cost effectiveness. Some important installations have however suffered from hail/gale damage (as indeed have other polythene and glasshouse structures in the South East in 1987).
Polycarbonate has gained in popularity as it is lightweight and extremely strong. Transmission rates on approximately 80% as opposed to glass at 90%. The big advantage of these products is their ability to insulate the glasshouse and retain heat overnight thus saving high cost heating bills.
Acrylic sheets are also used with a transmission quality close to that of glass.
Modern nurseries are so highly mechanised with artificial lighting, automatic ventilation and irrigation controls that electricity would be essential. The cost of installing electricity must be allowed for in any budgets.
Accessibility and Transport
Modern nurseries are often situated near main roads or motorways because of the importance of distribution throughout the country. Previously, the site of a nursery was chosen for the suitability of the soil in the area, but now many crops including tomatoes, pot plants, and shrubs are grown in containers or other artificial media, such as, nutrient film, rockwool and vermiculite. The quality of the local soil is irrelevant to the crops grown.
In the early days glasshouses and frames were the sole providers of protected cropping situations, but the advent of polythene has extended the areas which may be protected.
The basic concept of a glasshouse’ is its capacity to trap the longwave radiation given off by warm ‘bodies’ like people or pot plants all below ‘red heat’.