Plastics for Greenhouses and Frames

New Zealand Journal of Agriculture, Volume 100, Issue 5, 16 May 1960, Page 465

Plastics for Greenhouses and Frames

By

W. BRANDENBURG,

Horticultural Instructor, Department of Agriculture, Christchurch, and S. CHALLENGER, Lecturer in Horticulture, Canterbury Agricultural College QLASTIC films are being increasingly used by commercial horticultural ■ producers. This article discusses the information gained in New Zealand to date on the use of transparent plastic film as a substitute for glass on greenhouses and frames. As the information is incomplete as yet and does not rest on solid experimental evidence, the article is intended to be a guide only to those planning to use plastic films so that they may have some idea of what can be expected from their use.

KJJANY types of chemically and J- physically distinct plastics are available today. The following are transparent plastics in commercial production, though not all are in the form of film, which is the most useful horticultural formulation. Trade name Material “Garnite” “Visqueen” Polythene “Thermoplus” “Arnathene” “P.V.C.” Polyvinylchloride ■■Perspex" P “* ‘‘Cellophane" «~ted “Polystyrene” Polystyrene “Celawrap Q” Cellulose acetate

Of these materials, polythene is the plastic film most readily available in New Zealand. The uses of plastic sheeting in horticulture and any merits or disadvantages it has were investigated in the Hamilton and Christchurch districts. The main use so far has been for covering greenhouses and frames and making cloches, generally using clear polythene of 5/1,000 in. thickness as a glass substitute. The number of structures observed is unfortunately still small. At present eight structures, some with more than two years’ use, are being ' closely observed. The possibility of error in this kind of general observation is therefore still large, and the following report is confined to features that have been consistent and clear in all or most of the structures observed. Where possible these observations have been correlated with the recorded physical properties of polythene and other materials. Construction The way in which glass and a glasshouse are of value to the crop inside is worth discussion as a preliminary. First, the warmth from the sun and sky, as short-wave radiation heat and light, penetrates the glass and warms the soil, air, and plants inside the house. Secondly, the glasshouse insulates the materials within from heat loss due to convection. Thirdly, the glass itself is opaque to the reradiation of heat from within the house. Heat given off by the glasshouse contents as low temperature, long-wave radiation is reflected back by the glass and retained inside the house. (This is distinct from heat loss from the glasshouse due to direct conduction through the glass.) Thus, the heat increase inside a glasshouse is due to the minimisation of convection heat losses during the day and to the retention at night of heat reradiated from the soil and plants. Any substitute for glass must act similarly to be effective. Glass is brittle and heavy, though it has an indefinite life. Plastic films are light and pliable and their general adaptability makes them ideal for glasshouse construction. Polythene 5/1,000 in. thick, for example, is onesixtieth the weight of glass, so that a 100 x 30 ft house, which would require 6 tons of glass, would need only 2 cwt of polythene. This means that structures for plastic can be much lighter (and therefore cheaper) than for glass. Stability of House Nevertheless, it would be dangerous to assume that as the structure of the house or frame is lighter, the foundations can be lighter too. Foundations

of plastic structures have a reverse function to those of glass structures; they do not support so much as hold down. Skimping on the foundations can easily mean destruction of the entire house, and it definitely pays to trust nothing but a reasonably heavy, continuous concrete sill. Anything less substantial may fly off with the house in a gale. For plant frames the posts supporting the sides must be sunk well into the ground. The house must be well tied down to the foundations also, and it is here that an improvement on glasshouses can be made. Instead of a sill plate, which may rot, strips of heavygauge galvanised sheet iron can be used; the strips are folded lengthwise at a suitable angle and one side is bolted on to the concrete sill, the other side standing up to have the uprights of the house directly screwed on to it. All upright timber is kept an inch or more clear of the concrete sill. No dampcourse is needed and a source of decay is eliminated.

A “glass” house gets an important part of its stability from weight, but the plastic greenhouse lacks this weight and for that reason the framework must be adequately strong and well braced. As there is little weight to carry, however, most of the stresses will be pull and bending movements and a lot of the bracing can be done safely with fencing wire and wire strainers. Galvanised sheet iron can play an important role in bracing corners and simplifying construction. However (and this is an important consideration) the plastic covering must be nowhere in direct contact with metal parts of the structure. During summer metal parts, including even No. 8 gauge wire, will heat sufficiently in the sun to cause failure of plastic touching it. This happens even when the plastic carries shading. Ventilation Plastic houses are generally perfectly airtight and the air in them

tends to be humid. Condensation is a problem. Under direct sunlight the temperature in a plastic house also tends to be higher than in a glasshouse. Both these problems can, however, be greatly minimised by proper ventilation. Continuous side vents consisting simply of a hinged 6 in. board under the eaves, and a continuous ridge ventilator, if possible about 12 in. wide and of the type that lifts up completely, appear the best, and can be built into most structures easily. It has been shown that the houses can be kept free of condensation with this type of ventilation if handled properly. Frames seldom give ventilation problems.

The surface texture of plastics as well as the increased humidity causes drips. Water condenses on plastics in the form of droplets, not as a film as it does on glass. Consequently a plastic house should have a steeper roof angle to ensure water run-off and to reduce drips. Doors should be light, and if the more convenient sliding doors are used, they must be held firmly when shut. If they blow out, serious damage results quickly.

The size and shape of the bars on which the plastic is fastened are immaterial. Depending on support and length, a range of different timber sizes has been found satisfactory. The roof must be able to support as a minimum the weight of two men working close together, as the plastic must be renewed regularly, but extra strengthening is required if the roof also acts as crop support. Battening the plastic on to the bars, nailing at least every 6 in., appears to be the most satisfactory way.

The timber surface of the bars and battens where in contact with the plastic must be left rough sawn. Plastic slips readily from between dressed timber surfaces. It is also advisable to use a soft timber such as pinus, as its slight sponginess ensures a better grip. Rusty nails hold better than new ones, and, if available, the double-headed type may be preferred for easy replacement. Stapling the plastic or screwing the battens has not been satisfactory.

English-made plastic is 5 ft wide, and New Zealand-made 6 ft wide. Both are too wide to leave unsupported. Bars at slightly under 2 ft 6 in. and slightly under 3 ft respectively are most satisfactory. Characteristics of Plastic The life and cost of plastics are important. Polythene (0.005 in. film) costs about 2.8 d. per square foot and glass 20 x 20 in. is about Is. 4d. per square foot or five and three-quarter times the cost of polythene. A short life can be tolerated if the cumulative replacement cost is low.

It has been proved at Hamilton and Christchurch that polythene sheeting on the roofs of structures cannot be relied on to last more than one year. Slow oxidation gradually reduces the strength of the film and it then tears easily. Sheeting on the sides of houses appears to last longer and may last two seasons. Replacement of roof films is therefore an annual task and it should preferably be done on a hot day when the film has the greatest stretch. The season when the plastic is applied affects the life of the plastic, and films replaced in autumn may have a longer life owing to the reduced amount of ultra-violet light to which they are exposed in their early life. Oxidation occurs in the presence of ultra-violet light, and the rate of oxidation is roughly proportional to the exposure to sunlight. Unfortunately, the New Zealandmade types of plastic are all sold on rolls 3 ft or less wide. The 6 ft-wide kind generally used is sold folded double on a 3 ft roll. This fold is a cause of premature failure as the plastic splits along it. The effects of this can be minimised by applying the plastic to the house lengthwise, with generous overlaps, working from the bottom up as if weatherboarding a

house. The older method of applying it in strips vertically from top to bottom results usually in this fold running over long unsupported stretches. However, the most important peculiarity of polythene and its biggest disadvantage is that it offers little or no protection against radiation frosts. A series of observations of frost damage under polythene shows that quite a few growers who wanted to use the plastic as protection in winter have been disappointed in the results. This peculiarity may be explained by an examination of the heat and light transmission properties of glass and plastics. The range of ultra-violet light is less than 0.4 microns in wavelength; light used in photosynthesis, which is approximately the same as that visible to the human eye, lies between 0.4 and 0.75 microns. The warmth and light from the sun lie in the short wavelengths between 0.3 and 3.5 microns, while the heat radiated by low temperature bodies such as the earth is in the long wavelengths from 6.5 to 16 microns. Glass is transparent only to the range of short wavelengths between 0.32 and 3.5 microns, and then only to the extent of about 50 per cent. Polythene, on the other hand, is about

90 per cent transparent to almost the whole range of wavelengths between 0.23 and 16 microns. The diagram above shows that it does exclude transmission at about 3, 7, and 14 microns, but these points are only a small proportion of the range 0.23 to 16. It can be seen from this that glass is opaque to the long-wave soil radiation which occurs in radiation frosts, while polythene is 90 per cent transparent to it. The transmission of visible light is about the same in both, with polythene allowing the passage of more ultra-violet light than glass. Thus, though polythene admits adequate light for photosynthesis, it has no ability to prevent loss of heat which has been bottled inside a plastic structure during the daytime as a result of short wavelength heat transmission from the sun. However, it is interesting to note that a water film 0.002 in. thick retains radiation heat much better than polythene, so that polythene with a film of moisture over it is almost as effective as glass. The increased amount of ultra-violet light passing through polythene compared with glass is sometimes stated to be an advantage, but ultra-violet light is of little importance to plant life, except that it is slightly detrimental. Some of the more modern plastic films, such as “Kodapak”, “Linapol”, “Mylar”, and “Melinex” (which are not at present available in New

Zealand), have much better heatretention properties than polythene. All are expensive, howeveralmost the same price as glass—so that the future of these materials, assuming they have adequate weathering properties, depends on the extent to which their use can reduce the cost of construction of the framework carrying them. When heating plastic houses sources of heat relying on radiation, such as hot-water pipes, are practically useless according to New . Zealand experience. The heat radiated passes straight out of the houses. Probably hot air methods are the most efficient form of heating, since they rely on convection only. A fan working during cold nights without the addition of actual heat may improve frost protection. A plastic house is no worse than a glasshouse for conduction heat losses, and is probably better, from the point of view of convection heat losses, as there is no leakage through laps between panes. Growing Methods and Crops Growing methods in plastic houses may require slight modification compared with those in glasshouses. Reduced watering and greater ventilation will be required owing to the retention of water vapour, and the higher humidity may be a problem. Foliage or flowers should. not touch the plastic or decay is very likely. The following crops have been tried under plastic:

Tomatoes: Successful, subject to adequate ventilation and proper heating arrangements. Tomato leaf mould has been found to occur later in plastic houses than in glasshouses, possibly owing to the effect of the increased transmission of ultra-violet light, which is recognised to be detrimental to fungi. Cucumbers: Ideal. They are very much at home in the close, humid atmosphere under plastic. Lettuce: Successful, subject to adequate ventilation and disease control. Cut flowers: Very good for summer protection to ensure unspotted quality blooms. Structures with open sides are preferable. Bedding- plants: Heavy losses were experienced by the Christchurch City Council, which tried to overwinter such plants as geranium, iresine, fuchsia, etc., under plastic. High day temperatures, high humidities, and frost at night were too much for the plants. Pot plants: Successful with cyclamen. No trouble with damping off or spotting of the blooms. Summary In any judgment of the usefulness of plastic film on structures used for growing plants in commercial horticulture, economics play a decisive role. On the other hand, this side of

the picture is the most difficult to judge, as so many unknown factors are involved. We could, for instance, try to compare the costs of building a conventional glasshouse and a polythene house of similar dimensions. In both cases the cost is strongly influenced by the amount of work the grower can do himself or with his own staff. Also, variations in the actual purpose of the structure—for instance the requirements of crop support—have an important bearing on the strength and therefore the cost of the structure. There is not nearly enough information available to make a reliable overall estimate of the economics of the subject and any suggestions offered here must be taken with considerable reserve.

Cost of materials for a standard 30 x 100 ft glasshouse in Christchurch is between £6OO and £BOO. Cost of materials used in the structure alone of a plastic house of the same dimensions would be between £l5O and £3OO, depending mainly on the amount of crop support to be incorporated. In addition, the cost of the plastic alone for covering would be between £5O and £6O, and this would be an annual expense. As already stated these figures . must be taken with a large amount of reserve and they apply to the Canterbury district, only, no figures being available from other parts of the country.

They illustrate a trend, however, and that is: for permanent structures, because of the annual cost of recovering the houses with new plastic, a plastic house will in eight to 10 years have cost just as much as a glasshouse. On present knowledge it seems best to use plastic only in the field for which it is inherently most suitable and that is for light, temporary structures.

The use of plastic film for temporaryshelter could be very profitable when the reduction of convection heat losses or losses due to wind is more important than reduction of radiation losses. Spring or autumn use would be excellent to hasten a crop or prolong a season. Plastic - covered structures which could be moved from place to place would be of especial value to the beginner with very little capital. It is felt at the moment that it is in this field that plastics are most likely to be valuable, rather than as alternatives to glass in orthodox glasshouse construction. ACKNOWLEDGMENT The authors thank growers in the Waikato and Canterbury areas who have freely related their experiences and permitted observations in their plastic structures.

Photographs by

Mr R. C. Blackmore.