November/December – 2004
Author: Steven Carruthers
The Purpose of greenhouse covers is to separate the inside from the outside environment and to transmit as much light as possible. The challenge for the plastic manufacturers is to develop greenhouse films that last longer, and optimise light transmission and plant growing processes. STEVEN CARRUTHERS looks at a new generation of greenhouse covers specially engineered to improve light transmission and crop production.
Greenhouse film technology has improved significantly compared to 10 years ago, giving commercial growers access to plastic covers that last up to twice as long, improve light transmission, and are specifically engineered to lift crop production. This has followed a demand for technological improvements worldwide within the expanding greenhouse industry.
Unique among these covering materials are multi-layered polyethylene films, often referred to as ‘smart films’ that include UV inhibitors and colourants to filter or enhance light quality, and additives that offer anti-drip and anti-disease properties, as well as cooling and thermal benefits.
Among the new generation of smart films available on the Australian market are:’Kritifil’, manufactured in Greece by Plastika Kritis and available from Monbulk Rural Enterprises; ‘Celloclim’, manufactured by French-based Polyane and distributed by David Gill Greenhouses; ‘Duratough’, distributed by Redpath in New Zealand and Australia;and a range of films and screens from Svensson, distributed by Living Shade & Climate Control Pty Ltd.
LIGHT QUALITY AND PLANT GROWTH
In order to understand how smart films work, the effect of the solar spectrum on plant growth needs to be understood. Plant growth is stimulated by the presence of water, nutrients, temperature, carbon dioxide, and photosynthesis. During the process of photosynthesis, plants depend on the total amount of visible light, or more specifically, photosynthetically active radiation (PAR), the group of wavelengths of light between 400-700 nanometers (nm).
The distribution of individual wavelength intensities, not only the total combined PAR intensity, influence plant growth and development responses (photomorphogenesis). Wavelengths of non-visible energy known as infra-red (600-700nm) can inhibit stem elongation and branching of plants, and leaves are sometimes smaller and thicker. This results in a lessening of flowering in short-day plants, but promotion of flowering in long-day plants. Far-red radiation (700-800nm) leads to an increase in stem elongation, and eliminating the far-red part of the solar spectrum can produce shorter plants. Flowering can also be inhibited in some plants. At the other end of the light spectrum, UV wavelengths below 380nm have detrimental effects on plant growth and cause blackening and burning.
Ultraviolet (UV) light and excessive infra-red (IR) or heat radiation reduce the longevity and performance of plastic covers. The challenge for plastic manufacturers is to develop greenhouse films that limit UV degradation and IR heat radiation absorbency without interfering with light quality and the development of plants. The light transmission, performance and longevity of films can also be affected by placing inhibitors, stabilisers, colourants and other additives in the plastic, making them a less virgin resin base.
Plastica Kritis was the first company in the world to introduce three-layer greenhouse films (1983), which consist of three inseparable layers of different polymers and additives with useful properties to improve plant growth, protection and productivity.
It was also the first company to manufacture all its additives in-house that give their films special properties. This gives Plastica Kritis the flexibility of producing a wide range of films adapted to specific requirements. Their films are noted for toughness and longevity with optional features such as anti-dripping, anti-fogging, anti-disease, and other useful characteristics that transform the film into an active contributor to plant production processes.
Plastica Kritis recently introduced a new family of ‘super tough’ films that use high-strength polymers and offer additional benefits in areas with very strong winds. These films contain 15-30% more UV stabilisers in one layer of the film to protect the film from the harmful effects of UV light and heat for long periods. Film lifetime is guaranteed from one to five seasons, depending on the type of film and the climate under which the film is exposed.
Cooling and thermal effects
In Australia, there is a growing requirement for films that cut-down excessive heat during the day and maintain a cooler environment inside the greenhouse. Plastica Kritis manufacture special films that reflect and/or absorb the IR radiation, thus reducing the heat entering the greenhouse during the day. “The higher the outside temperature, the larger the temperature difference achieved by using these cooling films,” said Michael Arnott, the principal of Monbulk Rural Enterprises. “On cold days, these films do not have a notable adverse effect,” he added.
Plastica Kritis manufacture special three-layer thermic films that combine EVA (ethyl vinyl acetate) polymers and infrared additives that absorb IR radiation and reduce heat losses during the night. These films offer protection from frost and low temperature, and slow temperature drop to give higher nighttime temperatures, thus reducing energy costs for heating.
The most recent offering from Plastica Kritis is a clear cooling film that also has thermal benefits at night. The outer layer of the patented film has a UV additive that gives the plastic a metallic silver-grey appearance that is still quite transparent. The three-layer film reflects radiation heat away from the greenhouse, but still allows 75% of PAR light through. “Another strong benefit of this film is its very good longevity,” said Michael Arnott.
He added that this new film will cost about 10% more than standard cooling and thermic films.
Droplets that form on the inside of greenhouse plastics due to condensation reduce light transmission by 15-30% and have negative impacts on plant quality and growth. Condensation also increases the risk of disease.
Plastica Kritis manufacture anti-dripping films that eliminate droplets by forming a continuous thin layer of water running down the sides of the greenhouse. Some greenhouses capture this condensate for reuse.
These films are mainly recommended for well-ventilated and/or heated greenhouses with adequate inclination of the roof. The anti-dripping effect only lasts about two years, because the additives function by migrating to the surface of the film and are slowly washed out by water. Under certain circumstances, there is a heavy fog (mist) formation in greenhouses, which usually occurs at sunset and dawn. However, this fog is undesirable and could damage plants. Growers using these films need to ventilate and/or heat the greenhouse to remove the fog.
To allow the use of anti-dripping films, with all their benefits, without fear of the fog, Plastica Kritis has developed anti-dripping films with anti-fogging characteristics;however, no warranty is offered concerning their effectiveness and durability.
Disease control films
There are several ‘types’ of disease control films targeted at certain pests and diseases. All of them contribute to better IPM management inside the greenhouse. An important type is a UV-blocking film that works by blocking UV light up to 390nm. These films reduce populations of whiteflies, thrips, aphids and other greenhouse pests by omitting those wavelengths needed for insects to thrive and survive. Disease control films are sometimes called anti-insect films because of this feature. They are also called anti-viral films, because they reduce viruses vectored by these pests.
Yet another type of disease control film is the manufacturer’s latest release, a clear film that is 98% resistant to sulphur. It is well known that sulphur, halogen and other substances contained in pesticide residues that are diffused in film, react with UV stabilisers and deactivate them. Hindered Amine Light Stabilisers (HALS), a class of UV stabilisers widely used in greenhouse films, are particularly sensitive to such attack. Even nickel , which is relatively inert to attack from sulphur, is known to be sensitive to chlorine.
“Sulphur is a major contributor to plastic degradation and the life of the plastic is reduced by at least 50%. While most films use a standard class of UV stabilisers or nickel, this new film uses an additive that is almost completely resistant to sulphur, ” explains Michael Arnott.
He added that this ‘breakthrough’ film is expected to cost up to 20% more than other sulphur-resistant films on the market. However, replacement costs for film damaged by sulphur and other agrochemicals, will mean a short payback period. There are also crop health and productivity factors to be considered from covers that perform under specification owing to agrochemical damage. As sulphur condenses out, it also accumulates around greenhouse gutters, causing rust.
Plastica Kritis manufacture a range of special films that incorporate colourants (pigments) to modify the light spectrum entering the greenhouse. These photo-selective films make it possible to promote or retard plant growth, causing elongation or dwarfing of the stems.
They are mainly used by flower growers to improve colouration and promote special characteristics demanded by market fashions, such as long-stem roses. They are sometimes used by tomato and capsicum growers to enhance fruit colour and quality.
Monbulk Rural Enterprises was the first Australian company to trial films with red pigments for rose production. The Australian distributor offers a range of red and blue photo-selective films manufactured by Plastica Kritis, including a specific rose film.
For further information contact:
Michael Arnott, Monbulk Rural Enterprises Pty Ltd
Lot 8 Old Emerald Rd Monbulk, Victoria 3793
Polyane is the first company to develop bubble film and has developed a new family of smart films under the concept they call ‘The Plant Effect’ with an emphasis on improving the parameters for plant growth. Of special note is the Celloclim clear bubble plastic, a three-layer film manufactured to suit Australia’s harsh climate with its large temperature differences between day and night (the diurnal range). “The clear Celloclim is durable and perfectly adapted to all types of climate and every type of crop that we can find in greenhouses here,” said Polyane Export Manager, David Ribot.
Celloclim has been specially engineered with multi-layer co-extrusion technology that sandwiches an expanded polymer layer with gas micro bubbles between two plain polymer layers. This film favours photosynthesis by improving the light diffusion inside the greenhouse. Light passing through the bubbles break up the light inside the greenhouse into a multitude of light rays, bathing plants in intense but not direct light. According to David Ribot, the patented bubble film improves light quality. “The gas bubbles diffuse the light to give more uniform light radiation throughout the greenhouse, and the shadows below the plant canopy become less distinct,” he said.
Polyane call this “util” (from utility) or ‘useful’ light. Instead of giving 90% of available light on only 20% of the crop surface, the diffusing effect gives 85% of light on 80% of the crop surface. The multi-directional spread of light also eliminates growth towards the light source (phototropism).
Clear Celloclim includes a proprietary system of UV absorber additives that cuts the light wavelengths from 300up to 375nm, which helps to minimise the blackening of rose petals. The same wavelengths that burn sunbathers. Importantly, the additive doesn’t block those UV wavelengths responsible for plant colouration (375-400nm). “These wavelengths are responsible for brighter colours and better contrast in bicolour flowers, including roses.” said Ribot.
The introduction into Australia of the clear Celloclim bubble film follows six years experience by Polyane across a wide range of vegetable and flower crops, and in diverse climates – from north Ukraine to Spain, and from north to southern Africa. Polyane also produce their own resin granules and proprietary additives, and have the ability to ‘formulate’ greenhouse films for different climate conditions.
Polyane is a part of the ADELPRO group, one of the largest plastic extruders in Europe, manufacturing 65,000 tonnes of polyethylene film per year, representing a turnover of 100 million Euros. Fifty-five percent of their market is agribusiness.
Polyane was also the first company to develop sulphur resistant films. This followed three years experience in the Kenyan rose market where growers burn sulphur for 12 hours during the night to minimise fungal diseases. “When the sulphur evaporates in the greenhouse atmosphere, it combines with humidity to form sulphurous acid, which destroys the UV stabilisation system of greenhouse film over a short period of time,” explains Ribot. “Sulphur-burning also causes leaf yellowing, damages the quality of the crop, and eliminates the option of using biological pest controls,” he added.
Today, most Kenyan growers confine sulphur burning to specific periods of the year when diseases are more common.
Cooling and thermal effects
For the Australian market, Polyane have produced a specific Celloclim film because of its close proximity to the ozone hole over Antarctica and high UV radiation levels, a point not lost on European growers who are experiencing a 30% thinning of ozone over their own continent. Celloclim 4S not only blocks harmful UV radiation and improves light quality, but has a ‘cooling effect’ during the day and slows temperature drop at night (the ‘thermal effect’). “As outside temperatures rise, the cooling effect becomes noticeable once external temperatures reach 25°C, with the inside temperature up to 7-10° cooler than under a standard film,” explains Ribot. “This cooling effect reduces plant stress and evapo-transpiration, which improves fruit sugars and taste. The cooling effect also reduces irrigation requirements, ” he added.
Other benefits of a cooler environment include better fruit production, reduced pest and disease control treatments, lower energy costs for mechanical cooling, and a more comfortable environment for workers.
At night, the thermal effect of Celloclim kicks in with nighttime temperatures inside the greenhouse between 2°C and 4°C higher than outside temperatures. The reflection and absorption of IR radiation slows the temperature drop compared to standard plastic coverings. “Australia has hot days and cold nights, so there is a large thermal gap. If you use standard film you will increase this temperature difference, while Celloclim reduces this gap,” said David Ribot.
Should Australia be allowed to import bumblebees onto mainland Australia, or native blue-banded bee become a commercial reality, this film permits those UV wavelengths needed for navigation to penetrate the greenhouse, which will improve bee pollination. “With a proprietary system of UV absorbers and additives, this film will let bees work harder and for longer, ” David Ribot said.
Celloclim has a warranty between 33 and 45 months, depending on where the film is installed.
For further information contact:
David Gill Greenhouses,
PO Box 20, Huntly, Victoria 3551
Fax:+61 (03) 5448-8846
The Australian and New Zealand greenhouse film importer/distributor, Redpath, have been developing their range of greenhouse films for both markets. Within New Zealand, the interest has mainly been from export flower growers constantly looking to improve colour and stem length, and vegetable growers who want to improve fruit production.
A good example of this is one of New Zealand’s longest established exporters of calla lilies (Zantedeschia). Graeme and Dorothy White of Fantail Grove commented that the Redpath Duratough film provided an immediate gain in vibrancy of colour to their crop once installed, which almost matched the colour gained from outdoor production on the property. “The strength of this colour could only be put down to the Duratough greenhouse cladding, as production techniques were identical in our cropping structures other than this,” Graeme White said.
For vegetables, Redpath Director, Glen Williams, said that the steady introduction of bumblebees into the New Zealand greenhouse industry has led to a change in the formula specification for the Duratough range of greenhouse films. “Bumblebees require a clear film that transmits light in the range of 350-385nm, which they use for navigation,” he said.
He said that while pest-reducing films are moderately effective, one should keep in mind the difficulties that may be experienced in achieving effective pollination ratios by the reduced performance of friendly IPM insects, including bees. This is a consideration even in Australia, if and when bees become available to growers.
These aspects have led Redpath to concentrate on a film that is a little more complimentary to modern production techniques such as bumblebee technology and high-cropping production. “There is a fine balance in blocking out light spectrums that the crop would normally be naturally exposed to, without interfering with the development of plants,” explains Williams.
He warns of the negative impacts that can result from placing various components in the make-up of a greenhouse film, including reduced life expectancy. As films become less pure, there are also impacts on light transmission. The Redpath experience has found that additives can also reduce tensile strength and elongation of the film.
The three-layer Duratough film incorporates the basic features that growers require, and exceeds the requirements of most. The outside layer is drenched with UV inhibitors for superior resistance to Australia’s high level of UV, and incorporates a higher percentage of EVA polymer to improve light transmission and film-fold pliability. Film elasticity becomes especially important when the cover is exposed to strong winds. “The Duratough film better resists cracking and shearing compared to previous films,” said Williams.
The middle layer is also UV stabilised and incorporates a ‘modern resin filler’ that helps retain heat at night. “The thermal efficiency is restricted to this layer to maximise the performance of the film, and to minimise transparency haze that can occur in other thermic-style films,” Williams said. “Thermal efficiency is important during cold winter nights and hot summer days when the reflectivity of the film provides a barrier to the effects of external temperature extremes,” he added.
The middle layer is constructed from a linear low-density polyethylene (LLDPE), which has exceptional elongation and break strength. This provides greater tear resistance, allowing installers to be a little more ruthless when covering the structure. “Its break strength is 20% greater than film supplied in previous years,” said Williams.
The inner layer is coated with a long-life anti-drip agent, which offers benefits equal to or greater than the life of the film. The coating also gives a substantial gain in light transmission. “A crop lost to disease and the time invested in disease control sprays easily justifies the cost of this anti-condensate feature,” said Glen Williams.
The long-life Duratough film has a four-year pro-rata UV factory warranty.
Redpath also offer their Hortiplus X30 range of films in Australia, which has improved resistance to chemical attack. The Hortiplus three-season film uses a chemical resistant nickel-base “sequencher” stabiliser, rather than the industry standard HALS (Hindered Amine Light Stabiliser) UV additive.
Hortiplus X30 is also a multi-layer, co-extruded film and incorporates a yellow tint for light diffusion and spread of light.
Hortiplus is targeted toward the grower that has shorter cropping schedules in igloo-type structures and requires a simpler and more cost-effective film compared to Redpath’s higher specification Duratough brand.
Hortiplus films are supplied in Australia from $0. 88csq/m.
‘Polyweave’ reinforced greenhouse film is Redpath’s super-strong, non-rip style greenhouse cladding. This smart polyweave is of a woven construction and is a non-extruded product. It has a centre construction of woven leno weave clear fabric and this is laminated on each side with a layer of clear polyethylene film. Often used for side-wall ventilation screens, end walls and lean-to type structures, if damaged or cut, the polyweave will not continue to tear like normal extruded films. This greenhouse cover offers very reliable long-life performance.
For further information:In Australia, contact:
Glen Reidy, Redpath Greenhouses,
Rohs Rd RSD4, Bendigo East, Victoria, 3539
Fax:+61 (03) 5441-4164
In NZ, contact:Glen Williams, Redpath Pacific
PO Box 9058 Terrace End, Palmerston North.
Ph:+64 (06) 508 733-728
Fax:+64 (06) 353-5956
The greenhouse films, screens and weaves from Svensson are an exciting addition to the family of smart covers. The Svensson Solar Ultra film is seven times stronger than other extruded plastics, giving growers the best protection for their money.
The Svensson Solar Ultra film is made in three layers of high density poly (HDPE). The outside layer absorbs 90% of solar radiation with IR heat radiation wavelengths reduced by 60%. This provides a cooler greenhouse environment, reducing crop temperature by up to 5°C, and slows temperature drop at night.
The middle layer includes additives to strengthen the film and to increase its longevity. The tensile strength is twice that of normal reinforced plastics and seven times greater than extruded plastics. This means fewer replacements and greater security against hail. The crop is therefore often able to be insured.
The inside layer includes an anti-condensate agent that reduces drips, as well as improves light transmission. The transparency of Solar Ultra with direct light is up to 86%, and 75% with diffused light. For crops that require reduced light, the green Solar Ultra shades plants by 45%.
Solar Ultra covers are made to order to any size. The film is manufactured to four metre widths and can be welded to any manageable size. However, fewer welds means easier make-up and walls without joins, as well as lower costs.
No treatment of smart covers would be complete without a review of Svensson screens, distributed in Australia by Living Shade and Climate Control Pty Ltd. This company has been at the forefront for more than a decade, providing growers with dynamic solutions to problems they encounter during the growing process.
The Svensson range of smart screens provide cooling and thermal effects, as well as significant energy savings. Light, temperature and humidity can all be controlled with the aid of these screens, which are installed on a moveable slide system so that growers can open and close them, depending on the outside weather conditions and the needs of the crop.
Irrespective of the climate and type of greenhouse you have, you can install Svensson screens in it. The screens are flexible with an easy-folding structure that minimises shadows and makes maximum light transmission possible. If the sun is too harsh for growing, these screens provide shade and reduce plant temperatures. The aluminum content of the screens reflect up to 90% of solar radiation, optimising the shading and cooling effect on the crop. The more aluminum in the screen, the cooler the crop will be.
Notable Svensson screens include the OLS 6°Cfor medium shade on a moving slide system. At night, when the screen is closed, it will increase the crop temperature by up to 3°C and protect crops against light frost. The screen offers 34% light transmission, 30% diffused light, and up to 20% in energy savings. It is easy to install and comes with the Svensson three-year guarantee against UV degradation outside, and five-year guarantee inside the greenhouse. With production increases of up to 20%, most growers can expect a full investment payback within two years.
The Svensson SLS Ultra Plus 10° screen will help growers control the amount of light coming into the greenhouse, as well as humidity. This screen lowers ambient temperature inside the greenhouse by 3° to 5°C during warm sunny days and reduces crop temperatures by up to 10°C. Because every 15thstrip is open, the screen can breathe to help balance relative humidity in summer.
Svensson have tested over 100,000sqm of this screen to deliver some impressive results. When closed, the SLS Ultra Plus 10 reduces heating costs by up to 37%. In unheated greenhouses, it increases the temperature at night. By keeping the leaf temperature higher, dew and related diseases are also reduced.
This lightweight screen helps optimise relative humidity and extends the seasonal use with exceptionally high light transmission – 88% direct light transmission and 81% diffused light. In summer, it cools the crop on warm sunny days by 12% direct shade and 19% diffused shade. UV transmission is 28%, and it is resistant to regular chemicals used in the greenhouse. In hotter climates, SLS Ultra Plus 10 is used in conjunction with the open solar screen, ‘XLS 15F Firebreak’.
Most growers using Svensson smart screens can expect a full investment payback within two to three years.
Svensson manufacture a smart anti-insect plastic yarn made from UV stabilised polyethylene and acrylic yarns, and so are strong and long-lasting. Of special note is ‘Econet T’ for thrips and whitefly, ‘Econet M’ for aphids, and ‘Econet L’ for miner. Growers can choose a screen according to which pest they might be having a problem with. However, if you choose the finest mesh to combat tiny insects when you only needed one to combat small animals, then you will be sacrificing ventilation levels unnecessarily. The Econet range of anti-insect products are sold by the metre or in rolls.
For further information contact:
LS Climate Control Pty Ltd,
2/43 Leighton Place, Hornsby NSW 2077
Mobile 0419 249-570
Not all components of the light spectrum are useful for plant growth and development. In general, UV and excessive IR heat radiation can be harmful to plants and should be avoided as much as possible. Plants use visible light, or more specifically PAR as the main wavelengths during the process of photosynthesis. Therefore, greenhouse films should have a high transmittance for PAR to optimise plant growth and development.
Wavelengths of non-visible radiation at both ends of the light spectrum can inhibit or promote stem elongation and branching of plants, among other growth characteristics. Films containing colourants exploit these wavelengths and are mainly used by flower growers to increase stem elongation and improve colouration.
However, there can be negative impacts placing colourants and other additives in the make-up of greenhouse films, including reduced life expectancy. As films become less pure, there are also trade-offs in light transmission.
Nonetheless, smart films, screens and polyweaves do offer growers cost-effective benefits. They offer growers the opportunity to better control light transmission and plant growing processes. While the new generation of smart coverings come at a higher price than standard greenhouse claddings, growers’ returns are improved product quality, higher production yields, and lower energy costs for mechanical heating and cooling.