Making light work of growing

What the Grower Magazine had to say in a recent article …

Thumbs up for Smart Films

Reproduced by kind permission of The Grower
Peter McGrath reviews progress with new age films for crop covers

Research into different materials and additives for polytunnel covers has produced a range of high-tech films with different characteristics. These additives either block or allow through different wavelengths of light, a process known as spectral modification, which creates biological effects on the plants, pests and diseases growing beneath them.

Already near or actually on the market are films that claim to reduce the disease incidence of botrytis and other diseases, and the plant height of ornamentals. Although these 'new-age' films can be 5 to 30% more expensive than the conventional alternatives, growers can benefit because of the reduced need to apply fungicide and plant growth regulation chemicals.

Both Visqueen and XL Horticulture - leaders in the development of 'smart' films - offer a thermal heat block (THB) film. These contain an additive which 'scatters' the visible wavelengths of light as they pass through. The films also block heat-producing infra-red rays, so reducing the chance of plants becoming scorched.

"This allows us to achieve two things", says Visqueen's technical director Dr Richard Henbest. "The scattering effect means we can get more light into the glasshouse or polytunnel and, by making it more diffuse the photosynthetically active visible light is more available to the plants."

First marketed in France Visqueen's Luminance THB has been available in the UK for the past couple of years. Last year XL re launched its Sterilite THB film with new specifications.

The main difference between the two films is down to a trade off. The greater percentage of the valuable diffused light which penetrates further into a crop's canopy, the lower the total amount of light that enters the polytunnel Luminance THB transmits 88% of photosynthetically active light, of which over 90% is diffused. In comparison, SteriLite THB allows through 91% of the light (89% guaranteed minimum), of which 62% is diffused.

Richard Henbest says: "Under most growing conditions, trials show that we can get about a 10% increase in the growth rate of ornamentals under our Luminance THB film".

According to Les Lane, in order to create viable spores, most fungi need the stimulus of wavelengths in the ultra-violet range of light, between 350 and 380nm. "All polythene and glass block up to about 350nm, but if you take out light up to 380nm you can stop diseases from spreading", he says. XL's Sterilite HDF, for example, is claimed to reduce levels of both botrytis and powdery mildew and can be used over four seasons.

Insects too
Research from Israel also suggests that UV-blocking films - in this case made by Israeli company Ginegar Plastics - can control insects such as thrips and whiteflies and the viruses they transmit.

Les Lane also claims that Sterilite films can help control aphid populations. "We tested it on pansies in our own nursery," he says. "In three or four spots we had colonies of about 30 to 40cm across but they didn't spread throughout the house as they would normally have done." The reason for this, he believes, is that many insects use UV light for navigation.

By blocking the UV to 380nm, the insects are dissuaded from moving around. "But we want SteriLite to be part of an IPM system," adds Les Lane, "so we only block the light to 380nm. We have anecdotal evidence that beneficial insects move around during the brightest part of the day and do the work they have to do."

Blocking the UV light to 400nm could cause problems, claims Les Lane, as they can see just 20% of the light available to them, compared with 60% if it is blocked to 380nm.

"It was a consideration that we would get substandard pollination," admits grower Angus Davison, MD of Haygrove Tunnels and chairman of Kentish Garden, "but so far we have not found it to be a problem." He believes the main reason is that trials with Visqueen's Anti-Botrytis and XL'sSteriLite film used tunnels with high legs that were not sealed in the middle, so bees had enough natural light available for navigation.

Growth regulation
Other films work by screening out light at the red end of the spectrum, around 650nm. XL's SuperGreen and SuperBlue and Visqueen's Solatrol films all adjust the ratio of red to far-red light, with pronounced effects on the morphology of plants. However they have been designed for different uses. XL products, for example, are used as tunnel covers. SuperGreen has been available for several years as a film suitable for growing plants which prefer a shady environment, and SuperBlue covered tunnels are being tested commercially for the first time in the UK this year.

In contrast Solatrol is recommended for use within tunnels and glasshouses as an internal screen between the light source and the crop during just part of the growing season. HDC funded trials carried out at the University of Reading, have shown that the control of plant growth was not consistent enough to be able to eliminate the use of growth regulators. However in other trials Solatrol has been used successfully over pansies and poinsettias. Although it is not yet commercially available, limited quantities of Solatrol are available for evaluation purposes.

What's next?
XL Horticulture has already introduced films containing metalocene, a co-polymer that adds extra strength to the polythene. This means that the films can be made thinner and lighter.

The new 600 gauge films are 150 microns thick compared to the more typical 180 micron, 720 gauge films. Adding metalocene also makes the films 50% more resistance to tearing claims Les Lane. XL will also soon be trialing anti-algae film.