The AHDB Horticulture-led tour formed part of AHDB Horticulture-funded project PO 019 ‘The Bedding and Pot Plant Centre – new product opportunities for bedding and pot plant growers’.
The findings highlighted by AHDB Horticulture are as follows:
Paul van Gils, account manager at Mardenkro
, said diffuse light is "a light ray that is transmitted through a surface and broken up and scattered into different directions. However, there is a minimum change in angle from the straight required before the light ray is deemed to have been scattered. Some sources deem this to be 2.5° or more, others specify only 1.5° or more. The amount of light which is redirected determines the haze-factor, the higher the haze-factor the greater the level of light scatter.
"But to complicate matters further, a new method of calculating (as opposed to directly measuring) light scatter, termed F-scatter (forward scatter), has been developed by Wageningen University and Research Centre (WUR), which also takes into account the redistribution of the light as well as the amount redirected. The correlation between the haze-factor and F-scatter and what it means in practical terms is still being discussed by researchers at WUR and the manufacturers of materials which diffuse light."
The level of haze-factor is important because when light enters directly into a glasshouse it is focused towards the top of the crop canopy or growing point of the plant and causes temperature increases and, as the stomata in the leaf close in response to minimise water loss, there is increased potential for physiological and physical damage to the leaf, expressed as scorch. When light is scattered as it enters the glasshouse, a greater proportion is redirected towards the base of the plant reducing stress levels at the top and allowing improved levels of photosynthesis throughout, this in turn allows the crop to be exposed to higher light levels without damage, this is one of the main factors which drives the yield increases found in edible crops.
Ralf Derksen, product manager at Saint-Gobain/Cultilene said: "As a rule of thumb, an increase of 10% in the haze-factor leads to a 1% yield improvement."
But diffuse glass has a lower light transmission than ‘normal’ glass and lower light levels, as a result of simple plant physiological responses, will lead to yield reductions. Where there is a need to maximise light transmission too, the diffuse glass can be covered with an anti-reflection coating (a thin coating based on silicon dioxide and a small percentage of organic beads), either on one side or both, pushing light transmission towards 85-90% (untreated basic float glass is around 90%).
Ralf Derksen said diffuse glass can also be created with different haze-factors, Saint-Gobain offer a low (20%), mid (50%) and high (80%) haze-factor glass. The high haze-factor glass is created for areas of high light intensity, the mid-value glass is more suited to the production of summer crops in the UK and the low value glass to winter crops.
Derksen added that another benefit of treating the inside of the glass with an anti-reflection coating is that it makes the glass surface hydrophilic, instead of hydrophobic, meaning that condensation gathers as a film rather than as beads. Water in the form of beads can lead to light losses up to 1.5%, while films of condensation actually lead to improvements in light transmission.
So which material to select to diffuse light?
Glass is the obvious, permanent choice and with the addition of anti-reflection coatings the light transmission can be maximised. However, in reality this is the most expensive option, diffuse tempered glass is twice the price of ‘normal’ glass, even without anti-reflection coatings. As the use of tempered glass is not a legal requirement in commercial production houses in the UK (unlike in the Netherlands), a price saving can be made by using non-tempered diffuse glass, but generally speaking in the UK, diffuse glass only currently tends to be considered when new structures destined for edible crop production are being erected.
At the other end of the ‘spectrum’ are glass coatings such as ReduFuse and other similar coating materials. These have the advantage of being relatively inexpensive materials (around 20-25p/m 2 ) that can be quickly applied when needed to glass or any other structure. However, they still need to be applied and removed each year, they need to be applied uniformly to achieve a consistent impact on the crop and further research is still ongoing to examine their effects on the light spectrum and the growth of a range of crops.
A third alternative is the use of diffuse screens. A range of screens are now offered with differing degrees of light scattering capabilities, along with energy saving capabilities. At around £6-7/m 2 (including the mechanical hardware) they are more costly than coatings, but are more flexible (they can be pulled back in dull weather to maximise light levels) and can be easily retrofitted to older glasshouses, unlike diffuse glass. On some of the nurseries visited as part of the study tour, multiple screens were employed, including a combination of a diffuse light screen, an energy
saving screen and a black out screen.
A fourth option, not explored on the study tour because of the glass-based nature of the protected crops industry within the Netherlands, is the use of plastic diffuse materials for polythene tunnels. A range of diffuse plastics, with life expectancies of around 7 years, are now available from 80p-£1/m 2 (material only).
On many of the nurseries visited as part of the study tour more than one option to diffuse light entering the glasshouse was actually deployed. In the case of crops such as Phalaenopsis orchids for example, which need shade, both diffuse glass and diffuse screens were used; this permitted higher levels of light scatter but it also meant that when vents were open the screens diffused any light that entered the
glasshouse directly through them, avoiding plant scorch.
Filip van Noort, crop specialist at WUR, commented that he found three main benefits during his research
on a number of pot plant crops:
1 -Better crop quality (especially for shade loving crops such as Phalaenopsis and Anthurium) as there is less chance of leaf scorch. Plants also tend to have a better habit as a result of the light reaching the lower parts of the plant.
2 -Heavier plants, the diffuse light led to ‘bulkier’ plants which were up to 25% heavier. In a similar vein, Ralf Derksen presented independent research work which showed a 0.8% increase in the fresh weight of cut roses and a 0.6% increase in cut chrysanthemums when grown under diffuse glass.
3 -A shorter production cycle, in the case of the plants examined in WUR trials (Anthurium, Bromeliad and Phalaenopsis) the production period was cut by 25% from an average of 22 to 16 weeks.
There may also be other potential gains too. During discussions with the presenters for example, Neil Bragg of Bulrush raised the issue of increased pot temperatures on the south side of batches of plants as a result of the black pots being exposed to longer periods of direct sunlight. This can lead to poor root development and uneven crop growth. It may be that by diffusing light entering the structure that this increase in the temperature of the growing media within the container can be minimised resulting in a better crop grade out at marketing.
Ultimately the benefits of diffuse light for (non-shade loving) ornamental crops will never be as straight-forward to calculate as the yield increases for edible crops.
Plant quality is a basic requirement of the customer, so generally doesn’t generate an extra premium, increases in plant size may not always be beneficial and a shorter production cycle is only of benefit if another crop can be grown or savings can be made in terms of inputs into the crop. The financial decision is therefore often calculated based on parameters which are difficult to put an exact value against, such as better growing/working conditions, better plant habit etc., and a smaller number, such as crop turn-around times, which do have a more defined value to the business.