Judging by the number of hits on the Horticulture Week website last time we published an article on LEDs, there must be a lot of growers and nurserymen who find it an interesting topic and want to know more.
In the past, plant lighting such as SON-T lamps have been used. While providing light, traditional lamps tended to be expensive to buy and run. They consumed a lot of energy and, being slow to turn on and off, did not respond quickly to changes in ambient conditions. On top of that, they produced excessive heat — not only a waste of energy but also making it difficult to achieve optimum growing conditions, to the point where cooling might actually be needed.
Plus, rapid light loss tended to occur after 12,000 hours runtime and the light might not be evenly distributed due to the nature of the bulb and the fitting. But above all, these traditional lamps often did not match the requirements of the crop, typically hitting a small band of 560-630nm and thus falling far short of the wider PAR optimum.
LEDs, or light-emitting diodes — the same as the light flickering away on your computer — only emit light of a designated narrow band wavelength. Combining LEDs of different wavelengths gives the ability to control the colours that plants need and thus to control the plant in terms of photosynthesis, growth, flowering and fruiting. LEDs could be considered to be better than sunlight because they
Now add in the fact that LEDs last longer, are cheaper to run and produce less heat, meaning the lamps can be placed closer to the plants and have less effect on temperature. Plants are cooler and transpire less. The light itself needs no ballast — no bulky box. It is not surprising then that LED lighting has been something of a hit for horticulture, both in research applications and for production in controlled environments, and their use has dramatically increased over recent years.
Making the switch
One facility to make the switch from SON-T lamps to LEDs is the University of Bristol Unigro-installed rooftop Grodome at the Life Science Building. When horticultural greenhouse manager Tom Pitman showed me around the horticultural research facility this time last year, 6,000W high-pressure sodium SON-T lamps were used for plant-growth purposes. At the beginning of the design process back in 2011 it was thought that LEDs were not the right offer. But now is a different story.
Pitman and the university’s sustainability manager (energy) Chris Jones recognised that SON-T lamps use a high level of energy and saw an opportunity to reduce energy consumption and costs by changing to modern LED plant-growth lighting. Pitman and Jones were aware of the work carried out by Phytolux at Rothamsted Research and other universities, and arranged for a trial to investigate the potential energy saving of LEDs.
A comparison trial was set up using the Phytolux Attis-7, winner of the Horticulture Week award for innovation at the 2015 Four Oaks Trade Show, to measure the effect on the quality of plants grown compared to the use of traditional SON-T lights already installed in the Grodome. Compact fluorescent lamps were also considered as a low-energy alternative control.
The Phytolux lights performed well, with growth more vigorous and plants stockier and darker than their counterparts under SON-T and fluorescent lighting. Energy saving was calculated at 71 per cent, beyond the 60 per cent target set by Pitman and Jones, and the predicted carbon saving amounted to 44 tonnes per year.
The payback period on investment was calculated as 2.5 years on energy alone. The Attis-7 LEDs require minimal maintenance and have a low heat output, meaning additional savings will be made on maintenance costs and substantial savings on air-conditioning.
As a result of the trial, the University of Bristol replaced its SON-T lights with Phytolux Attis-7 LED units in July 2015. But it is not just research facilities that are benefitting from the latest lighting technologies.
Three years ago UK strawberry grower Wallings Nursery started trialling LEDs in its production houses in Lawford. It was able to supply Sainsbury’s with UK-grown strawberries at Christmas thanks to LEDs from Phytolux.
Similarly with tomatoes, the latest LED trials at the GreenQ Improvement Centre, a modern glasshouse complex in the Netherlands, a yield of 100kg/sq m was achieved with the help of intensive light from Philips GreenPower toplighting and interlighting. Impressive results have also been achieved in ornamental production.
Philips LED toplighting trials at Kernock Park Plants showed dramatically higher propagation success rates for patio plant plugs as well as seven- to 10-day faster finishing times for perennials. Philips claims up to 46 per cent energy saving compared with high-pressure sodium lighting as well as cheaper installation and maintenance costs.
In the trials, Kernock Park Plants improved the propagation success rate for many of its crops, with one variety almost tripled. Faster rooting and improved Botrytis control were also recorded as well as an energy saving of 30 per cent.
Yet another application, and one often covered in daily newspapers, is that of vertical growing. There are many examples of "multilayer" cultivation taking place and Philips has developed the GreenPower LED production module as a complete package to give controlled, uniform, high-quality plant output.
At Peterborough Regional College, horticulture and hospitality students spent the autumn collaborating with Local Roots to reduce food miles associated with some of the ingredients used in the college’s Parcs Restaurant to zero. Led by sustainability experts Dr Ian Tennant and Gloria McNeil, Local Roots provides expertise and a fully serviced solution to allow caterers and organisations to grow their own food on site.
The company set up a small version of a new vertical farming system, VydroFarm, recently launched by hydroponics firm HydroGarden, based in Coventry. Installed in the college’s on-site restaurant, the horticulture department participated in a three-month trial to grow fresh ingredients for use at the facility.
VydroFarm is a fully controllable environmental system featuring vertical racks on tracks that can be moved together to save space and maximise production density as well as enabling easier harvesting.
The system can be monitored and controlled remotely via smartphone, tablet or PC and optimum lighting conditions for crop growth are achieved with Valoya LED lights. The units are fully scalable, which makes them useful for on-site production for restaurants or for commercial growers.
"Utilising VydroFarm in a catering environment is just one of the applications through which it could make a significant contribution to providing sustainable solutions for future food production," says HydroGarden commercial sales manager Stephen Fry.
"The controlled environment makes it perfect for herbs, microgreens, edible flowers and even strawberries. The consistency of the growth and quality achieved across the crop will be higher, resulting in less wastage."
Crops with a low profile — such as lettuce, other salad crops and many herbs — can be grown in multiple tiers with LED systems enabling maximum production.
From propagation to the production of food, ornamentals and flowers, the applications for LEDs in horticulture are many. No doubt more will be discovered in future. This is definitely one to watch.
Back to basics: encouraging photosynthesis
Photosynthesis fuels plant growth. The process requires carbon dioxide, water and light energy. Natural sunlight is the cheapest source of light energy but it is not always available when needed or in the quantity and quality required to give the yields demanded by growers and plant producers.
Only part of natural radiation is used for photosynthesis. Plants require the part of the spectrum between 400nm and 700nm. We call it photosynthetically active radiation, or PAR light. The number of light particles (photons) in the PAR region received during the day over an area of one square metre is called the daily light integral (DLI). Outdoors the DLI averages between five and 60mol per square metre per day depending on location and season. Inside a greenhouse or polytunnel, light transmission is reduced and the figure can drop to 3mol/sq m/day or less.
DLI is important because it affects shoot growth, cuttings’ rooting, stem thickness, branching and flower abundance. The rooting of cuttings is understood to require 4-6mol/sq m/day while bedding and pot plant production needs 10-12mol/sq m/day. Plant quality can generally increase, with greater branching and flower number, when the DLI increases — although different crops tend to have different requirements.
To overcome any shortfalls of natural sunlight and to boost yields or crop quality, we can use grow lamps. The advantages of supplementary and artificial lighting are the ability to manage quantity, intensity, day length and wavelength according to the needs of the crop. Lights also add consistency and uniformity.
It is also believed that plants have other reactions to light as well as photosynthesis. For example, for radiation longer than 700nm wavelength, photomorphogenesis takes place. Plants may also be affected by radiation shorter than 400nm. Recipes can be calculated to give different crops the right stimulus to perform in the ways needed to produce high-quality yields — and profit.