Greenhouse LEDs
There have been a number of advances in greenhouse LEDs.
Light-emitting diodes (LED) have become increasingly popular with growers and nurseries for both edible and ornamental plants.
Far from being an experimental technique, it has proven to be an effective way to increase yields at a much lower cost.
Crop output for some has increased by as much as 50% simply by altering the incidence of red, blue, and sometimes green light, and increasing CO2.
What is LED?
The diode in a LED is a semi-conductor that lets electricity flow in only one direction.
It’s made of two kinds of material: One which doesn’t conduct electricity, and a very small amount of another that does.
The colour of the light emitted depends on the material used in the semi-conductor.
Controlled-Environment Lighting Laboratory, University of Michigan (Used with permission of Dr Runkle.)
What is light?
When we speak of light, we mean that which the naked eye can see; but that is a very small part of the total spectrum.
Some animals, such as spiders, for example, can see ultraviolet light, and snakes can see infrared.
Both of these wavelengths are beyond the ability of the human eye in most cases, though very intense infrared has been seen as green by a few people.
The different types of light are defined by their wavelengths and measured in nanometres.
What is sunlight?
Sunlight, in the visible spectrum, consists of violet, indigo, blue, green, yellow, orange and red light.
Blue and red light are essential for plant growth, though green light when added in small amounts can also be effective in some plants.
The wavelength of blue light is shorter than that of red light.
That’s why the sky is blue.
As the sun rises from the horizon, more and more blue light is able to penetrate the atmosphere.
At sunrise or sunset, only the red light has wavelengths that are long enough to get through to us.
That’s because the angle of the sun is such that there’s more air to pass through in the early morning and evening than there is during the day.
How LED helps growers
Controls the intensity & colour of light
The primary advantage for growers is that LED enables them to control the intensity and the colour of the light, rather than just switching it on or off.
This gives them much greater control over their growing schedule, especially as it’s rather common to have more than one crop during the year.
In the days before this technology took hold, growers only concerned themselves with “managing” sunlight.
During the winter, they couldn’t get enough.
In the summer, however, interior temperatures as well as the intensity of the sun created the need for shading and ventilation.
LED has obviated much of that.
Growers can now use lights that enable them to apply exactly the right qualities of light to give them the greatest benefit.
Even though blue light is the light of day, by itself it’s insufficient to give the right combination of light needed for plants.
Some red light is needed, as is some far-red, which is invisible.
The best results have come from a combination of the LED blue and red light, and natural daylight.
Blue seems to be better for keeping plants from spreading out, while red light is beneficial for gaining height.
Green light also balances growth.
Some plants need more hours of darkness than others.
This, too, can be simulated with LEDs.
No added heat
A second advantage of LEDs is that while the lamps create light, they don’t give off heat.
More sunlight generally means higher internal temperatures.
LEDs create light in a different way from standard lamps.
Tungsten or other lamps rely on heat that builds up in its filament to generate light.
With LEDs, the light comes from the conductible material through which the electrical current is passed.
In other words, heat isn’t required to produce light.
This makes them exceedingly efficient to use.
The lamps last much longer than conventional bulbs, too.
Fifty-thousand hours, or more than five years of continuous use is not unheard of.
Doesn't attract insects
A third advantage of LEDs is that they don’t attract insects.
Tungsten lamps seem to attract every hexapod for miles around.
For more information on ongoing horticultural LED research, visit the website of Professor Erik Runkle at the Department of Horticulture at Michigan State University in East Lansing, Michigan.
He and his colleagues are collaborating with American Hort on research into the use of LED on ornamental and high value plants.