So What Is The Right Spectrum? So clearly, when you look at these graphs, plants are best off with red and blue light, and you should stay away from green light. LEDs with red and blue light should be the most efficient answer, right? Well, that is what many people think. Let’s look at the sun first.
Our Baseline: The Sun When discussing sunlight, I always ask what people think is the most abundant color in the sunlight spectrum. Mostly, the answer is yellow or red, but hardly ever do they know the right answer - It is green light! Now how is it possible that plants, according to studies, don’t make use of all that green light? You can actually see that plants don’t use green light, they are green, so they reflect green light, right? Wrong!
When you have followed the discussion about light quality in the professional horticultural world over the last five years, there was a lot of discussion about red, blue, and far red, initially due to the upcoming red/blue LED technology. Green was then added to the discussion, and more recently, there is a lot of discussion about UV and wide spectrum light. Research follows technology. We know a lot about the plant responses to light, but we do not know everything. There is still a lot to be learned. During a recent conference about light and spectrum, with many manufacturers in the audience, the moderator, a plant scientist, asked who could present a spectrum that would perform 10% better than any other, or guarantee 10% more yield. Not one arm was lifted. In 1994, Bruce Bugbee, professor at Utah State university, performed an experiment in which he lit plants with 6 different sources: Low Pressure Sodium (LPS), High Pressure Sodium (HPS), Incandescent (INC), Metal Halide (MH), Cool White Fluorescent (CWF), Red Light-Emitting Diode (LED), and Solar on a clear day.2 So very different spectrums of light, all administered at the same intensity to compare the efficiency of those spectrums. The conclusion was put in the following headline: “PLANT GROWTH IN SOME SPECIES IS SURPRISINGLY LITTLE AFFECTED BY LIGHT QUALITY.”
Fig 3 – spectral diagram of sunlight
This is not best practice, of course, for all types of plants, and will not create very healthy crops in most cases, but it does show that there is much confusion and misinformation.
I have written about this before - Green light may be even more efficient than red or blue light.1 It will not influence the photoperiodism mechanism in plants which makes them start to produce flowers in short days, but it does have a great effect on photosynthesis.
Morphogenesis
Was McCree wrong? No, he wasn’t. He was measuring photosynthesis on a leaf disk at very low intensities, and didn’t really look at a plant as a system. In high intensity white light, green light is very efficient.1 It travels through the plant, has a greater effect deeper in the leaf, and reaches the lower part of the plant more easily than red and blue light which is absorbed by the top canopy.
Researchers discovered that the shape of a plant is influenced by the color of light which can lead to a croppy plant, an open plant with a large node distance, a dense plant, a plant with more shoots or less shoots, bigger and smaller leaves etc. This is called morphogenesis, or the shape of the plant. An optimally positioned leaf intercepts light much better than a covered leaf. Therefore, having too many leaves can cost the plant a lot of energy, because plants that do not intercept light have to be maintained by the plant, costing the plant energy.. In recent years, growers are much more aware to use the shape of the plant as an indicator to steer the yield of a plant.