Why are red and blue LEDs used for grow lights anyway?
Up to a few years ago only Red and Blue LEDs were available for 'high power' (3 watt, 5 watt) applications. So early grow light manufacturers used the LEDs available and made the first LED grow lights. To support the use of only blue and red LEDs for horticulture they reference Chlorophyll absorbance graphs.
These graphs are not the entire picture and are misleading. They are based on absorbance of light by chlorophyll extracted from leaves and tested in a laboratory test tube. They make the case that green light is not absorbed by plants at all. This is incorrect and is an argument constructed to support the sale of red and blue LED grow lights. In fact, all colours of light in the PAR spectrum contribute to photosynthesis and growth, just at slightly different rates. An experienced grower will know this. For example HPS (High Pressure Sodium) bulbs have been the preferred grow light source for decades. HPS bulbs generally output about 50% of their light in the green spectrum. If green light did not photosynthesise then 50% of the light output of HPS bulbs would be ineffective. It's simply not the case.
What light do plants really absorb?
In the 1970s a scientist called McCree determined the wavelengths of light a plant can absorb. He assessed the relative quantum efficiency of different wavelengths of light on photosynthesis. In other words he recorded the rate of growth of plants exposed to each wavelength or colour of light. McCree discovered that plants use all parts of the PAR spectrum but at different rates. The quantum efficiency for red is close to 100% efficient whereas blue and green were less efficient, ranging from 65% to 75%. McCrees action spectrum identifies what colors of the visible spectrum are best for photosynthesis. McCrees action spectrum is the correct reference for assessing the suitability and effectiveness of a grow lights spectrum output.
Experimental tests of the effects of Blue, Red and White (Full spectrum) light
We setup three grow chambers with flowering plants and productive edibles. We ran a grow comparison under blue, red and full spectrum light over three weeks to see what growth would result. The light intensity in each growth chamber was the same. We tested each grow chamber with a PAR (Photosynthetically Active Radiation) meter and adjusted the grow light output and hanging height to make sure each chamber had the same average PAR level. The results were very interesting...
The effect of Blue light on plant growth
The flowering plants under blue light continued to flower but with less vigor than under red or white light. The flowering plants did not grow as much and had fewer and smaller petals and leaves. The growth rate of the lettuce was very low but the growth was compact and the colour a deeper green. Overall the yield was less than 50% of the other grows.
The effect of Red light on plant growth
The red light was very good for the flowering plants and they had most flowers and the largest leaf and petal size. However the other characteristic of plants grown under red light is stretching. The flowering plants and the lettuce both stretched compared to the other grows. This means the leaves were longer and the distances between nodes or branches were longer.
A plant with long branches and spaced out flowers will not produce as much yield in a small space as a plant with short and dense growth. Despite the stretching the lettuce had high growth rates and the yield was the best of the three tests. However the leaf thickness, colour and compactness was not as good as under the full spectrum light.
The effect of Full spectrum (white) light on plant growth
So, What Colour Light Do Plants Grow Best In?
The test results demonstrate that some Blue light (about 15%) is needed to keep growth Dense and healthy. With regard to Red and Green it appears that there is little difference in the effects on plant growth. This demonstrates that, although uncomfortable to look at, the Red and Blue only (Burple) LED grow lights and Full spectrum grow lights are closely matched for growing efficiency.