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What is the best grow light spectrum?

This is a simple guide to the best grow light spectrum. We have taken the information from a recent & comprehensive scientific study of grow light spectrum. We present a summary of their experiment findings in an easy to understand way.

Details of the experiment and results are at the end of the article

The setup of the experiment

The scientists tested eight different grow light spectrum at two different light levels. They tested the growth characteristics of the light spectrum on seven plant species. Light intensity was maintained at the same level in each growth chamber so that the effect of each light spectrum on plant growth could be compared precisely.

The growth characteristics examined included leaf area, Chlorophyll levels, petiole length (leaf stem length), stem length and dry yield. In this summary we have extracted the two most significant growth characteristics:

  • Dry mass yield – We all want the best yields from our grows. The test results show you the productivity level you can expect from each grow light spectrum.
  • Stem Length – Generally growers want short stem length (distance between branches) and dense growth. The results demonstrate the effect each grow light spectrum has on stem length.

The grow light spectrum used in the tests

Eight different grow light spectrum were used in the test. Below we breakdown each of the grow light spectrum into the percentage of Blue, Green and Red light in each example.

Grow light spectrum

Light spectrum in % Blue, Green and Red for each grow light spectrum tested

The Red, Green and Blue Spectrum are grow spectrum examples that are not generally available but are useful for demonstration. The rest of the examples are among the most common grow light spectrum used today.

Please note we have inserted ‘MIGRO’ over the 3500K spectrum result as it is a very close match

The effects of the different grow light spectrum on yield

We have taken the average dry yield result for the 7 plants (under the higher light intensity, 500 PPFD) and show you the result for each grow light spectrum.

The average dry mass yield result for each grow light spectrum

The most productive spectrum tested was the Red/Blue and is the benchmark at 100% and the other results are compared.

From the experimental result we learn that:

  • Yield results are very close for ‘Red/Blue’, ‘Warm (3000K)’, ‘MIGRO (3500K)’ & ‘Neutral (4000K)
  • Green is almost as effective as Red in photosynthetic efficiency
  • Adding more blue results in less productivity
  • Using only Blue, Red or Green light is not efficient (mixing with other spectrum is better)

The effects of the different grow light spectrum on Stem Length

We have taken the average stem length result for the 7 plants (under the higher light intensity, 500 PPFD) and show you the result for each grow light spectrum

Stem Length vs grow light spectrum

The average stem length result under the eight grow light spectrum

The shortest stem length was achieved with the ‘Cool, 5000K’ spectrum and is set at 100%. The results for the other spectrum are compared.

The results are also very interesting and reveal the following:

  • A minimum percentage of 12% Blue will result in short stem length
  • Red/Blue, Neutral (4000K) & the MIGRO (3500K) result are comparable and close to the optimum

Conclusions from the experiment and most importantly..

The most significant findings from the experiment are:

  • The Red/Blue and MIGRO (3500K) spectrum are best for balancing yield and short stem length
  • Green light and Red light are closely matched for photosynthetic efficiency (within 5%)
  • You need at least 12% Blue light to prevent stretching
  • As you increase Blue light over 15% the stem length reduces but yield also reduces

The experiments have shown that a mix of spectrum is best. From the mixed spectrum you can chose to target shorter plant height or higher growth rate. This can be a matter of preference as to what suits your situation. For example if you have a small space you may wish to sacrifice a little yield so that your plants stay short with dense growth. In this case you would chose a spectrum with more blue light..

Last but most importantly: Grow light system efficiency

The photosynthetic efficiency of your grow light spectrum is important but the efficiency of your grow light system is much more significant. We have tested grow light systems from all the technologies available on the market today (see how to evaluate grow light system technology in our article Grow Light Efficiency Calculator tool).

HID efficiency has plateaued at 1.4 µmols/m2/sec/Watt (PPFD/watt) and all boxed Red/Blue LED systems tested deliver a maximum of 1.5 PPFD/Watt.

Much higher system efficiency of 2 PPFD/Watt is possible with the latest COB LED technology with lenses. 

The maximum efficiency of grow light systems tested. HID efficiency (includes HPS, Metal Halide and Ceramic Metal Halide). LED efficiency is based on our extensive testing of products available on the market today.

 

Overall Conclusion: The combination of grow light system efficiency and photosythetic efficiency demonstrate that COB technology with full spectrum light in the 3000K – 4000K is ideal for healthy growth and maximum yield.

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Seminal research paper on the effects of different light spectrum on plant growth

The reference for this article is the recently published scientific paper ‘Sensitivity of Seven Diverse Species to Blue and Green Light: Interactions with Photon Flux’ by M. Chase Snoden, Kevin R. Cope, Bruce Bugbee, 2016. The research paper can be downloaded for free here:  http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0163121

This research article is a study of the effect of different mixes of light spectrum on plant growth. The mixture of Blue, Green and Red light were varied and tested on seven species of plants. The light intensity or PAR level was kept at two different PAR levels (PPF) in each grow experiment so the different light colour mixes could be studied.

The eight spectral treatments and characteristics.

The eight spectral treatments and characteristics.: Blue, green and red values are percent of total PPF (400 to 700 nm). UV-A is percent of total PPF. Phytochrome photoequilibrium (PPE) was determined as described by Sager et al. (1988). Symbols correspond to the color for each treatment and shape represents the two PPFs (200 and 500 μmol m-2 s-1), which are associated with DLIs of 11.5 and 29 mol m-2 d-1. Symbol shape and color are consistent in all figures

 

The plant growth characteristics were recorded for each plant species.. Each plant species was grown under the various light spectrum and at two different PAR levels.

Overhead view of cucumber plants at harvest.

Overhead view of cucumber plants at harvest. Includes all eight LED treatments at both light intensities arranged from low to high BL fraction. There were four plants per treatment and three replicates per study. Note the difference in coloration with the green and red treatments at the 500 PPF level (DLI = 29 mol m-2 d-1).

All of the results were recorded and collated. The average of these results have been used in our article.

The effect of percent blue light on dry mass (DM) gain for seven species under two PPFs.

The effect of percent blue light on dry mass (DM) gain for seven species under two PPFs.

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