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Experiment: Red Light vs Blue Light -How Spectrums Affect Plant Growth- LED vs CFL

Blue light vs Red light. Which is better for vegetative growth??? Instinctively, the answer might seem obvious: Blue light is for vege! That basic mantra has been plastered all over the internet. But if the issue was so simple, then why is it that so many LED grow lights are sold using such a large ratio of red to blue? Why don’t we see many pure blue “vege” lights?

Wouldn’t nurseries that are growing seedlings want all blue LEDs? Wouldn’t indoor lettuce farms prefer all blue light since they don’t want their lettuce to bloom (bolt)? Are horticulture LEDs high in red light because most of the artificial lighting market is targeting weed growers who want marijuana producing massive buds?


I decided to stop making assumptions and to test this for myself! Specifically, today’s experiment tests the following HYPOTHESIS: Blue light fuels vegetative growth. Thus lettuce plants grown in the absence of blue light will grow slowly and develop less plant mass than lettuce grown under blue light.

I set up several grow chambers using 4 types of artificial light sources: Full Spectrum 6500k CFL, Pure Blue 450-460nm LED, Red/Blue 460+630+660nm LED and Pure Red 620-630nm LED. One hydroponic (Kratky’s Method) lettuce was placed under each light.

1:44 – Lights Specs for This Test
2:14 – Specs for Hydroponic Solution
7:27 – Final Results of Lettuce Growth
8:03 – Photosynthetic Absorption Spectrum
8:10 – Photosynthetically Active Radiation (PAR)
10:53 – Recommended Light Levels for Lettuce
12:02 – PPFD Light Readings for the Lights


It turns out that red light was able to grow just as much plant mass as the blue light. An added advantage to the red LED light was that it generated just as much PAR light, while consuming 29% less electricity.

The red light might have produced a large plant, but the quality of the plant was not desirable. The stem and leaf growth was very elongated. It was similar to a lettuce plant that is bolting. The leaves were pale, indicating low chlorophyll levels. They were very weak, easily separating from the stem.

The blue LED showed similar dry weight as the red, but it had higher chlorophyll levels, exhibiting a much greener color and stronger tissues. Growth was still undesirable though, as the plant stretched vertically, rather than staying compact. It might appear that light levels were too low, but the PPFD PAR numbers were actually too high. Also, the blue light did not generate as much root mass.

Check these sources for appropriate PPFD levels for lettuce:

The Red / Blue LED resulted in a more natural, compact growth pattern. It would seem that the lettuce plants require a mixed ratio of reds and blue in order trigger a compact growth characteristic.


Many growers will focus on chlorophyll A + B adsorption charts, assuming that other wavelengths of light are useless to plants. However, PAR (Photosynthetically Active Radiation) spans the entire range of light between 400nm and 700nm. All of this light can fuel photosynthesis with varying degrees of efficiency. Additionally, photosynthesis is not the only concern of a grower.


We also must account for photomorphogenesis, which deals with the way plants grow in response to various wavelengths of light. This can even include wavelengths beyond the realm of PAR, such as UV light. Plants will display various morphologies or developmental traits in response to combinations of light. A classic example is the ratio of red to far red light.

In this test, we saw that including blue light at a ratio of 1:3 (blue to red) was able to prevent the plant from growing in an elongated fashion. This photomorphogenic response explains why we see the popular red / blue mixture of LEDs that are so common in grow lights.


Full spectrum light is the best way to ensure that plants are getting everything they need. But producing such light in LEDs can reduce efficiency. Red and blue LEDs are the colors that offer highest levels of efficacy. Their light is readily absorbable by chlorophyll and easily drives photosynthesis. In a proper ratio of blue to red, most plants respond with natural looking growth.

What about green light? Do plants use green light, or it wasted energy?
Find out at my site:

#ScienceExperiments #LEDGrowLights #VegetativeGrowth #IndoorGrowing #ArtificialLightings #HydroponicLettuce #UrbanGardening


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