Many indoor gardeners and urban farmers are interested in using LED grow lights to supplement natural sunlight for their plants. LED lights provide many benefits over traditional grow lights like fluorescent or high pressure sodium lamps – they use less energy, generate less heat, and allow you to select specific wavelengths of light. But when it comes to LED grow lights, one question often comes up – does the color of the LEDs matter for plant growth? Specifically, do purple LED grow lights provide any advantages compared to other colors?
Purple LED grow lights have become very popular in recent years. The reason behind this has to do with the light spectrum that plants use for photosynthesis. Plants primarily absorb light in the blue (400-500 nm) and red (600-700 nm) regions of the visible light spectrum. They absorb less green and even less infrared and ultraviolet light.
Why plants use blue and red light
Plants have evolved to capture blue and red light because this is the spectrum of light that reaches the Earth’s surface. Shorter wavelength blue light is absorbed by chlorophylls and carotenoids and supports vegetative leaf growth. Longer wavelength red light is absorbed by phytochromes and supports flowering and fruiting. Together, blue and red light drive photosynthesis and allow plants to produce sugars and energy.
Some LED grow light manufacturers claim that adding wavelengths in the purple or ultraviolet range provides additional benefits to plants. The theory is that these wavelengths contain higher amounts of energy that can boost plant growth and influence plant morphology. But is there any scientific evidence to support these claims?
Research on supplemental UV and purple light
A number of studies have looked into how UV light and specific purple wavelengths affect plant growth:
| Study | Wavelengths Tested | Effects on Plants |
|---|---|---|
| Lee and Kim 2016 | 400-500 nm (blue) 530-590 nm (green/yellow) 640-690 nm (red) |
No differences seen between blue, green, yellow and red LEDs for leaf lettuce |
| Muneer et al. 2014 | 365 nm UV-A 385 nm UV-B |
UV exposure increased antioxidants in Brassica microgreens but reduced biomass |
| Samuoliene et al. 2012 | 375-400 nm UV-A | UV-A enhanced mint phenolics but caused oxidative stress |
| Snowden et al. 2016 | 405 nm purple 435 nm purple |
No differences seen between purple and red/blue LEDs for tomato seedlings |
These studies found that UV wavelengths (below 400 nm) and specific purple wavelengths did not improve plant growth or quality over standard red and blue LEDs. Exposure to UV often caused oxidative stress and reduced yields. The purple wavelengths tested did not provide any measurable advantage for the parameters studied.
Why purple LEDs remain popular
If research does not show major benefits to supplemental purple light, why are purple LED grow lights still so popular? There are a few possible reasons:
– Aesthetics – Some people prefer the look of purple lighting over red and blue light. The purple hue is less harsh to work under.
– Mimicking HPS lighting – Plants grow well under traditional high pressure sodium (HPS) grow lights, which emit a broad spectrum with spikes in the blue, green, and red regions. The mix of wavelengths in purple LEDs attempts to mimic this HPS spectrum.
– Marketing – Advertising specific purple diodes is an easy marketing tactic to make LED grow lights seem more advanced. Terms like “ultraviolet” and “photosynthetic” make the lights sound high-tech and effective for plant growth, whether or not there is evidence to back up those claims.
So in summary, while purple wavelengths might not provide any physiological advantage, growers may opt for purple LED lights based on subjective preferences for their color. But you can grow big, healthy plants under standard red and blue LEDs without the need for exotic purple diodes.
Optimal wavelengths for indoor plants
Based on the available research, the ideal wavelengths for an LED grow light are:
– Blue (400-500 nm) – supports leafy growth
– Green (500-600 nm) – aids in photosynthesis
– Red (600-700 nm) – drives flowering and fruiting
– Far red (~730 nm) – aids in flowering for some long-day plants
A ratio of approximately 2:1 red to blue light provides a good balance for most indoor plants. No UV or specific purple diodes are required to achieve optimal growth.
Here is an example of a suitable wavelength ratio from a commercial LED grow light:
| Wavelength | Light Color | Percentage |
|---|---|---|
| 440-470 nm | Blue | 12% |
| 620-630 nm | Red | 5% |
| 650-670 nm | Red | 27% |
| 730 nm | Far red | 1% |
| White | (green/yellow) | 55% |
This provides roughly 40% blue, 30% red, a small amount of far red, and white light delivering green and yellow wavelengths. This is an effective spectrum for growing most common garden plants.
Choosing full spectrum vs. purple grow lights
When shopping for LED grow lights, you will see models advertised as “full spectrum” and models featuring heavy emphasis on purple diodes. Here are some factors to consider when choosing between the two options:
| Full Spectrum | Purple LED | |
|---|---|---|
| Light Color | White and bright | Muted purple hue |
| Light Distribution | Even mix of blue, green, red | Blue and red peaks |
| Plant Response | Balanced, normal growth | Possible blue/red imbalance |
| Light Intensity | High intensity across all colors | Less green/yellow wavelengths |
| Cost | Higher upfront cost | Lower upfront cost |
Full spectrum lights deliver a continuous band of wavelengths that better mimic natural sunlight. This balanced energy typically leads to healthier plants.
Purple lights concentrate on peaks in the blue and red regions. They often lack intensity in the green/yellow portion of the spectrum. The skewed spectrum could lead to abnormal or stretched growth.
In the end, choosing between normal white/full spectrum LEDs or heavy purple LEDs comes down mostly to personal preference in the color aesthetic. Both can grow healthy plants if calibrated and used properly. But full spectrum delivers a more complete, natural light energy.
Conclusion
In summary, research does not show any major advantages to using LED grow lights focused on purple or UV wavelengths. While these exotic wavelengths might sound intriguing, plants actually thrive under a balanced full spectrum that includes blue, green, red, and white light. This mimics natural sunlight filtered through the atmosphere.
Well-designed full spectrum LED grow lights will provide all the wavelengths plants need without requiring special UV or purple diodes. Focus on delivering an optimized balance of cool whites, warm whites, and reds for the healthiest growth under LED lighting.
