Beyond PAR: A Grower's Guide to UV and Far-Red Light in Advanced Horticultural Lighting
13 Nov 2025
For years, horticulturalists have focused on Photosynthetically Active Radiation (PAR) as the gold standard for grow light spectrums. While the 400-700nm range is crucial for driving photosynthesis, cutting-edge research and commercial cultivation have revealed that what lies beyond PAR is key to unlocking a plant's full genetic potential. To truly master your craft, it's essential to understand the entire spectrum. For a foundational understanding of core lighting principles, we highly recommend reading The Ultimate Guide to LED Grow Lights: From Photon Science to Commercial Yields. This article will take you a step further, exploring the powerful roles of UV and Far-Red light in advanced horticultural lighting.
Understanding the Light Spectrum Beyond PAR
While PAR is the engine of photosynthesis, light at the edges of this range—specifically Ultraviolet (UV) and Far-Red—acts as a powerful signaling tool, influencing plant development, morphology, and the production of valuable secondary metabolites. Think of PAR as the fuel, and UV and Far-Red as the precision steering that guides the plant to its desired destination.
Modern LED systems, like those from Groxlum, are now engineered to provide targeted doses of these specific wavelengths, giving growers unprecedented control over their crops.
The Power of Far-Red Light (700-800nm): More Than Just a "Sleep" Signal
Far-Red light is a fascinating tool that directly communicates with a plant's phytochrome photoreceptors, which regulate key developmental processes.
The Emerson Enhancement Effect: A Photosynthetic Boost
One of the most significant benefits of Far-Red light is the Emerson Enhancement Effect. In the 1950s, Robert Emerson discovered that when Far-Red light (wavelengths >680nm) is applied alongside Deep Red light, the rate of photosynthesis is significantly higher than the sum of the rates from each light used individually . This synergy occurs because the two wavelengths excite different photosystems (Photosystem I and II) in the plant, allowing them to work in tandem more efficiently and boosting overall energy production.
Shade Avoidance and Strategic Growth
In nature, a high ratio of Far-Red to Red light signals to a plant that it is being shaded by a competitor. This triggers the "shade avoidance response," leading to:
•Stem and Petiole Elongation: The plant stretches to outgrow its neighbors and reach direct sunlight.
•Expansion of Leaf Area: Larger leaves are produced to capture more of the available light.
•Early Flowering: The plant accelerates its reproductive cycle to ensure survival.
While uncontrolled shade avoidance can be detrimental, a controlled application of Far-Red light at the end of the light cycle can be used strategically to increase plant biomass and encourage flowering.
Unlocking Potential with UV Light (UVA/UVB): A "Positive" Stressor
Just as a vaccine triggers an immune response, a small, controlled dose of UV light can act as a beneficial stressor (eustress), prompting the plant to produce compounds that enhance its quality, potency, and resistance to pests.
Boosting Secondary Metabolites: Terpenes, Flavonoids, and Cannabinoids
UV radiation, particularly UV-B (280-315nm), is known to significantly stimulate the production of secondary metabolites. The plant produces these compounds as a natural "sunscreen" to protect itself from DNA damage .
"UV-B radiation influences the yield of SMs [secondary metabolites] in medicinal plants, impacting the biosynthesis of phenolics, terpenoids, and alkaloids..." - BioResources, 2014
For growers of medicinal herbs, cannabis, and aromatic plants, this is a game-changer. Increased production of these compounds leads to:
•Enhanced Aroma and Flavor: Higher concentrations of terpenes.
•Increased Potency: Greater accumulation of active compounds like cannabinoids.
•Improved Pest and Mold Resistance: A stronger natural defense system.
Strategic application of UV-B can lead to a measurable increase in these valuable compounds, directly impacting the market value of a harvest.
How to Implement an Advanced Spectrum with Groxlum
Integrating UV and Far-Red light requires precision. Too much can be harmful, while too little will have no effect. This is where engineered solutions become critical.
Groxlum's full-spectrum LED fixtures are designed with dedicated UV and Far-Red diodes on separate, controllable channels. This allows growers to implement advanced lighting strategies with confidence.
Here is a simple guide to getting started:
1.Vegetative Stage: Focus on a PAR-dominant spectrum with minimal UV and Far-Red to build a strong plant structure.
2.Flowering Stage: Introduce UV-B for short periods during the light cycle to begin stimulating secondary metabolite production.
3.End-of-Day Treatment: Apply Far-Red light for 15-30 minutes after the main lights turn off to promote flowering and the Emerson Effect.
4.Late Flower/Ripening: Increase the duration and intensity of UV-B exposure to maximize terpene and flavonoid expression just before harvest.
Conclusion: The Future of Lighting is the Full Spectrum
The conversation around horticultural lighting is evolving. By looking beyond PAR and embracing the signaling power of UV and Far-Red light, growers can move from simply growing plants to truly designing them. This level of control enhances crop quality, increases commercial value, and creates a more resilient final product.
Ready to harness the full power of the light spectrum? Dive deeper into the science and explore the technology that makes it possible by visiting The Ultimate Guide to LED Grow Lights: From Photon Science to Commercial Yields and discover Groxlum's range of professional, full-spectrum lighting solutions.