Indoor cannabis cultivation has always been a game of controlling every variable — temperature, humidity, nutrients, airflow. But light is the one lever that keeps surprising researchers. A new peer-reviewed study published in April 2026 in the journal Horticulturae by scientists from the University of Bologna reveals that adding a specific wavelength of light — far-red, invisible to the naked eye — can significantly boost vegetative growth in cannabis without disrupting its flowering cycle. The implications for commercial indoor growers could be substantial.
Key Takeaways
Continuous far-red light supplementation increased cannabis plant height by 9% and leaf biomass by over 16% compared to standard red-blue lighting. Both far-red and end-of-day far-red treatments boosted chlorophyll content by 9%, improving the plant’s photosynthetic capacity. Critically, flowering time was completely unaffected by light spectrum — cannabis blooms on schedule regardless. End-of-day far-red is a practical, lower-energy compromise for growers who want the benefits without the electricity costs of running far-red all day.
What Is Far-Red Light, and Why Should Cannabis Growers Care?
Visible light spans roughly 400 to 700 nanometers — that’s everything from violet through red that human eyes can detect. Far-red light sits just beyond that, at 700 to 750 nanometers, in a zone we can’t see but plants absolutely can. For decades, horticultural scientists have known that far-red plays a crucial role in how plants sense their environment, particularly whether they’re growing in shade or in open sunlight.
Plants detect far-red through specialized proteins called phytochromes, which exist in two forms that flip back and forth depending on the light they receive. When far-red dominates, phytochromes switch into a state that tells the plant it’s growing in shade — beneath a forest canopy, for example — and triggers what scientists call the “shade avoidance syndrome.” The plant starts stretching upward, expanding its leaves, and ramping up its photosynthetic machinery to capture more light.
Indoor cannabis growers, of course, are not growing in a forest. But as this new research shows, strategically triggering those same responses with LED lighting can produce measurably healthier, more productive plants. It’s worth noting that spectrum manipulation doesn’t stop at growth — we’ve previously covered how specific far-red protocols can influence THC accumulation as well, making light quality one of the most versatile tools available to modern cultivators.
Inside the Bologna Study: How They Did It
The research team from the University of Bologna set up a controlled experiment using cannabis cuttings (a CBD-dominant variety called Mango) and tested three lighting conditions over a 49-day growth cycle. The first group received only standard red and blue LED light — the conventional horticultural setup. The second group received the same red-blue base with continuous far-red light added throughout the full 12-hour photoperiod. The third group received red-blue light for 11 hours, followed by one intense hour of far-red at the end of the day — a technique known as “end-of-day” (EOD) treatment.
All three groups received the same total daily light dose, so any differences in plant development could be attributed purely to light spectrum rather than light quantity. After an 18-day vegetative phase in a greenhouse, plants were moved into enclosed grow boxes where a 12-hour light/12-hour dark cycle was used to trigger flowering.
The Results: Far-Red Makes Plants Bigger and More Productive
The plants grown under continuous far-red light were measurably taller — about 9% taller than control plants — at an early measurement point, and their leaf stems (petioles) were 17% longer. Longer petioles mean leaves spread out further, increasing the canopy area available to capture light. More importantly, the leaf dry weight — a reliable measure of actual biomass accumulation — was 16.3% higher in far-red plants than in the control group, and 9% higher than in end-of-day plants.
When the researchers analyzed plant physiology using a sophisticated multispectral imaging system, the differences became even clearer. Plants grown under both far-red conditions had chlorophyll levels roughly 9% higher than control plants, along with higher scores on the NDVI — a standard index of vegetation health and photosynthetic vigor. Interestingly, the control plants grown under red-blue light alone accumulated more anthocyanins, pigments associated with plant stress. This suggests that without far-red supplementation, cannabis plants may experience a degree of light-quality stress that diverts resources away from productive growth.
The Most Important Finding: Flowering Stays on Schedule
For commercial cannabis growers, perhaps the most reassuring result of the study is what did not change. Despite the dramatic differences in plant morphology and physiology, flowering time was identical across all three groups. Every plant, regardless of light spectrum, began flowering at exactly the same time once the photoperiod was shifted to 12 hours of darkness — and remarkably, all plants across all treatments showed pistil emergence within the same two-day window.
This confirms something plant scientists have long suspected but not thoroughly tested in cannabis: floral induction in this species is controlled by photoperiod — the length of the dark period — not by the quality of light. You cannot accidentally trigger early or late flowering by adding far-red. That’s good news for growers who may have been reluctant to experiment with spectrum, fearing disruption to their production schedules.
End-of-Day vs. Continuous: What’s the Real Trade-Off?
One of the study’s most practically useful contributions is the direct comparison between running far-red all day versus just for the final hour. End-of-day treatment consumed slightly less energy than continuous far-red (1.47 versus 1.51 kWh per square meter per day), and it still delivered meaningful physiological benefits — higher chlorophyll content, improved canopy development, and better plant health indices — compared to the red-blue control.
However, the continuous far-red treatment was more energy-efficient when measured as grams of dry biomass produced per kilowatt-hour consumed. In other words, it cost a little more to run, but it produced proportionally more plant material, making it the more economically sound choice if maximizing vegetative biomass is the priority.
The researchers also point out an intriguing practical opportunity: end-of-day treatment happens at the end of the light cycle — often in the evening — which in many electricity markets, including Italy where the study was conducted, corresponds to off-peak tariff periods with lower energy prices. Scheduling that final hour of far-red enrichment during cheaper electricity windows could make the technique essentially cost-neutral while preserving most of its physiological advantages.
What This Means for the Cannabis Industry
Indoor cannabis production is one of the most energy-intensive agricultural operations in existence. Lighting accounts for a massive share of operating costs, and even marginal improvements in energy-use efficiency translate into significant savings across large facilities. This study gives growers a concrete, scientifically validated tool for improving plant performance without increasing energy costs substantially — and without any risk of disrupting the flowering schedule that commercial operations depend on.
The findings also open new questions. The study used a single cultivar and focused on the vegetative phase; the researchers themselves note that the effects of far-red during the flowering stage — particularly on bud development, cannabinoid content, and trichome formation — remain unexplored. Light is just one piece of the puzzle: other pre-harvest interventions are also gaining traction in the research community, and we’ve reported on how salicylic acid applications in the days before harvest can meaningfully elevate THCA and THC levels, pointing to a broader toolkit growers can draw on to improve cannabinoid profiles without changing genetics. There is also meaningful variation between cannabis genotypes in how they respond to light spectrum, meaning that growers using different strains may see somewhat different results.
Still, the core message is clear and actionable: far-red light, whether applied continuously or strategically at the end of the day, is a legitimate lever for optimizing cannabis cultivation in controlled environments. In an industry where marginal gains in plant performance and operational efficiency can determine profitability, that’s a finding worth paying close attention to.
FAQ
This particular study did not measure cannabinoid concentrations, as it was designed to assess early vegetative and early flowering responses rather than final chemical profiles. However, prior research has found that excessive far-red light or low red-to-far-red ratios can reduce secondary metabolite production, including cannabinoids. The researchers deliberately limited far-red exposure during the flowering phase to avoid this risk. Further studies specifically examining far-red effects on THC, CBD, and terpene content during flowering are needed.
In principle, yes. Far-red LEDs are available from specialty horticultural lighting suppliers. The key parameters from this study were 20 micromoles of far-red per square meter per second for continuous supplementation, or a much higher intensity (220 micromoles) applied for just one hour at the end of the photoperiod for end-of-day treatment. However, the optimal settings may vary by strain, growth stage, and your existing light environment, so gradual experimentation with measurement is advisable.
This study found no effect on flowering time. All plants, regardless of spectrum, flowered simultaneously when placed on a 12-hour dark cycle. An earlier study did report slight acceleration of flowering with extended far-red end-of-day treatment (up to four hours), but the Bologna team’s shorter, one-hour end-of-day treatment did not replicate this effect. Cannabis flowering appears to be governed primarily by photoperiod, not light spectrum.
Yes — it’s arguably more practical for smaller operations than commercial facilities. A simple timer controlling a supplemental far-red LED strip during the final hour of the light cycle requires minimal additional investment and, based on this research, can meaningfully improve chlorophyll content and canopy development. The energy cost difference compared to standard red-blue lighting is minimal.
Disclaimer: This article is intended for informational and educational purposes only. Cannabis cultivation is subject to varying legal frameworks across jurisdictions. Always ensure compliance with your local laws and regulations before growing cannabis. The findings described here are based on a single scientific study conducted under specific controlled conditions and may not apply universally across all cultivars, growing environments, or cultivation systems. Consult qualified agronomists or lighting specialists before making significant changes to your production setup.
