What if an Eight-Week Flowering Cycle Didn't Have to Take Eight Weeks?
Introducing the P-Sol Flowering Cycle Methodology
For decades, cannabis cultivation has operated under a simple assumption: flowering takes eight to twelve weeks, and there is little anyone can do to change it.
Lighting technology has become more efficient. Environmental controls have become more sophisticated. Genetics continue to improve. Yet one constant has remained: the biological clock governing vegetative growth and flowering.
At Black Dog Horticulture Technologies & Consulting, we began asking ourselves a question:
What if we stopped designing lighting schedules around Earth's 24-hour solar day?
That question led to years of discussion, planning, and ultimately experimentation. The results surprised even us.
Our initial research demonstrated that photoperiod-sensitive cannabis can complete flowering approximately 25% sooner under a shortened light/dark cycle while producing flower quality and quantity comparable to conventionally-grown control plants.
That's two full weeks removed from a normal 8-week flower cycle.
"Diva" strains that normally require 12 weeks could be harvested in only 9.
If these results continue to scale commercially, the implications extend far beyond shorter flowering times. They could fundamentally change facility design, production planning, electrical utilization, breeding programs, and annual profitability.
This paper explains what we discovered, why it appears to work, and where we believe this research could lead.
In short, we believe we've uncovered a cultivation methodology with the potential to reshape how commercial facilities think about flowering, and we'd like to help determine what aspects of this methodology make sense for your unique facility.
Two Weeks Changes Everything
Removing fourteen calendar days or more from every flowering cycle doesn't simply shorten one harvest.
It could change the economics of entire facilities.
If this approach ultimately proves commercially viable, it could provide your facility:
- More annual harvests
- Better utilization of expensive cultivation facilities
- Improved cash flow
- Higher ROI on cultivation infrastructure
So, how does it work?
Re-thinking a Long-Held Assumption
One of the more empowering aspects of being an indoor cultivator is controlling the daily light cycle of plants to manipulate their vegetative and flowering phases. As cannabis growers are aware, giving the plants 16-24 hours of light each day maintains vegetative growth. Switching to a 12/12 cycle with 12 hours of light and 12 hours of darkness initiates flowering.
But each light/dark cycle still adds up to 24 hours.
Over 12 years ago, during one of our many discussions surrounding lighting and plant physiology, Kevin Frender- Black Dog Chief Science Officer and life-long plant geek with over 45 years' experience growing indoors- proposed a deceptively simple question:
Does flowering actually require a 24-hour day?
The question sounds almost trivial.
It isn't.
Photoperiod-sensitive plants don't measure the length of daylight to determine when to flower. They measure the duration of uninterrupted darkness, and once this reaches a critical threshold, hormonal changes trigger flowering.
Since darkness and not total day length is the true biological trigger, what if we provided the critical 12-hour dark period, but shortened the illumination period to create a less-than 24-hour "day"?
That single question became the foundation of everything that followed.
Geeks Love a Good NASA Name
Very quickly we ran into a language problem: using the term "day" when discussing this idea quickly becomes confusing. We often use the word "day" to refer to a 24-hour period, but also to the daylight portion of that period.
To avoid similar confusion when coordinating schedules on 2 different planets, in 1976 NASA scientists coined the term "sol" to refer to the 24.66 hour long solar day on Mars.
Borrowing from that, we adopted the term Plant Sol (P-Sol) to describe any repeating light and dark cycle that is not 24 hours long.
The Science Behind Our Theory
Plants' cellular metabolism and division are at least partially governed by their circadian rhythm: a natural, roughly-24-hour cycle, which itself is continually synchronized to their environment by exposure to light and darkness- the "circadian clock".
This gets rather complex and is outside the scope of this article, but if you would like to learn more on how this works, search for information about phytochromes and cryptochromes- as plant geeks we find this stuff fascinating.
If it was possible to continuously re-synchronize the circadian rhythm to a less-than 24 hour cycle, it may be possible to accelerate plants' metabolism and cellular division, ultimately speeding up growth and maturation.
Once you wrap your head around the idea, you quickly realize that running 6 hours illuminated and 12 hours of darkness for cannabis could cut 25% off the time it takes to complete a regular flowering cycle- if it worked. Essentially, every "day" for the plants becomes 18 hours long instead of 24- hopefully accelerating their growth and maturation compared to natural days.
When we first thought of this we naturally explored existing research.
The only study we found that tested a non-24-hour clock cycle was from Dodd AN, Salathia N, Hall A, Kévei E, Tóth R, Nagy F, Hibberd JM, Millar AJ, and Webb AA in 20051. Rather than examining the effects of non-24-hour cycles on photoperiodicity, this experiment focused on aligning individual cultivars' natural genetic circadian rhythms with a similar-length P-Sol.
The study identified cultivars of Arabidopsis thaliana, the "lab rat" of botanical research, that naturally had circadian rhythms of 20 hours, 24 hours and 28 hours when the rhythm was not being reset by the circadian clock. Then they exposed each of the 3 identified cultivars to 10/10, 12/12 and 14/14 hour schedules: a 20-hour P-Sol, normal 24-hour day, and a 28-hour P-Sol.
Their findings indicated that cultivars performed better when given light/dark cycles matching their natural circadian rhythm.
That result has interesting implications of its own, but this study did not answer our question as to whether photoperiod-sensitive plants can perform with an altered light and dark cycle designed to re-align their circadian clocks and rhythms. It only indicated that cultivars naturally aligned with a shorter-than and longer-than 24-hour P-Sol performed better when their default circadian rhythm aligned with the photoperiod provided.
We're aware of many other research studies exploring non-12/12 flowering schedules for cannabis, for example using a 13/11 schedule to increase yield. But these are still using a cycle that repeats every 24 hours.
P-Sols are divorced from a 24-hour cycle completely.
The Challenges
Initially, our largest question was if it is really possible to trick a plant into accelerating its metabolism and maturation by using a less-than 24-hour P-Sol, which only experimentation could answer.
But there are real-world challenges to running a P-Sol schedule.
Biological and Engineering Challenges
There is a change in environmental needs. Most thermostats and other environmental control gear are firmly based on 24-hour days. Cycle timers, such as those used in hydroponics, can run a simple repeating cycle for lights, but HVAC and related equipment is more difficult to control.
As lighting experts, we recognized that with a reduction in the duration of the lights-on period, the DLI (Daily Light Integral- the total amount of light given to plants in a day, or in our case a P-Sol Light Integral or PLI) would drop. To offset this and avoid a reduction in yield, a commensurate increase in the light intensity is likely necessary. For example, if you were running 1000 PPFD with a normal 12 hour flowering light cycle, you would need to provide 2000 PPFD to attain the same DLI in a 6 hour period. For those unfamiliar, PPFD (Photosynthetic Photon Flux Density) measures the immediate intensity of light on a crop and is usually measured in the number of photons (in micromoles) in the 400-700nm range going through one square meter each second, expressed as μmol/m2/s.
However, light intensity cannot simply be arbitrarily increased- 2000 PPFD under high pressure sodium lights would quickly burn the leaves.
The light spectrum matters here, as some spectra heat leaves up much more than others. We published research on this over 12 years ago while we were perfecting Black Dog LED's spectrum, showing how different spectrums affected leaf surface temperature. We have since run research cannabis grows with our spectrum at over 2600 PPFD with almost no bleaching or other negative side-effects. This spectral tuning allowing extreme light intensity without harming plants is a key to accelerating the plants' metabolism in shorter-than 24-hour P-Sols.
Yet another issue with lighting is the physical size of fixtures. Most existing LED "bar lights" simply cannot provide 2000 PPFD average illumination over an area the size of the fixture, and they cannot be "stacked on top of each other" without blocking light, overheating the fixture, or both. Black Dog LED's compact, actively-cooled PhytoMAX-4 lights can be deployed to achieve intensities exceeding 6000 μmol/m2/s average PPFD, enabling tests with extreme illumination.
But, if the biology of P-Sols works, the engineering problems can be solved. That's exactly what Black Dog HTC has spent years doing.
Practical Challenges
The final major challenge is purely practical, but potentially a deal-breaker: a non-24-hour cycle can lead to days where the lights are not on during normal working hours. Having run plenty of grows ourselves, and with 16 years' experience consulting on commercial facility workflows, labor-balancing, and power-balancing, we knew this could create a nightmare for commercial growers.
An oddly-offset non-24-hour schedule could also cause electrical problems in facilities where flower rooms are "flipped" so that one flower room has lights on while another has lights off to reduce total amperage loads.
But there's one special less-than 24-hour P-Sol that has some unique properties.
The Goldilocks P-Sol
An 18-hour P-Sol could provide the "just right" fit of accelerated harvests and practical scheduling. Having 6 hours illuminated and 12 hours of darkness allows a light schedule that always has at least one hour of lights-on in the room during a normal 9-to-5 8-hour workday.
A visual representation of an 18-hour "Goldilocks" P-Sol Schedule- note that the 56th P-Sol (the equivalent of 8 normal weeks) occurs on the 42nd calendar day (6 calendar weeks).
This 18-hour P-Sol would also allow rooms to be "flipped" not just across 2 rooms but 3: one room with lights on and two other rooms off at all times.
A schedule showing 3 flower rooms "flipped" with each other using the 18-hour Goldilocks P-Sol. Note that the schedule repeats every 3 days, and can be adjusted so that all 3 rooms are accessible during an 8-hour workday.
But cutting the illuminated period in half of the "normal" 12 hours seemed rather ambitious- surely the plants would stress out?
It would also require doubling light intensity to keep the same number of photons nourishing the plants, and under most grow light spectrums plants start burning or bleaching when intensity is doubled to 1800-2400 μmol/m2/s from a "normal" level of 900-1200 PPFD.
But we had already designed and tested our lights' spectrum to run 1800-2600 PPFD without causing plant stress, so providing a normal DLI / PLI for the Goldilocks P-Sol is possible.
Given the "just right" balance of the 18-hour Goldilocks P-Sol, we recognized that this could be the foundation of a very different commercial production strategy.
But only if it actually worked.
Part 2 coming tomorrow, August 26!
Black Dog Horticulture Technologies & Consulting
References:
1: Dodd AN, Salathia N, Hall A, Kévei E, Tóth R, Nagy F, Hibberd JM, Millar AJ, Webb AA. Plant circadian clocks increase photosynthesis, growth, survival, and competitive advantage. Science. 2005 Jul 22;309(5734):630-3. doi: 10.1126/science.1115581. PMID: 16040710.
2: Ahrens A, Llewellyn D, Zheng Y. Longer Photoperiod Substantially Increases Indoor-Grown Cannabis' Yield and Quality: A Study of Two High-THC Cultivars Grown under 12 h vs. 13 h Days. Plants (Basel). 2024 Feb 1;13(3):433. doi: 10.3390/plants13030433. PMID: 38337966; PMCID: PMC10857075.