Can Cannabis Growers Optimize Light Capture?

Editor's Note: Cannabis Business Times published another version of this article in August 2023.

Commercial cannabis growers face an ever-evolving market and constant uncertainty. As a result, we must continually challenge our own thinking and build upon what we think we already know about growing. Across the industry, a primary objective is to reduce cost per pound while increasing quality and yield, to generate more revenue per square foot, per turn and per year. But how we pursue that goal will be essential to survival and success. 

In the past, cannabis activists pushed for information sharing regarding cultivation practices, but growers are hesitant with the commercialization of cannabis and everyone thinking they have hit the goldmine with a special secret sauce.

Nowadays, there is still a lack of solid commercial data from growers about successful, cost-effective and progressive cultivation practices that could help all cultivators as a group. It is essential to explore ways to build awareness and encourage the sharing of scientifically valid and commercially proven information within the cannabis community and industry to further advance the market as a whole. 

With that said, StateHouse, a California-focused, vertically integrated cannabis company, would like to share an innovative approach to optimizing cannabis yields by improving photon conversion efficiency while potentially reducing total energy use required per gram of biomass produced.

Photosynthesis Fundamentals

As a grower myself, I’ve come to believe that channeling energy directly to cannabis flowers during production—what we at StateHouse now call Flower Direct Cannabis Cultivation (FDC²)—may hold great potential for the goals we aspire to achieve. 

While lighting is just one aspect of cultivation, optimizing light is crucial to FDC² success. Understanding some fundamentals of photosynthesis is key to implementing this new growing approach.

There’s been a lot of attention given to the relationship between light and yield—specifically the commonly cited metric that 1% more light equals 1% more yield. This is widely assumed not only in cannabis but also in other crops, including commercial tomato, cucumber and leafy green production. This rule is a good baseline, but the linear relationship of light to yield is not exactly that simple. That’s where understanding photosynthesis comes in.

Photosynthesis is the process by which green plants and other organisms convert photosynthetically active radiation (PAR) energy from the sun into chemical energy in the form of organic compounds, mainly glucose. During photosynthesis, carbon dioxide (CO₂) from the atmosphere is combined with water to form glucose, which can then be used by the plant for energy and growth.

In that process, the carbon in CO₂ is partitioned into the glucose molecules produced. The glucose can be used immediately for energy, or it can be stored in the form of starch or other complex carbohydrates for later use. These carbohydrates serve as the building blocks for biomass and yield in cannabis production.

Once we appreciate the fundamentals of photosynthesis (light + CO₂ + H₂O), we can work to optimize the growing environment’s impact on it. This involves examining cultivation performance via key environmental measurement metrics, including daily light integral (DLI), photon conversion efficiency (PCE) and leaf area index (LAI), along with other FDC2 metrics. Being able to calculate these metrics and compare them against yields clearly conveys both the facility infrastructure’s latent yield potential or capabilities and, separately, a grower’s performance.

Key Metrics for Optimizing FDC2

Two well-known adages are fitting here: “You can’t manage what you don’t measure” and “Don’t be lazy in learning.” Keeping the simplicity, relevance and importance of these two quotes in mind can help as we examine light-related metrics and look at new opportunities to maximize yields in cannabis flower production systems.

The Optimal Daily Light Integral (DLI)

In partnership with the American Flower Endowment, Dr. Jim Faust and Dr. Joanne Logan (from Clemson University and University of Tennessee, respectively) created interactive daily light integral (DLI) maps and explained DLI this way: “Daily Light Integral represents the total photosynthetically active radiation (PAR) accumulated over one day (24 hours). Since plants are accumulators of solar radiation, this measurement is extremely useful when describing solar radiation as it affects plant growth. Daily light integral has become a familiar measurement for plant scientists and commercial growers.”

Photosynthetic photon flux density (PPFD) is light intensity or instantaneous light measured in micromoles per square meter per second (μmol/m2/s). In contrast, DLI is a cumulative measure, a function of both light intensity and duration, measured as moles per square meter per day (mol/m²/day).

It’s important to note that natural DLI is tied not only to the sun’s seasonal intensity but also to your natural day length as it changes throughout the year. Keep in mind this addition or loss of light throughout the year directly impacts photosynthesis and yields as discussed earlier. When growing in a greenhouse, this is especially critical to understand.

Research out of Utah State University’s Crop Physiology Laboratory (note: StateHouse is a funding partner for the USU Lab) shows that cannabis can be continually productive up to DLIs as high as 70 and potentially above. However, DLIs in the United States do not reach this level naturally, even during peak summer, and most indoor growers shoot for between 51 and 55 moles per day or approximately 1,200 umol/m2/s.

As growers, our goals are not always aligned with Mother Nature. To achieve the highest yields possible year-round, growing indoors or in a greenhouse with supplemental artificial lighting is necessary to achieve those DLIs.

The maps on p. 22, created by Drs. Faust and Logan, reflect the variation in DLI based on U.S. geography and seasons. January is on the top, and July is below. The brightest reds on the July map represent DLIs of 55 to 65, while the pinks and purple on the winter map correspond to 5 to 15 mol/m2/day. Depending on your location, your natural DLI can change by 40% to 65% throughout the year—having a significant impact on cultivation performance.

StateHouse’s cultivation facility in Salinas, Calif., is in a region with some of the highest DLIs in the United States, but the change between summer and winter is significant. The natural DLI outside the greenhouse changes by 60% throughout the year. At StateHouse, we use 60 moles as a practical, cost-effective target for optimization of yields, and even in California, we have opportunities to improve light capture and utilization.

Let’s say you’re a commercial greenhouse grower growing under glass in a tall Venlo greenhouse. When adding supplemental LED top lighting to optimize your grow, there are many considerations—including fixture form factor, fixture efficiency and spectra. But focusing on DLI as a function of light intensity you want to make sure your crop never receives a DLI less than 30 mol/m²/day at any given time of the year—not including your natural ambient light conditions. This means you would need to outfit your greenhouse with no less than 700 μmol/m²/s of supplemental LED top lighting when growing under a flowering photoperiod (approximately 12-hour day length).

Photon Conversion Efficiency (PCE)

Photon conversion efficiency is another important parameter to consider and measure when trying to maximize yield with light, as it directly affects the efficiency of a growing system and therefore should be considered a new key cultivation performance indicator/metric. Photon conversion efficiency (PCE) reflects the efficiency with which light energy is converted into biomass. More specifically, it’s the ratio of the number of photons absorbed to the number of photons converted into useful energy.

A higher PCE means that more of your light energy is producing useful energy, which leads to higher overall system efficiency and yield. In photosynthetic systems, a higher PCE means more of the absorbed photons are being utilized to drive the photosynthetic process. This results in a higher rate of biomass production from the same amount of light energy.

When discussing cannabis flower production or biomass yield in general, the unit of measure for PCE is grams per mole (g/mole) of photosynthetically active radiation (PAR) light. You can apply this to a single square foot, cycle or year (g/mole/ft²/cycle). To maximize yield with light, it is important to design photosynthetic systems that have high photon conversion efficiency. This can be achieved by optimizing the system’s materials, structure, light design, defoliation techniques, crop density and other operating conditions.

Currently, most growers only apply lighting from above the crop—and most commercial lighting is designed for that application. Obviously, this approach is due to plants being naturally adapted to capture light from the sun. But upper leaves are not the only surfaces that can drive photosynthesis. Again, when thinking about crop production systems, our goals as growers are not always the same as Mother Nature.

Leaf Area Index (LAI)

LAI is a key metric used in many fields, including ecology, agriculture, forestry and climate science, to provide valuable information about plant canopy structure, plant growth and ecosystem functioning. For cannabis growers, LAI is a critical tool in optimizing photon capture during production.

Typically expressed as a ratio, LAI quantifies the amount of foliage or leaf surface area present in a defined area. More specifically, LAI is the ratio of total leaf surface area—including both the upper (adaxial) and lower (abaxial) surfaces of leaves—to the ground (or bench) surface area.

Whether they know it or not, cannabis growers use this metric when filling in their canopy space during flowering. But there are many different approaches to maximizing canopy area. Although different approaches may achieve the same “total” yield result, they come with extremely different costs, system efficiencies gained or lost, and varying quality results.

For example, one way to fill in your growing area is to place more plants at it. Many assume you can increase yields easily by adding more plants. However, in some systems, labor cost has a 1 to 1 relationship with plant count, along with other direct costs, like materials (pots, substrate, fertilizer, water, etc.). More plants can require more handling; more defoliation contributes to additional labor and other costs. More plants also mean additional total wet weight going into drying, so a grower must remove more water from non-flower biomass. This poses many challenges, especially when growers have fixed and limited drying space, limited water removal capacity and limited dry time.

From an efficient drying and quality-ensuring perspective, growers can try to implement strategies that reduce total wet weight being introduced into drying at one time, enabling more precise control over the drying process. There are operators with systems as high as 1 plant per 1 square foot (1p/ft²) with the same yield as 1 plant per 3 square feet (.3p/ft²)—with the exact same crop time and schedule, but with vastly different finished flower quality, biomass category ratios, PCEs and costs per pound. In other words, less, i.e., fewer plants, can be more—and more efficient. 

Flower Direct Cannabis Cultivation Strategies

As growers understand and appreciate daily light integral, photon conversion efficiency and leaf area index, the bigger question becomes how we can optimize these metrics to maximize production, while also minimizing cost per pound. This is where FDC² strategies and technologies, like those that follow here, come into play.

Light Diffusion

In lighting, diffusion refers to the process of scattering or softening light rays to reduce harsh shadows and create a more even and uniform distribution of light. Greenhouses are subject to shading and, as the sun rises and sets, growing areas receive varying amounts of light throughout the day. By diffusing the light, the crop/growing area can receive more consistent light levels throughout the day while possibly accumulating more total light (DLI) during a single day.

Light diffusion also can help with temperature/heat management issues caused when wasted light concentrates on non-plant surfaces and contributes to radiant heat levels. With that said, light diffusion is also why most indoor grow rooms are white. Flat white can reflect and diffuse up to 85% of light, ensuring the same benefits seen when growers deploy proper light diffusion strategies in a greenhouse. The same is not true for gloss white, which can reduce uniform light diffusion compared to flat white by up to 50%. These small details matter.

While this may sound new to some growers, the proven strategy of maximizing diffusion and increasing light accumulation in plants in a single day—increasing single-day DLIs—has been integrated into traditional greenhouse production system designs for more than a decade. Understanding the benefits of diffusion, we can appreciate the possibilities when plants are able to absorb photons from all angles, not just from above. 

Moving Beyond Top Lighting

Chlorophyll, the pigment that gives plants their green color, is present in the chloroplasts of plants, algae and some bacteria. It is essential for photosynthesis. In fact, the importance of chlorophyll in photosynthesis cannot be overstated.

Chlorophyll is the molecule responsible for capturing light energy and transferring it to other molecules in the photosynthetic pathway. Without chlorophyll, plants and other photosynthetic organisms would not be able to produce the energy they need to survive nor convert that energy into biomass.

Knowing the importance of the role of chlorophyll in photosynthesis and its direct impact on plant growth/biomass production, it’s also critical to acknowledge that cannabis flowers are green. Yes, photosynthesis in fruits and flowers is also significant, not just leaves; green tomatoes are just one example. This principle may be crucial to maximizing your PCE during cannabis flower production.

It’s time to now ask the question: Would applying light directly to all flowers on a single plant (throughout the whole vertical and horizontal profile) increase photon capture, therefore increasing photosynthesis, resulting in increased PCE and greater yields? Here at StateHouse, we think it does.

Dr. Bruce Bugbee at Utah State University’s Crop Physiology Laboratory offers additional support for this idea, including research by USU Ph.D. Candidate and Graduate Research Assistant Mitchell Westmoreland.

“Cannabis flower photosynthesis is proportional to chlorophyll and surface area of the flowers. Mitch has data indicating that the flowers contribute up to 30% of the total photosynthesis at harvest. This is based on canopy photosynthetic measurements after removal of the leaves,” Bugbee says. Consider that again: Flowers contribute up to 30% of the total photosynthesis at harvest.

To date, much of the research exploring more direct application of light to cannabis flowers, central to Flower Direct Cannabis Cultivation, has focused on supplementing top lights with intercanopy lighting (ICL), sometimes called intracanopy lighting, which places supplemental lighting into the canopy itself. Bugbee’s lab is not alone in acknowledging ICL’s potential for improving plant growth and yield.

Many other universities have also studied this promising lighting technique, including the University of Arizona’s Controlled Environment Agriculture Center; Wageningen University & Research (Netherlands); University of California, Davis, Department of Plant Sciences; Michigan State University’s Department of Horticulture; and Purdue University’s Department of Horticulture and Landscape Architecture. The latter three have also developed numerous lighting systems now being used in commercial greenhouse operations.

Research: Intercanopy Lighting Validation Group (ICLVG)

Based on StateHouse’s own robust internal experiments and data from related research, the company, partnering with the Cannabis Research Coalition and others, explored FDC² lighting principles through the application of a specialized form factor of lighting fixture designed specifically for cannabis production.

The specialized fixtures have two levels and surround the crop while also sitting on the bench, enabling trellis connection and bracing of the plants. These fixtures create a three-dimensional cube around the crop.

As the crop grows, a grower can tweak the placement and direction of the light to channel directly on the flowers of the crop.

The objective of the study was to determine the impact of redistributing the same DLI (same amount of energy used) between top lighting alone versus the combined application of top lighting plus the surround canopy fixture on cannabis crop biomass yield, biomass category ratio and finished flower quality. Read more details about the study and the findings, published in Cannabis Business Times, here.

As VP of cultivation for StateHouse, Travis Higginbotham oversees a 230,000-square-foot cannabis greenhouse operation, running all cultivation, postharvest and bulk sales.