The Key to Maximum Yield

When you don’t know where "optimum" is, it’s hard to get there. We made up the term “Optimum…ness” to describe a state that a lot of growers reach, where they are just not sure whether or not they can squeeze any more out of their facilities. They have tweaked and dialed the operation up to everyone’s satisfaction, and they know they are close to optimal.

But the real question is: How close are they to optimal?

The answer starts with understanding the nature of each cultivation variable’s optimal point and how to make the best use of it.

In 2008, researchers at the University of Mississippi published a paper (“Photosynthetic response of Cannabis sativa L. to variations in photosynthetic photon flux densities, temperature and CO₂ conditions") that validated the fundamental optimal growing conditions for cannabis cultivation to be light levels of 1,500 PAR [photosynthetically active radiation] and a leaf temperature of 86 degrees Fahrenheit.

To us, those numbers are design requirements that apply to every cannabis operation in the world. When you think about it, these numbers ultimately define the capital, labor and energy costs of growing at or near optimal conditions. These numbers tell growers where the holy grail of optimal cultivation is. It’s up to the grower to find a path to it.

How Do You Like Your Greens Cooked?

The rate of photosynthesis can be measured easily with available tools, allowing us to discover important information such as the relationship between photosynthetic rate and leaf temperature shown in the graph on this page.

Measured at 1,500 PAR illumination, the data shows a prominent peak around 86 degrees Fahrenheit and reflects the study’s findings that photosynthesis rapidly declined beyond the optimal temperature. The same is true of running cooler — 10 degrees cooler could see significantly lower photosynthesis. Four-season greenhouse growers take note.

Other plant species may have a broader operating range, but this data supports speculation that this species of cannabis is highly optimized around one temperature. The data suggests fairly tight control will be required to keep this plant operating near optimal levels and growers should be on the lookout and ready to act if swings occur.

Thanks to the Ole Miss researchers, growers don’t need an expensive photosynthetic measurement system, but they should have an infrared thermometer to measure temperature of their limiting upper leaves.

Leaf temperature, particularly upper-leaf temperature, is the facility’s real design point, not air temperature. The upper-most leaves in a plant get the full value of optimal light. To allow them to overheat would drastically drop the photosynthetic efficiency in some of the bigger contributors to a plant’s photosynthetic output.

Under the ideal 86-degree top leaves, temperatures inside the canopy will likely be lower unless canopy density and lack of air movement creates an oven inside. It is vitally important that the temperature of exposed leaves be the focus of temperature management. Keeping exposed leaves close to the optimal points is a primary goal. We’ll talk about under canopy shortly.

Leaf temperatures for plants grown indoors are usually as steady as the facility's light levels. In contrast, greenhouse lighting is much more variable. As a result, the grower needs to measure a crop's top-leaf temperature across daily and seasonal periods to understand the range of temperatures the plants can experience.

The Light Sweet Spot

The photosynthetic response curve the Ole Miss study produced is represented in the graph on page 90, where photosynthesis climbs to the 1,500 PAR sweet spot and declines, but it does not stop entirely.

It is important to note the study did not mention observing light saturation even up to 2,000 PAR, so it is unlikely that even outdoor cannabis plants get saturated and lose photosynthetic output. They will see days when they are not running optimally, but the graph suggests there may not be enough improvement to be gained by attempting to reduce light back to optimal levels.

Because light really does equal yield, growers can compare their current flowering light levels to the 1,500 PAR target. Moving from 1,000 to 1,500 PAR offers upwards of a 33-percent yield boost.

This would be an expensive upgrade, but cultivators won’t do it unless the numbers justify the cost. Predictable yield boosts of this size don’t come around often, so if this sounds hard to swallow, run the numbers.

Canopy Density

Optimal PAR and leaf temperature was determined by measuring individual leaves tested inside a special chamber that allows control of leaf temperature, light and CO₂, and the measurement of factors indicative of photosynthetic rate. In the field, so to speak, for actual crops, that optimal 1,500 PAR is blocked, reduced and reflected resulting in an overall light level and distribution within the canopy that is well below optimal levels.

Therefore, growers should have some idea of exactly how much less than optimal their “optimal” is. Whatever light levels are being delivered, plant shape and canopy density has an effect on the overall photosynthetic capability of plants.

Packed corn fields, close-cropped lawns and crowded fiber hemp fields are witness to structures and leaf arrangements that allow sunlight to penetrate deep into these tight canopies. Between skinny walking stick sativas and umbrella-like indicas and hybrids, it really is a battle getting photons to leaves.

Photosynthesis is a fascinatingly complex and beautiful process, but we don’t need a deep dive into its science (which can be found in "Plant Physiology" by Frank Salisbury and Cleon Ross) to leverage it. In pursuit of a way to compare photosynthetic capability or efficiency of different plant shapes, we offer a layman’s version of the equation of photosynthesis that many intuitively have been using for a long time: More light, more leaf area, more growth.

PAR Light x Leaf Area = Photosynthetic Potential

If the photosynthetic potential, derived by the formula above, is divided by the area of the floor space (square footage) the plant occupies, the result is the efficiency with which that plant collects light. (Note: Leaf area is typically measured by measuring several leaves in square inches and multiplying that by the plant's estimated total leaf count.) We call this a Photosynthetic Efficiency Index.

The index equation suggests plant shapes that can take advantage of height (tall, standalone plants) can be more efficient than ones that cannot (Sea of Green).

This is, of course, part of the rationale behind multilevel or vertical cultivation, and the recent Cannabis Business Times article on our old friends Colorado’s Best, Inc. (”10 Questions with Adam Hudgins and Tony Frischknecht” in the September/October 2016 issue) and its vertical cultivation scheme shows you don’t need equations to get results. But the equations do tell you whether your current conditions can be improved upon or not.

Because there are as many canopy shapes as growers, we built a spreadsheet tool to compute this index for different plant shapes and floor arrangements and concluded that properly shaped, standalone plants can have an index as much as 1.6 times that of Sea of Green plants with overhead lighting.

But before anyone throws Sea of Green out the window, know that there are factors that reduce this advantage, most notably that Sea of Green systems can easily be stacked vertically to improve efficiency. While this index does not make decisions for you, it does offer you more information. Feel free to download our spreadsheet tool at http://bit.ly/HortHowtoResources and model your current plants.

Anyone can see how well his current canopy intercepts light by taking a PAR snapshot of the canopy. Measure PAR values throughout the canopy, top to bottom. When plotted in 3D, the picture that emerges shows visually how much deep light is getting into the under canopy. The distribution that results is due to plant structure, height, canopy density, and light levels and direction.

Remembering that each PAR level has a rate of photosynthesis associated with it, this snapshot also reveals which leaves are contributing to yields and which may not be paying their own way. This visualization is a powerful tool for looking at a canopy to find opportunities for improvements in canopy shaping or maintenance.

However you decide to shape plants and canopy, you should keep in mind that leaves that develop in shade physically have less photosynthetic capability than leaves developed in high light. High-light leaves (think about sweet leaf sticking out of cola structures) are thicker, with the extra thickness due to the presence of more photosynthetic structures.

This little tidbit suggests shaded leaves should only be retained if they receive a relatively high level of light. It also helps solidify our thinking about the goal of the vegetative period: to build plant structure that allows the "deployment" of as many high-efficiency solar panels (leaves) directly to the lights during the two or three week high-light transition period.

Optimal conditions will vary from strain to strain, and further investigation will document the range of optimal conditions across cannabis species; but the conditions the study reported could become chiseled in stone as the first two commandments of cannabis cultivation. We might suggest a third: Always know exactly where optimum is and what it costs to achieve it.