Driven by the need to save energy and reduce operational costs, indoor growers are turning to alternative means of illuminating their cannabis crop. In more and more indoor grows, high-intensity discharge lamps (HIDs) are being swapped out for light-emitting diodes (LEDs). The reasons to do so are numerous, not just energy savings, but the increasing ability to tailor the light spectrum for getting the most out of the plant. LEDs will play an important role in grows of the future, where networked lighting and environmental sensors are integrated into a comprehensive cultivation platform. The time to try LEDs is now.
As I discussed in the last installment of this column (1), an important part of a cannabis cultivator’s environmental obligation is minimizing our grows’ energy consumption. It’s not only good for the planet, but also for the balance sheet. There is more to the selection of a light source than just picking the low-cost option, so sometimes to save money you must spend money. Although light-emitting diode (LED) fixtures are more expensive, they can pay for themselves in electrical cost savings in three years or less. Also, because I believe light is the first nutrient of the plant, your selection shouldn’t rely entirely on finding the most energy-efficient fixture but also the one with the right spectrum. There are a large number of LED horticultural lighting options out there. I have chosen to focus on three major brands—Lumigrow, Illumitex, and Fluence Bioengineering—as examples of the industry. Rather than just relying on manufacturers claims, I have spoken to the growers themselves who are cultivating with these LEDs to gain insights into how to best use these examples of the current state of the art in indoor cannabis lighting. My goal here is not to recommend one product over another; rather it is to share my insights to help cultivators successfully transition to this energy efficient technology and further optimize the yield and quality of their production strains.
Science the “Bleep” Out of It
I consider light to be the first nutrient of the plant. It provides the energy needed to absorb and process all the other nutrients of the plant. Before we go any further, it is important to lay down the definitions of key terms used when discussing lighting for cultivation. Bear with me here, because it can get a little heavy going with these terms. However, once you learn them, you will use them with ease and be an impressive expert in lighting terminology.
Light levels for residential, commercial, and industrial applications are expressed in terms of lumens. This term is intended to quantify light brightness as seen by the human eye. I won’t go into the scientific unit, because all you really need to know is “lumens are for humans.” A lumen is the brightness of one birthday candle located one foot away from you. Normal lighting in a room for your home is about 1000 lumens. While we need to know this because it is an everyday term, we need to become more specific about lighting and lighting measurements for plants and plant growth. These are discussed below.
It’s difficult to describe light in layman’s terms; however, for simplicity, sometimes we refer to light behavior in terms of a wave and sometimes we refer to it as behaving more like a particle. For the purpose of explaining, photosynthetically active radiation (PAR) using the notion of light as a wave provides the best tool for understanding PAR. First, you have to think of the spectrum as the entire rainbow of colors from infrared through reds, yellows, greens, blues, violets, and ultraviolet wavelengths. PAR is the part of the light spectrum absorbed by a plant’s pigments, such as chlorophylls and carotenoids. It drives the formation of glucose—the energy-rich end product of photosynthesis. PAR covers the range of wavelengths, from 400 to 700 nm, where chlorophyll absorption can occur, with peak absorption efficiency occurring in the red (665 nm) and blue (465 nm) parts of the spectrum. While other parts of the spectrum are useful to the plant, they are not directly involved in photosynthesis. When we talk of the quality of the LED spectrum, we are referring to the relative ratios of light in the PAR range of the spectrum. Equally important is the intensity of the light. This is best understood by thinking of light as a particle (called a photon), not a wave. When discussing the effects of light upon the plant canopy, I find the most useful term to be photosynthetic photon flux density (PPFD). PPFD is the number of photosynthetically-active photons falling on a given area each second, with measurement expressed as micromoles per square meter per second. A micromole is one-millionth of a mole—and a mole is an extremely large number, approximately 6.02 X 1023. A mole is a unit for measuring amounts in chemistry and is not something to worry about, but you can bring it out at the next party and impress the crowd with your detailed knowledge. When choosing a lighting fixture, look for one delivering to the top of the canopy a PPFD of at least 700 micromoles per square meter per second (2).
PPFD also allows the calculation of another important number: daily light integral (DLI), which is a measure of the total number of photons falling on the canopy over a 24-h period, expressed as moles/square meter/day (3). Expert cannabis growers like to see their plants receive 30 to 40 moles of light a day, sometimes even higher. Translated, this means they want to see a DLI of 30–40.
So, just to recap: knowing these three measurements—PAR, PPFD, and DLI—for your growing situation will help you determine the best lighting system to maximize health and yield.
LEDs: From Space Science to Cannabis Science
A brief history of LEDs can explain why they’re now coming to the fore in the cannabis industry and indoor and greenhouse cultivation in traditional horticulture (vegetables and ornamental flowers).
One of the first cultivators to use LEDs were National Aeronautics and Space Administration (NASA) scientists performing ground-based research for future flight experiments. The scientists wanted LEDs on spaceflight because of their benefits compared to other types of lighting. For one, LEDs are comparatively small, which is essential on a spacecraft where room to operate is limited. Also, the diodes are shatterproof, eliminating the fear of an exploding light sending slivers of glass into a zero-gravity environment. LEDs run on direct current (DC), which is important because spacecraft run on DC power. Lastly, for greater control over photosynthesis in plant experiments, LEDs can be manufactured to emit a specific spectrum. (On a personal note, I first became familiar with LEDs in plant-growth systems, working with them for space experiments in the mid-1990s.)
Eventually, the horticulture industry, with cannabis growers among them, took note of LEDs. However, early adopters found the technology to be underpowered and expensive. As a result, growers were skeptical of the value of LEDs to improve their crop performance.
- R. Kern, Cannabis Science and Technology 1(4), 18–20 (2018).
- Y. Park and E. Runkle, PLoS ONE 13(8), e0202386 (2018).
- A.Torres and R. Lopez, “Measuring Daily Light Integral in a Greenhouse” (Purdue University, Purdue Extension Program handout, 2012).
- H. Wollaeger and E. Runkle, “Why should greenhouse growers pay attention to vapor-pressure deficit and not relative humidity?” (Michigan State University Extension, July 2015). Available at: https://www.canr.msu.edu/news/why_should_greenhouse_growers_pay_attention_to_vapor_pressure_deficit_and_n.
- S. Chandra, H. Lata, I.A. Khan, and M.A. Elsohly, Physiol. Mol. Biol. Plants 14, 299–306 (2008).
Dr. Roger Kern is a scientist and technologist who cares deeply about the cultivation and health of plants in the cannabis industry. With his PhD in microbiology from the University of California, Davis, Plant Growth Laboratory, he solves the most challenging problems in hydroponics, from studying the root microbiome to developing nutrients and lighting systems to ensure plant health and a disease-free lifecycle. He spent 22 years at NASA’s Jet Propulsion Laboratory as a scientist, technologist, and research leader before becoming the President of Agate Biosciences, a consulting firm for project management, systems engineering, and science in CEA for the past eight years. He leads developments to optimize sustainability, consistency, quality, and yield without compromising plant health. Direct correspondence to: [email protected]
How to Cite This Article
R Kern, Cannabis Science and Technology 2(2), 20-24 (2019).