Cultivating with LEDs: Past, Present, and Future

April 4, 2019

Volume 2, Issue 2

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).

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.

Enter Haitz’s Law for LEDs, which is similar to Moore’s Law. Just as the latter observation accurately has forecast the increase in computing power over the years, the late scientist Roland Haitz predicted that every decade the amount of light generated by an LED would increase by a factor of 20 while the cost would drop by a factor of 10. Haitz’s Law has held up over the years, and if there’s any inaccuracy it underestimates the rates of progress.

In short, LEDs are less expensive and brighter than when they first became available to growers-a combination that has piqued the interest of today’s cannabis cultivators. Today’s LED fixtures are every bit as powerful as the high-intensity discharge lamps (HIDs) they are starting to replace. The LED fixtures are roughly 40% more efficient than their HID counter parts.  

A Change in Lighting System Means Careful Planning and Time to Adjust

LEDs can be great, but adapting the crop growing conditions to them requires some forethought and detailed planning. When a lighting “change-out” occurs in grow rooms, the differences between LEDs and traditional lighting systems are significant enough to cause a rethinking of the cultivation protocols in even the most meticulously designed and monitored grow rooms. Successful growers have had to re-educate themselves and adjust their tried and true cultivation approaches. In short, you just can’t replace the fixtures and expect the same or better results. You have to take some time to prepare, plan, test, and implement your system in a methodical and careful manner to achieve the results that you desire.

For example, LEDs produce far less radiant heat than their HID cousins. This means that adjustments in the cultivation environment must be made to ensure crop health. Without the radiant heat generated by HIDs, the temperature of the grow room and the leaf surfaces drops significantly-and lower temperatures means higher relative humidity in the grow room. When the plants enter a night cycle, the reduced heat input reduces nighttime transpiration and slows the plant growth rate. A grower can address this two-fold issue: 1. Heat the grow room to reduce the relative humidity, resulting in an increase in the water saturation level of the air, or 2. Increase the HVAC system’s dehumidification capability. Some growers choose to do both.

To have a deeper understanding of how this can impact the plant, I need to introduce a concept called vapor pressure deficit (VPD) (4). This is a calculated value based on three parameters: air temperature, relative humidity, and leaf temperature. Every professional grow room should always have the first two measurements visible. For reading leaf temperatures, those pistol-like infrared thermometers cost less than $100. (As an aside, a healthy leaf’s temperature should be lower than the air temperature by about 4.2 °F/2 °C with the difference due to the cooling effect of transpiration.) Once cultivators know those three measurements, they can find free online VPD calculators that will tell them the difference between the level of water vapor in the leaf (always 100%) and the water vapor in the grow room’s air. Because this is a measurement of a pressure difference, the standard unit is kilopascals (kPa); for reference, the pressure of air at sea level is 101.3 kPa.

Managing VPD over the course of the cannabis life cycle is critical to getting the most out of the plant. If the VPD is too high, the transpiration rate will not be enough to keep the leaves from drying out, which stresses the plant. Ultimately, the plant’s response is to shut down its transpiration rate and diminish the flow of nutrients, compounding the problem. If the VPD is too low, transpiration will be slowed, resulting in less cooling of the leaves and less uptake of nutrients, which means slower growth and lower yields. To get the best outcome, it’s essential to maintain the optimal VPD range over the lifecycle of the plant. For propagation and the early part of vegetative growth, the VPD should be kept between 0.4–0.8 kPa. During late vegetative and early flower growth, the VPD should be maintained at 0.8–1.2 kPa. Finally, VPD is increased further during mid- and late-flower growth at 1.2–1.6 kPa. Yes, VPD management comes at a cost of a bit more energy, but the value of doing so is well worth the price.

Allison Justice, vice president of cultivation at OutCo in San Diego, California, uses fixtures from Fluence Bioengineering to illuminate their indoor cultivation facility, cutting costs significantly. However, she cautions that the 40% savings in electricity use for lighting was negated partially by the need for dehumidifiers to be in the grow room. In my estimation, actual energy savings of the overall LED based cultivation operations are closer to 30%.

Additionally, cultivators quickly may learn just how much their plants needed the radiant heat from the HIDs to grow as desired. Then it’s a matter of compensating by raising the air temperature in the grow room, although experts disagree by how much-or if it’s even needed. Dr. Daniel Hopper, chief cultivation officer at Nevada’s Silver State Relief dispensary in Sparks, Nevada, using Lumigrow lights, said when he compared plants grown in a room kept at 82 °F/27.8 °C with only LEDs versus 78 °F/25.6 °C, he noticed no difference. But Justice said to raise the plants’ temperatures she runs LED rooms warmer than rooms equipped with high pressure sodium (HPS) lights, perhaps into the low 80s during the vegetative stage.

Another factor often overlooked following a conversion is light intensity. After cultivators realize they’re no longer at risk of burning their plants from a too-close HID, some bring the LED right next to the leaves. This, of course, cranks up the PPFD-which some genetics can handle while others can’t. Justice said one of OutCo’s best strains, Grape Pie, starts to foxtail once the PPFD reaches 900 micromoles, and when the PPFD crosses the 1000-micromole threshold, the flowers start to change morphology and look “finger-y” and not as “trichome-y.” In fact, she noted, the resulting plant takes on a shade of brown, as if it was grown outdoors. However, the lack of aesthetics is made up for by a higher yield and greater potency. On the other hand, she added, the strain “Tangimal” can take 1200 micromoles without ill effect. You need to optimize each strain to get the most out of your LED lighting fixture.

Many growers find it is easier to over-water LED grown plants than HID grown. Growers often modify their nutrient formulations when growing under LEDs. The nutrient mix is often run at a higher mineral salt concentration (electric conductivity [EC]) than they were using before. Tom Haffly, director of production at Temescal Wellness in Worcester, Massachusetts, using Illumitex lighting, said he’s found success from increasing salt in the nutrient content, bumping EC from 2.0 to 2.7. Haffly also raises the carbon dioxide levels when using LEDs compared to HPS, going from 800–950 ppm to 1000–1200 ppm. While Haffly has seen some growers boost their CO2 levels to as high as 1500, he doesn’t believe yields increase once past 1200–1300 ppm.

Optimizing the spectrum during the vegetative stage can change the overall morphology of a plant that’s tall and lean, with a weak stem and not many leaves to a smaller more robust plant that is better suited to production. This can be accomplished by using a fixture that is relatively high in the blue part of the spectrum. During flowering, a relatively high level of red light helps the growth of large, dense buds. (Of course, if a cultivator is growing cannabis for a product that doesn’t need photogenic buds [for example, concentrates or edibles] then the absence of red light is not of consequence.) Silver State found that flowers given a “No Red” light treatment in the final three days of production showed an increase in terpenes with almost no effect on the final cannabinoid content.

But in a macro sense, different genetics simply react differently to LEDs versus HIDs. Hopper said his grow rooms with a “checkerboard” pattern of the two types of lights are good for the strains Bio-Diesel and Pineapple Express, while doing better in the all-LED rooms are, among others, White Widow, J1, and Durban Poison.

Haffly has his own lists of which strains do well under LEDs (Sour Candy, Jelly Sherbet, Blue Dream, as well as all Punch and Platinum genetics) and those that don’t (Super Silver Sour). But for the latter category, he notes, it may be less a matter of plants reacting poorly to LEDs than those plants needing different environmental parameters (for example, temperature, CO2 levels, and concentration of nutrients) while under LEDs.

Lighting the Path Forward

I expect over time the cost of LED lighting fixtures will continue to drop and, based on theoretical considerations, there is room for them to improve dramatically in efficiency. In short, LEDs are here to stay and I expect them to capture the lion’s share of the market in the not too distant future. But I expect these fixtures may evolve into more than just a light; we may be able to establish a fully-sensored lighting platform. What has me, and others, excited about LEDs is how they can be integrated into the grow rooms of the future. Imagine a set-up in which the lighting, irrigation, and climate control are networked with these fixtures that can also host tiny cameras and sensors surveilling every plant for wilting, foxtailing, mildew, pests, and other signs of distress. With everything connected, an alert can trigger an immediate and appropriate response by the cultivation system as a whole. With enough computing horsepower, such a platform can use the collected data to be predictive, enacting proactive solutions rather than reactive fixes.

Specific to lighting, LEDs will be finely tuned to direct cannabinoid profiles, with the goals of delivering consistent results harvest after harvest or bringing out compounds subtler than tetrahydrocannabinol (THC).

In my estimation, the time to switch to LEDs is now. If you are operating a facility, start by purchasing a few fixtures for one room and learn how to grow with them before embarking on a total conversion of your facility. Education is key, understand you will have to modify your environmental parameters: temperature, humidity, nutrient concentration, and watering rate. You may even want to change the strains you grow to optimize your production under LED lighting. In the end, this will result in energy savings and higher yields of more potency that will help your bottom-line grow.


  1. R. Kern, Cannabis Science and Technology 1(4), 18–20 (2018).
  2. Y. Park and E. Runkle, PLoS ONE 13(8), e0202386 (2018).
  3. A.Torres and R. Lopez, “Measuring Daily Light Integral in a Greenhouse” (Purdue University, Purdue Extension Program handout, 2012).
  4. 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:
  5. S. Chandra, H. Lata, I.A. Khan, and M.A. Elsohly, Physiol. Mol. Biol. Plants 14, 299–306 (2008).


About the Columnist

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:

How to Cite This Article

R Kern, Cannabis Science and Technology 2(2), 20-24 (2019).

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