Cultivating with LEDs: Past, Present, and Future: Page 2 of 3

April 4, 2019
Figure 1: Photosynthetically important part of the solar spectrum.
Figure 1: Photosynthetically important part of the solar spectrum.
Abstract / Synopsis: 

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.

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%.

  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).


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).