Which Light is Right: The HPS vs. LED Debate

What light is best for growing cannabis? Learn how HPS and LED lighting technology may impact bud structure, secondary metabolites, and MORE.

From cutting-edge nutrient formulations to energy-efficient lighting solutions, cannabis cultivation has seen significant advancements over the past few decades. As a result, cannabis can now be cultivated indoors with a level of control that was previously unattainable. 

This includes controlling factors such as humidity, CO2 levels, temperature, and of course, light. There’s still much debate, however, regarding the use of Light Emitting Diodes (LEDs) versus High Pressure Sodium (HPS) lighting. 

In partnership with legendary cannabis breeder, Josh Del Rosso – AKA Josh D, known for popularizing the original OG Kush – we decided to investigate how these light sources may impact cannabis development. During his 20+ years of experience, Josh D has observed visual differences in flowers grown under different lighting conditions, but was unable to quantify differences in their chemical profiles… until now.

Not only did the results of our experiment indicate that HPS lighting could promote gassier aromatics and LED lights tended to produce denser buds, but we also gained insights into how lighting conditions may impact the development of secondary metabolites.

Learn more about this novel experiment and how the results may help cultivators adjust their lighting technology to optimize resources and fine-tune the visual appeal of their cannabis flower.

Want to skip straight to the data? Dive into the technical details of the experiment in our White Paper, Which Light is Right: The HPS vs. LED Debate in Cannabis Cultivation.

Josh D: The Man Who Brought OG Kush to Light

Many breeders have decades of experience in cannabis cultivation, and that knowledge is invaluable when it comes to experiments like this. Josh D is a perfect example. Without his expertise and discerning palate for premium genetics, today’s cannabis industry would look entirely different. 

In his years of cannabis cultivation, Del Rosso developed growing techniques to control variables that less experienced horticulturalists may overlook. He’s traditionally used HPS lighting for growing, but has also experimented with newer lighting technologies to optimize growth and promote efficiency.

His expertise made him the ideal collaboration partner. Using clones of his original cut of OG Kush, we set out to answer the following questions regarding the use of HPS versus LED lighting methods.

1. Are there any variations in cannabinoid potency?  
2. If so, which lighting method yields a higher potency? 
3. Are there any differences in terpene concentration or composition? 
4. Which lighting method yields a higher concentration of cannasulfur compounds (CSCs), the key contributors to the plant’s distinct gassy and skunky aroma? 
5. Are there noticeable differences in physical appearance, such as bud morphology? 

A Light Review: Wavelength and Light Flux

In nature, plants absorb and transform sunlight into chemical energy in a process called photosynthesis. Any cultivator worth their salt, however, knows that light isn’t simply ON or OFF. 

Light comes in different colors or wavelengths. It travels from its source in waves, and the physical length between wave peaks is the wavelength. Each color of the light spectrum has a different wavelength, with red having the longest and violet having the shortest. The light that’s visible to the human eye makes up a very small part of the light spectrum, with a wavelength range between 400 and 700 nanometers.

Figure 1. Wavelengths of the visible and invisible spectrum of light.

This matters because evidence suggests that plant growth is impacted by the different spectrums of light.¹ Certain spectrums are more likely to influence vegetative growth, morphology, and other characteristics. 

Light also has a measurable intensity called light flux. This is the amount of light (photons) that a source can produce per unit of area. The intensity of the photons is measured in Photosynthetic Photon Flux Density (PPFD) and is expressed as μmol/m2/s. The sun, for example, has a PPFD upwards of 2,000 µmol m–2 s–1 at the equatorial equinox.²

Many plants, like tomatoes (and cannabis), require high flux, while others require low flux. However, if light flux is too high, plants might not be able to absorb all of that energy. This can break down chlorophyll, discolor the plant (AKA photobleaching), and inhibit plant growth.

HPS vs LEDs

Today, both High Pressure Sodium (HPS) bulbs and Light-Emitting Diodes (LEDs) are used to cultivate cannabis indoors, but there is much debate about which light is superior. 

For some cultivators, the light sources they choose are simply a matter of cost. HPS bulbs are less expensive upfront, but they require more frequent replacement and consume more energy. Conversely, LEDs can be costly, but they last longer and are more energy-efficient. When it comes to their impact on cultivation, several key attributes must also be taken into consideration. 

Today’s LEDs can be tuned to produce different wavelengths of color, a feature that cultivators are unable to achieve with HPS bulbs. This enables growers to assess the impact of various wavelengths of light on plant growth characteristics. Additionally, while HPS bulbs were previously superior in terms of light flux, the efficiency of light-emitting materials in LEDs has made them much more competitive.

One of the most important differences between them is the amount of heat they generate. HPS bulbs can produce a large amount of heat (enough to warm a room), while LEDs produce much less due to their high efficiency. Why is this important? Heat fluctuations can influence the humidity in a room, and additional heat can cause undesirable stress in plants, resulting in photobleaching and reduced yield. 

This means using HPS bulbs may also require additional temperature control measures, such as ventilation and air conditioning. Factors such as light spectrum, PPFD, heat, and humidity can have a profound impact on plant growth. That’s why switching from HPS to LEDs, or vice versa, isn’t as simple as it sounds.

Figure 2. A diagram depicting differences in heat generation between LED and HPS light sources.

Stepping Into the Light: The Experiment

OG Kush clones were transplanted into a hydroponically fed system in a room that was divided into a 50/50 split between LED and HPS lighting systems. LEDs were used in one half of the grow room, while HPS bulbs were used for the other half of the room. The lighting schedule for both was determined based on visual inspection:

LEDs

• Maintained at 50% power for the first 3 days.
• Increasing to 100% over the first two weeks.
• After flowering, and when plants were physically closer to the lights, LEDs were decreased to 80% power, resulting in a PPFD of ≈ 600.
• In week 7, power was further lowered to ≈ 70%, or about 500 PPFD. 

HPS

• Turned to 100% power (≈ 1400 PPFD) during the entire growth period of the plant’s lifespan until the final 10 days.
• During the final 10 days of flowering, the power was reduced to ≈ 75% (≈ 1050 PPFD).

For both groups, plants were maintained at a height of ≈ 48” tall, resulting in 16”–18” between the topmost inflorescence and the light source. Fans were installed near the vertical height of the HPS bulbs to disperse the heat, promoting a homogenous temperature between both HPS-grown and LED-grown plants.  

Finally, the plants were cut, dried, and cured over an average course of 16 days, at which point flowers were trimmed and stored in ceramic jars. Flower samples for analysis were selected from plants located at opposite ends of the room to minimize potential cross-contamination from light and ensure sample integrity. 

From here, we examined, identified, quantified, and basically went full nerd mode analyzing these samples.

Want ALL the technical information about our equipment, methodologies, and data? Get all the details in our white paper, Which Light is Right: The HPS vs. LED Debate in Cannabis Cultivation.

Results: Morphological Differences

During the flowering stage, striking differences in bud structure (morphology) and density were immediately obvious, and these physical differences continued into the final product.

Under HPS lighting, the buds tended to develop a “looser” structure characterized by elongated internodes and extended, slender bract clusters. This resulted in a bud architecture where the calyxes and sugar leaves were more widely spaced, giving the appearance of longer, threadlike “branches” within the bud.

In contrast, LED lighting with a higher proportion of blue wavelengths appeared to promote a more compact growth habit, reducing internodal elongation and resulting in tightly packed bracts and calyxes.

This morphological difference suggests that the increased stem elongation under HPS may contribute significantly to the less dense bud morphology observed. HPS lighting, which is usually richer in red (and infrared) wavelengths compared to LEDs, stem elongation can lead to longer internodes in both the vegetative portion and the bud structure. This results in a looser, more open bud morphology. 

On the other hand, LEDs with enhanced blue light tend to promote shorter internodes and a more compact structure, which contributes to denser buds.

It’s important to note, however, that these morphological differences from varying light conditions could be varietal specific. Essentially, while the OG varietal used in this study displayed these results, other cultivars with distinct genetics, such as Bacio Gelato or Super Lemon Haze, may have unique responses to varying light conditions.

Figure 3. Left: LED grown OG. Note the tight bud structure resembling that of other, typically exotic varieties such as Gelato. Right: HPS grown OG. Note the more loose, typical OG-like bud structure. 

Results: Differences in Secondary Metabolites

Our analysis revealed that the cannabinoid content was comparable for both LED and HPS flowers:

• LED samples had an average total THC concentration of 23.16 (±0.54).
• HPS samples had an average total THC concentration of 21.63 (±0.46).

The results of the volatile aroma compound (VOC) analysis were similar to those of the cannabinoid content, with minimal differences between the samples. The overall terpene profiles of the samples were nearly identical, with ß-Myrcene dominating the volatile profile by weight, followed by d-Limonene and Caryophyllene.

Lastly, two defining prenylated cannasulfur compounds (PCSCs), prenylthiol and prenyl thioacetate, were measured. PCSCs are a subset of volatile sulfur compounds (VSCs) and are the source of the OGs' signature gassy, skunky scent. While the HPS-grown flowers had negligibly higher concentrations of prenylthiol, they had almost double the amount of prenyl thioacetate.

This was verified by sensory analysis of the two flower types. Sensory descriptions of the HPS flower included “slightly gassier” when compared to the LED flowers. Considering the extremely low odor threshold of prenyl thioacetate, the difference in lighting types may explain the subtle, but clearly notable, aromatic differences.

Shedding Light on Future Cultivation Practices

So, did we find answers to all of our questions? Yes and no. Mostly, we came up with more questions, and that’s a good thing!

Were there noticeable differences in physical appearance? Yes. Were there differences in cannabinoid potency and terpene concentration/composition? Both were relatively similar between samples. However, the slight variation in a PCSC with an extremely low odor threshold ultimately contributed to a slightly gassier aroma in the HPS flower. 

This suggests that lighting schedules, flux, and spectrum can be optimized to yield comparable secondary metabolite production across both lighting types. That means cultivators who want to switch between light sources can hypothetically do so without a significant impact on the production of secondary metabolites. 

At the same time, since different light spectrums promoted clear differences in bud structure, cultivators have an opportunity to fine-tune bud morphology in OG varieties. This is valuable because appearances matter! While aroma, flavor, and sensorial effects are pivotal characteristics, the appearance of cannabis flower can also impact consumer preference and product selection. 

As we mentioned earlier, we still have a lot of questions. Like, WHY does LED lighting appear to influence bud morphology in this way? How do light-driven changes manifest across other cultivars? Could certain secondary metabolites be more sensitive to specific wavelengths or flux thresholds?

By continuing to explore questions like these, we can refine our understanding of how light quality, intensity, and spectrum impact cannabis development.

Abstrax Tech | Advancing the Cannabis Industry Through Advanced Chemical Analysis

Every time we learn something new about cannabis, we realize just how much we DON’T know. Every answer leads to a new question, and the research cycle repeats. For example, is there an optimal wavelength of light to encourage specific traits in different cannabis genetics? Is there a future where breeders cultivate cannabis with specific aromas by optimizing light factors?

Questions like these keep us up at night! That’s why Abstrax Tech pioneers the research of cannabis flavor and aroma. We will continue to conduct industry-leading, peer-reviewed research on cannabis, advancing the industry through enhanced education and collaboration. 

This is just the start! Want more details about our findings and methodologies? Get all the details in our white paper, Which Light is Right: The HPS vs. LED Debate in Cannabis Cultivation.

1. Mohamed, S. J., Rihan, H. Z., Aljafer, N., & Fuller, M. P. (2021, October 12). The Impact of Light Spectrum and Intensity on the Growth, Physiology, and Antioxidant Activity of Lettuce (lactuca sativa L.). Plants (Basel, Switzerland). https://pmc.ncbi.nlm.nih.gov/articles/PMC8538153/

2. Ritchie, R. J. (2010, October 15). Modelling Photosynthetic Photon Flux Density and Maximum Potential Gross Photosynthesis. Photosynthetica. https://ps.ueb.cas.cz/pdfs/phs/2010/04/14.pdf