HighScience - HPS vs LED: What temp do I keep my canopy explained

[ChadWestport]

Hey growers I have a habit of reading boring technical papers, so I thought I would start to summarize some of them and offer them to you all. I'm open for debate, I'm open to learn new things and I'm always happy to offer my opinion.

I hope you all enjoy the High Science

[ChadWestport]

#ChadWestport #Lighting #LED #HPS #HighScience

Publication: Plos One
Peer Reviewed: Yes
Authors: Jacob A Nelson and Bruce Bugbee
Published online 2015, Oct. 8
Title: Analysis of Environmental Effects on Leaf Temperature under Sunlight, High Pressure Sodium and Light Emitting Diodes


Why was I interested?
I’ve known that an ideal temperature for your canopy under HPS (High Pressure Sodium) lighting has been universally accepted as roughly 72 degrees. There are many factors that need to work in tandem for a perfect grow, but that temperature has been ingrained in my databank. LED technology has finally become an effective alternative and many home cultivators are using it, but why do users insist that the room temperature should be around 85 degrees? That would spell serious trouble if it were my HPS room.

The answer I got from online forums, was that the difference in desired room temperatures is due to LST (Leaf Surface Temperature). LST is important because increased leaf temperatures cause increased transpiration. In short, the plant sucks up water and nutrients faster. A slowing in the transpiration process would mean reduced yields, so that question was worth investigating.


The Study;
The goal was to compare leaf to air temperature differences among four radiation sources: clear sky sunlight in the field, sunlight in a glass greenhouse and indoor plants under either HPS or LED lighting. The plants used in the study are tomato, pepper, basil and broccoli. While Cannabis was not used in this study, the authors specifically note that their findings “would likely be representative of flowers, fruits and thick dense plant parts that have low transpiration rates, including high value products such as tomatoes, strawberries and Cannabis flowers”.

In the study, they took into account important factors such as PPF (Photosynthetic Photon Flux), stomatal conductance, wind speed, vapor pressure deficit and leaf size. Overall, I am satisfied with the process and considerations of this study and feel confident using this information going forward, until there is new evidence to dispute it. So why am I being told I need to run my room hotter with LED lighting?


The Why;
Professional grade HPS and LED lighting can generate an identical amount of thermal energy per photosynthetic photon but LED fixtures “dissipate much of their heat away from the plane they illuminate, while HPS fixtures dissipate more heat toward the plane they illuminate”. The greatest difference in terms of incident radiation between HPS and LED lighting was in the NIR (Near-Infrared) and longwave bands.

NIR is quantified in this study as the 700-2500nm spectrum and is found in high numbers with HPS lighting but very low numbers in LED. NIR light is poorly absorbed by leaves, although it is a significant source of energy it has little impact on individual leaves. “The effect of NIR on leaf temperature is relatively small”.

Longwave radiation however, had the largest effect on leaf temperature. “HPS lamps operate at higher temperatures and thus generate more long wave radiation in the same direction as the photosynthetic radiation”. Conversely, LED lights are typically thermally bonded to heat sinks and the thermal energy is directed away from the canopy.


The Results;
It’s a mixed bag. If the plants are not water stressed, leaves under both the LED and the HPS lighting systems were typically within 3.6 degrees of air temperature. In their “worst case scenario” which includes water stressed plants, low wind and high PPF, leaf temperatures would increase 10.8 and 21.6 degrees under LED and HPS respectively, compared to the air temperature. “When other environmental conditions are constant in the model, air temperature between 59 and 95 degrees has minimal effect on leaf to air difference”.


Conlusion of the study;
“The presented model indicates that the use of LED technology reduces leaf temperature by about two degrees compared to HPS technology under typical indoor growing conditions. While this is a significant difference for some applications, the difference is smaller than the difference between indoor and outdoor leaves”.


What are my take away thoughts;
There is some truth to running your room hotter using LED compared to HPS systems, although under ideal conditions it is only a few degrees difference.

Don’t get that confused, the LED won’t make your room hotter, in fact, HPS lighting operates at higher temperatures. If you are in cooler climates and keeping your grow room at the right temperature is an issue, HPS might help with that. Conversely, if you live in hotter climates, LED lighting might help keep your grow cooler comparatively.

The factor that seemed to have the largest impact on the question of air vs leaf temperature, is the transpiration and water stress level of the plant. An interesting note was added in regards to supplemental CO2. The authors noted that “supplemental CO2 can decrease stomatal conductance 10-40% and increase leaf temperatures”.

If I run an LED with a room temperature of 86 degrees as suggested online, and I happen to have water stress in my plants, the leaf temperature (without supplemental CO2) can reach almost 97 degrees. That scares me. If running a room with LED lighting, I’ll keep the air temperature around the upper 70’s, lower 80’s. I guess the real question now would be, what is the most efficient leaf temperature for Cannabis and does it vary by strain?



Link to study - https://journals.plos.org/plosone/ar...l.pone.0138930

[grouchyoldman]

Hi Chad, two thumbs up on the "Mister Science" approach, I look forward to future posts. Here are some useful resources:

Dr. Bruce Bugbee is a Golden Gawd of geeky cannabis science. His YouTube vids are fun and understandable. This one, "Cannabis Grow Lighting 101" should be required viewing for GWE fans. https://www.youtube.com/watch?v=ID9rE5JewVg

The Migro.com guys do a fantastic job on grow light comparisons and explaining the differences between them. https://www.migrolight.com/whats-the...ue-grow-light/

The Coco For Cannabis folks provide in depth grow light reviews. Their review of the ViparSpectra XS1500 just compelled me to open my wallet for this state of the art device. https://www.cocoforcannabis.com/grow...r-test-review/ Amazon has a 20% off deal on these until 12/31, look for the discount code in the reviews/comments. $130 is a soul deal IMHO.

Keep up the good work Chad!

-Grouchy

[ChadWestport]

Groucy, awesome post and thank you. You've mentioned sources that I consider to be reputable and a good source of quality information, I like it! Bugbee is like the lighting guru in the world of LED. Definitely someone to look towards for lighting science.

Migro is a source that I used to help determine my LED purchase. Non biased reviews that are pretty straight forward. He presents the results and lets you decide if it is a good buy or not. He just put out a video on what is the most efficient light he has reviewed thus far and had a chart with many of the lights we are all familiar with.

Dr. Coco, that is another pretty trusted person too. I listen to the Growcast with Jordan Rivers and Dr. Coco is often a guest. A podcast well worth tuning into.

Excellent taste and thank you for contributing the knowledge grouchyoldman lol, with a name like that, we should get along swell

[golfnrl]

My grow tent stays mid 70's daytime under an LED lamp...so, if I understand correctly my plants leaf temp should be no hirer than say, 80f?

[ChadWestport]

Is that at your canopy level? This is where I take my temp readings from. The leaves will be bit warmer than the ambient room temperature based upon a few factors, but the exact leaf temp, I haven't seen a study on cannabis and on a wide variety of cannabis strains in order to determine the most efficient leaf surface temp, although that study may exist. So, your leaves temp will be above 80 degrees, and that is ok, but the ambient air temp should be somewhere in that lower to mid 80's range, but given the ability of the leaf to run hotter if water stressed, I run a little below that 80's range.

[PLreef]

Great read. Thanks for doing the research Chad. Truth be told I only understood about half of it. I have a 5x5 with spectrum king sk402 (460 watts) and 2 optic XL ( 100 watt each). My tent around the canopy stays around 83° to 86° when the lights are on. Always thought that was a tad on the hot side but my girls are thriving.

[ChadWestport]

A rep for HLG that I have talked to a few times, suggests exactly that range (83-86 degrees) for LED. It was that conversation with him that actually made me go out and search for a paper like the one I reviewed. I'm glad your girls are thriving, I've always noticed that I get a little more rapid growth (but in the form of internal stretching) if I run things on the upper end of the hot side.

[bboyfromwayback]

Is your article stating the grow room itself should be 85 degrees? I certainly hope not because if I kept my grow room at 85 the temperature inside my tent would be in the 90’s. Think I’ll stick to my way of doing things.

[ChadWestport]

I'd agree, if what you are doing is working and producing good results, stay with it. I think we'll see a lot more science in relation to lighting and actual cannabis in the near future. So far, cannabis isn't used, or legal / allowed to be use in these tests. So, a lot of information we base our decisions on are how other plants are effected, although we can choose plants with close relations to cannabis.

[SoOrbudgal]

Me personally would not run a tent with room temps that are 86f would make tent too warm for my plants.

[ChadWestport]

I'd agree, this is why I dove into the topic. I've always run HPS systems at the recommended 72 degrees and 80 degrees is like the panic zone for temp control in that situation. So when I heard 86, I thought how can that be. Well, I guess it relates to leaf surface temp but I still somewhat disagree with the target of mid 80's.

That temp basically is contrary to everything I've practiced for so many years. Again, this is only in relation to LED lighting, not HID systems.

[ChadWestport]

The temp mentioned is for the temp at the top of your canopy, which will often be a few degrees hotter than the room temp, and it will be higher than the temp at the bottom of the room. Controlling the temp at canopy level is the important part because that is where you would record your leaf surface temperature.

Thank you for the feedback, input and questions GWE crew