Photoperiod vs Intensity: PPFD / DLI Experiment (Hydroponic Lettuce LED Lights)

Photoperiod vs Intensity: PPFD / DLI Experiment (Hydroponic Lettuce LED Lights)

Hi YouTubers! I’m AL Gracian from You’re growing plants indoors. You’ve picked out your lights, and selected
a good distance. But how LONG should the light cycle be? Should you run your lights twenty-four seven? Or do your plants need a “rest”? Also, is there any relationship between light
DURATION and light INTENSITY? In today’s experiment, we’re exploring these
ideas, specifically, with lettuce. It’s important to note that different plants
respond differently to light cycle durations. Some are classified as “long-day” plants. Some are “short-day”. And others are “day-neutral”. This gets into the concept of “photoperiodism”. The period of uninterrupted darkness can trigger
whether or not a plant enters its flowering or reproductive phase. So, long-day plants flower when night duration
decreases below their critical photoperiod. But short-day plants require longer periods
of darkness, perhaps more than 12 hours of night to induce flowering. Meanwhile, day neutral plants can flower regardless
of light and dark cycles. Well we’re working with a bibb lettuce and
a quick web search tells you that lettuce is a long-day plant. So we need to keep the day cycles short, to
prevent bolting… right? Or… can lettuce be grown under long light
cycles? What about… 24 hours of light with NO dark cycle at all!? Yeah, we’re gonna test that. In today’s experiment we’re trying 3 different
light durations but we’re also trying 3 different distances. Hold up! You can’t change more than 1 variable! Well we’re actually trying to MATCH a variable. We’re trying to create the same DLI for each
plant. DLI or Daily Light Integral is a sum of all
the photons reaching the plant canopy over a 24 hour period. Our target DLI for this test is 16. But we’re using different light cycle durations. Plant 1 will get 12 hours on and 12 hours
off. Plant 2 will get 16 on and 8 off. And plant 3? Twenty-four hours on! Imagine getting 3 inches of rain in a single
day. It might be a steady light rain, all day and
night. Or it might rain heavier for half of the day
then stop. Or there might be a torrential downpour within
a 2 hour window. The same number of rain drops in a single
day, but delivered in different ways. We’re doing the same thing here with photons! PPFD tells us how many photons hit an area
in 1 SECOND. With a little math we can calculate the total
amount per DAY (the DLI). Since each light will run for a different
period of time, each setup will require a specific PPFD. Longer photoperiods call for the lights to
be moved further and further away, reducing how many photons hit the plant per hour. The objective? To try to deliver the same number of daily
photons… each in a different way. How will this affect plant growth? With similar daily photon dosages, will they
grow at the same rate? Or will that dark cycle alter plant growth? And what about lettuce number 3? Twenty-four hours of light… Will it bolt? What do you think? For our test, the lettuce seeds were pre-germinated
and then transferred to their rooting plugs. Here we are at day one under our lights. On the left, is our 12/12 cycle, then the
16/8 in the middle and the 24 hour continuous light on the right. At day 9 all plants were growing but plant
3 was looking just a little larger. On day 16 all plants looked ok but plant 3
showed a little tip burn. Here we are on day 23. Plant 1 has its first signs of tip burn. Plant 2 has minimal signs of tip burn, but
the best looking growth. And plant 3 has severe tip burn & deformity. Also the deepest color of green in its leaves. Even after day 29, lettuce number 3 has still
hung in there with no signs of bolting. Of course, it shows the highest deformity
& tip burn. Twenty-four hours of light with NO break at
all. But the plant grew quickly with no vertical
elongation. The light intensity was 185 PPFD. Compare that to our previous experiment where
175 PPFD produced MUCH less growth! Why? Because we had a long period of darkness each
day. With no break in light exposure, this received
60% more photons per day. Oddly, in an earlier test, we DID observe
serious bolting in ALL THREE plants. The cause appeared to relate to light QUALITY,
not duration. Those plants were getting a custom blend of
red and blue wavelengths. The ratio of colors induced bolting. But switching to a broad spectrum white LED
has suppressed that response. So this particular lettuce variety responded
to light QUALITY more readily than light / dark DURATION. At day 30 we’ve pulled the plug. Here’s how the plants turned out. Similar in size. But all have at least some tip burn. A reduction in DLI would help with this. Perhaps 14 or 15 would be better for the next
trial. All data has been collected and here are the
results of our test. The longer light cycle produced the most growth. Let’s look closer at each plant weight. Obviously, this is a very small sample group. To get more meaningful data, a test could
be set up using several plants per photoperiod. Or the exact same test could be run several
times to see if we get the same results. Although we have a step-wise increase, plant
3 was only 21% larger than plant 1. There is a boost, but certainly not enough
to justify the added electrical costs. How much did it cost to grow these plants? These SANSI LEDs offer some good efficiency. But the shorter the light cycle, the more
growth we got per watt. Plant 1 was smallest, but that 12 hour photoperiod
made it the cheapest to grow. So what’s the take-away from today’s test? Do your lights seem a little weak? Sure you can move them closer, but you can
also try increasing the light duration to compensate. Check to see if your plant is long-day, short-day
or day-neutral. Or try an experiment and see what happens! Be aware though that custom red/blue LEDs
can cause unexpected growth responses. We’ve seen that excess light can cause tip
burn in lettuce. But that can be resolved in a couple of ways. We can try raising our lights to reduce light
intensity. Or we can also try reducing the light duration. Reducing the day cycle brings the added benefit
of less power consumption. Light intensity and light duration are both
key components that allow us to calculate the DLI. Low light plants, like lettuce, will fair
much better when we keep that DLI lower. So see how your plants are responding and
make adjustments as needed! Thanks for watching & thanks for your support
on my channel. Please subscribe if you haven’t already. And as always, Happy Gardening!

100 thoughts on “Photoperiod vs Intensity: PPFD / DLI Experiment (Hydroponic Lettuce LED Lights)

  1. Excellent presentation of all the information!! And really well thought out experiment. Very much enjoying these.

  2. Great experiment, the leds look pretty nice too. Thanks for all the work Al, and the best wishes to you ;o)
    Cheers !

  3. interesting video, as always, but I'm not sure about your cost comparison, I think that what you are actually measuring is the cost effectiveness off having your lights closer to your plants (the result of which is kind of obvious), rather than the cost effectiveness of keeping the lights on for a longer time. Maybe you should try it again with the lights at the same height but with a dimmer to even out the DLI. That probably wouldn't be a 100% fair comparison as LED efficiency is not constant because it varies with how much current you are feeding them but it should still be fairly accurate.

  4. So lettuce, at least the variety you grow, is not very sensitive to light cycle! Good to know! Thank you a lot for taking so much time doing this experiment!

  5. Great experiment, thank you for creating informative and entertaining content to share with all of us – awesome work!

  6. Loved the experiment and the info!

    But looking at your results table, I noticed that the 24-hour plant consumed 26% LESS solution than the 12-hour plant. That would imply 4x 24-hour plants could be grown for the same solution cost as 3x 12-hour plants! That would be top growth of roughly 372g vs 231g!

    Also, to get the lower PPFD, could not less expensive/dimmer lights, presumably with lower current draw, be used instead of simply raising the same light and losing half the photons per unit of time?

    Not sure if you would be able to do this or not, but I'd LOVE to see the experiment redone with 4 sets of 3 plants each with different sized lights at approximately the same height providing 14/day for 8/16, 12/12, 16/8, and 24/0 light cycles. Each set being 3 DIFFERENT plants by adding a short-day and a day-neutral plant to the long-day lettuce.

  7. you know, sometimes a person just plants their garden and lets it do its own thing ,but this experiment really makes you appreciate the true science involved in what we may take for granted. i really enjoyed the technical science, although i'll never remember the terms.

  8. Thanks for the great video and test. Can you please direct me to what the composition of your grow buckets are? How are you watering these and what nutrients does it need as well?

  9. The Inverse-Square Law of light

    This law states that for every foot below the reflector the light should loose 75 percent of it's energy due to dispersion. So 1000 lux at 1 foot is 250 lux at 2 foot. I have taken a lux meter and find that I am only loosing 50 percent per foot. I am thinking the reflective walls are keeping the numbers up. Also, if you are have multiple lights when you just below the reflector you only get light from it. As you move down you get light from multiple lights. This applies to HPS and Metal Halide due at the side bouncing of the wall is as bring as just below it. Where LED blow all their light down.

  10. You should do another set with lettuce using the inverse square law- 1,1.4, 2 for distance, same light cycle. I use cheap 5000K LED bulbs from Lowes for foliage growth. I tend to keep them close to the seedlings but the 14 hour light cycle also seems to contribute to growth if the intensity is not reduced too much. This light combination seems to work better than putting things out of doors where the light may be stronger but the nights are getting down around 50 compared to 65 in my basement.

  11. This document has a chart of day lengths for various plants:

  12. Хороший эксперимент. Но при увеличении высоты подвеса лампы увеличится площадь освещения, это означает, что можно высадить больше растений на этой площади.

  13. Hi, may I know what method of hydroponics you are using for growing? Do you prefer aeroponics option for commercial growing?

  14. Great experiment.
    There appears to be some additional light exposure on the plants due to short walls. Could this influence the results?

  15. Hi, like your informative content. Can you please specify the climatic conditions for lettuce to grow, ie the optimum humidity and temperature. Keep up the good work. Lots of love from Goa.

  16. What about splitting the cycle?… i mean, 8hr light, 4hr don't, 8hr light, 4hr no light?? ….. do you think that can reduce the leaf tip burn?

  17. It appears that lamp #3 also kept plants 1 and 2 in partial light 24 hours per day, nullifying equal accumulative lighting. Interesting experiment none-the-less.

  18. Thanks! Helped me a lot to understand things further. I would love to see such tests with Basil, as I'm struggling with my indoor setup and I'm sure I'm not watering to much / less and ferts should be good, at least in my opinion compared to outdoor gardening 🙂

  19. If nature had meant for lettuces to have 24 hrs of light, we would have 24 hrs of daylight on earth. Plants developed based on what nature gave them, the Four seasons.

  20. Fantastic work! Thank you for your well thought out mini experiments. I am fascinated with the idea of light intensity and light period. Have you tried any experiments with periodic darkness within a 24hr period? Say 12hr with 1hr on 1hr off. Or 4hrs on 4hrs off. Would more frequent pauses help with tip burn? Or will these pauses cause a different stress reaction?

  21. Hi, what was the starting pH and EC of your experiment ? Also, what nutrients did you use? Thanks so much for sharing this info. Also, have you done any experiments w shorter light cycles? 5 min on, 5 min off vs 5 min on 2 min off, etc ?

  22. Very interesting. And how can we calculate the right distance from the light to the plant? I mean how did you get figures 10,5 and 12,5 and 21 inches?
    By using PAR-meter?

  23. You should be a Professor! I learnt many concepts of lighting which i could not understand. The experiements were well designed and the resukts were explained clearly. Have you published your data? It has all the elemnents of a high impact publication.

  24. Doesn't tip burn happen when the plant grows faster than it can transfer water/nutrients to the tips? Maybe try multiple dark light cycles a day and see if it improves.

  25. i assume you are not topping up the nutrient solution with water, what i see happening is the plants are drinking the water leaving behind nutrients that is then burning the tips as it becomes more and more concentrated, you need to remember if plants grow bigger they consume more and so should be watered more so that they can keep growing. i would suggest doing this again but changing the water in all three buckets once a week

  26. Great science lesson. I'm really impressed!
    I already have a DIY aquaponics system and I'm trying to mimic your setup #3 (24hr).
    Is it a good Idea to add a diffuser on LED white lights instead of bringing it up? Just like an ordinary LED bulb with white plastic that covers it to diffuse.
    I have limited space and I can no longer move it up. Thank you!

  27. I have a few issues with this experiment. Number one what was your control? Number to it didn't seem as though the distance of the light had much to do with the experiment that you were doing although I understand the concept. I feel as though the experiment would have had much more relevancy if you had had the lights the same distance away from the plant it can probably go without saying that the plant that was on for 24 hours would probably have much more tip burn but most people don't take into consideration how much light their plant is receiving because they try to dump as much light into their plant as they possibly can. You need to have a control for every experiment that you run. But these are interesting numbers

  28. Hey Al (is this how your name is writen?), I googled a little on the optimum photoperiod of lettuce after watching your video. I found an article ( that I found interesting. They didn't just played around with how to devide the 24hrs in day and night but they were growing plants in periods of less than 24hrs. I'd love to read your thoughs on it or even see an experiment by you

  29. Great video! I would like to know how micro cycles might affect the growth. Specifically
    1 12 hours on / 12 hours off
    2 1 hour on / 1 hour off
    3 5 minutes on / 5 minutes off

    Do you think there might be a difference?

  30. Informative video +AlboPepper … I'm currently looking at the same issue to find the best way to increase productivity for lettuce, rocket and spinach in a grow room system and this explains and also prompts me to try new tactics and tests. Thank you … Sub'd

  31. Greate video.
    I'm so sorry, but could you tell me how did you get those values(0.0864, 0.0576, 0.0432)?
    Was it with a PAR sensor? Answer: The 0.0864 factor is the total number of seconds in a day divided by 1,000,000.

  32. At 7:49 ur xalculation for power consumption is wrong..since u used d same 15w light but for the third plant u increased the height f the light to reduce the amount of photon match the same amount f photon f light falling on it in comparison with others…but if u would have kept the heights all equal …ur third plant which is on 24 hours would jave yielded not 93 but more ..i think there is a flaw ….test is nice ..but i think two parameter thingy is flawing ur cost to power ratio

  33. I question the idea that "bigger is better" yet I think where I am at now in my current lifestyle, for my highest health benefit, my gut says to rather "eat" the plant on the far left.
    Cheers for now.

  34. I always knew light cycles affect the seasonal reactions to outdoor plants. I have so many vegetable and fruit types it is quite a challenge to make sure I have the science correct for each plant. Designing my indoor basement winter growing system has been a real challenge because of a) space available, b) plant type and size (squash and peas versus lettuce, kale) c) water cycle, d) lighting I use (I have 48" x 24" size x 4 T8 lamp fixtures), e) growing media; organic soil vs hydroponic solution, e) fertilizer strength and percentage variation per supplier, f) artificial ventilation (ie small fan to simulate wind), g) seed quality, h) and finally, the cost versus yield of my effort. Your video is very well done. I enjoyed learning the science behind your experiment. I guess trial and error are going to be my obvious best teacher. Write back in a PM if you can. I'd like to hear your response to my comments. Keep up the great work!

  35. Just to confirm, one needs to hold the sensor/probe of "PAR meter" to count PPFD at a specific location .. right ? There is no separate PPFD meter as such .. or I am wrong ?


  36. I found a chart that shows long day/short day/nuetral for a good number of edibles.

  37. Wow thank you so much really amazing.
    May I ask you please, ..
    What is the benefit of aluminum foil here?
    I want to start growing some veg but on the roof "i live in a building" and the easy method for me will be Hydroponic🌱.
    Please do you any Suggestion video i should watch it as beginner.
    I'll watch all your videos already sub, but you know start step by step.
    And any Suggestion book to start with.
    Thank you so much for your time effort Advices and helping us .. you're amazing ❤🌹

    Please Continue with this unique videos type of science
    Knowledge and studying ..📚💡

  38. There one thing i dont understand, from 10.25 ich there is a increase of 4 hours for same DLI at 12.5 but only and increase of 8 hours for same DLI from 12.5 to 21 inches. hours by inches are not proportional, I know they dont have to but in case of longer distance lees hours in proportion for same result???? WTF!!! what DLI you think is the optimal for lettuce?

  39. I love how you are bringing science into your videos. It would be awesome if you could have triplicates of each treatment 😀

  40. Great information. I also experimented with 12 varities of lettuce. Tom Thumb suffered tip burn.

  41. You should try with much less light. Lettuce and salad in general grow well in the shadow. Thanks for all your videos they're great!

  42. Hi Albo Pepper, where can I get the scientific data, for example light intensity, height, duration etc on specific plants that have been tested under a controlled environment?

  43. In this world people who have an opinion are a dime a dozen.  Those who put out the effort to find the facts are true gems.  Albo Pepper… you sir are a gem!  On a different note, have you ever watched my name is Earl?  You could pass as his brother with that mustache. Wishing you the best Karrma for all your videos and work.  Thumbs up!

  44. This experiment was all good till you compared cost/yield. It doesn't make sense to run a larger than necessary bulb, then move it further away from the plant; its just wasting light. I mean I understand you did that to avoid buying 3 different bulbs (which would add additional variables if they weren't otherwise identical apart from wattage); but it renders the cost/yield a moot point.

  45. Oh, that was so useful! I'm using 24h leds and got the same picture like yours, deformed leaves and burns. I definitely knew I made something wrong, but now I'm pretty sure I understand the reason.

  46. I tried tomatoes with 24 hours light they became chlorotic and then necrotic at 24 hours after 2 weeks,? they seem to do best with 14 hours light.

  47. The takeaway should have been that you should have adjustable wattage lighting so you could save money on it. Good research, bad conclusion. Next test should be cobs or quantum boards with an adjustable power supply then run the test with the lights at the same heights and adjust the power to match the ppfd you had for each. I bet the answer will be exactly the opposite you had because the efficiency goes up drastically on LEDs as you drop the power. Do 16hrs, 20hrs, and 24hrs instead to see where the best ratio is Between growth and power consumption.

  48. Watch PART 1 of this series for further context:

    Impact of Red / Blue light on plant growth:
    How does Color Spectrum / PAR Light impact plants?
    Calculating LED costs vs payoffs:

    LED Light growth tests:
    Infinity Linear (LED vs T5):
    UFO SMD vs Single COB LED w/ Time-lapse:

  49. Can anyone tell me what he says at 6:44? Some signs of tempering, was it? What does that mean for a plant?

  50. hi, the burn tip result of using red blue light at the same Walt compare with white light is the PPFD is the total difference between two bulls, the red-blue light is much higher than the white light according to the spectrum.

  51. OMG omg OMG. So I just ran my lights at 24 hours as an experiment. Deformed! 18/6 seems to be the cure. Also I agree with broad spectrum white light being better for growth. The PAR argument sounds good but LEDs makes my plants wonky unless they are already strong. Excellent novel research design.

  52. Ps. I wanted to say that your your teaching style is great. I find that you explain complex things in a very basic manner that anyone can understand. Even as a scientist by profession who understands most of these concepts, I'm not even mad at your pace. They always told me in my studies that you only truly understand something when you can make a layman understand which I think you do great. Subbed. Keep up the good work sir and keep these interesting experiments coming!

  53. You should have reduced the intensity of the light, not the distance (especially since you kept the foil that reflects the light). But I'm surprised that 24h plant survived at all. I thought they always need at least a few hours of darkness.

  54. Is it applicable to iceberg lettuce too??? please do an experiment on that kind and give results …we will much oblige …thank you…

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