Max Lumens before growth rate tapers

I have read tons of posts and grow guide topics concerning how many lumens per square ft to have in your grow room. Well, I am trying to dig deeper.

I understand that plants can use up to 10,000 lumens per sq ft. Minimum 2,000 lumens per sq ft and ideal is between 3,500-5,000.

At 4,000-5,000 lumens / sq ft. you have effectively maxed out without adding CO2.

Also, CO2 levels should be:
2,000 PPM w/ 5,000 Lumens / sq ft
1,500 PPM w/ 3,000 Lumens / sq ft
300 PPM w/ 1,000 Lumens / sq ft

Well, what about if you add CO2? If you go from 5,000 lumens / sq ft to 10,000 are you going to get twice the yield and twice the growth rate? At what point does growth rate taper off and at which point is most efficient vs. electricity/heat concerns?

In my case I am building a 12 square ft cabinet, vented with a 350CFM carbon scrubber ventilation, and an air cooled light tube lowered 8-10" above plants, CO2 tank, and a 400W 55,000 Lumen HPS (4,600 lumen / sq ft) in SOG. I am trying weigh out how much yield I will miss out on with 4,600 lumens (even though light will actually be much closer to plants with the cooling tube). If I add another light, I would have to scrap the cooling tube. So I could add another 150W HPS to bring me up to 5,920 lumen / sq ft. or 250W HPS addition to total 6,900 lumen / sq ft.

HOWEVER, beyond 5,000 lumens / sq ft. are we only talking about a 5% increase in yield/growth or a 75% increase? Which difference should one truly expect? When does yield/growth rate begin to taper off and heat/electricity bills keep rising? Anyone have a chart on this?



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B.Bugbee - EFFECTS OF RADIATION QUALITY, INTENSITY, AND DURATION ON PHOTOSYNTHESIS AND GROWTH
Effects of Light Intensity and Carbon Dioxide Concentration on Photosynthetic Rate of Soybean -- Brun and Cooper 7 (5): 451 -- Crop Science
Plant Metabolism
http://ag.arizona.edu/ceac/CEACeducation/PLS475A-575A_files/04.pdf

For HPS the saturation point is closer to 7500 lumens/foot^square. The 5,000 figure is for sunlight. HPS has a higher luminous efficacy which is perceived brightness but luminous efficacy and radiant flux per watt are different.

The actual saturation point for a C3 crop like pot is roughly 1000 umol/meter^2/sec which is about 6*10^20 photons per square meter per second.

Luminous efficacy - Wikipedia, the free encyclopedia

Pushing beyond saturation forces the plant into a condition called photo respiration where the plant tries to use oxygen in the calvin cycle rather than CO2.

Photorespiration - Wikipedia, the free encyclopedia
Calvin cycle - Wikipedia, the free encyclopedia

If you want to bust through the saturation point you can reduce the oxygen levels in a low pressure system.

Dr. Davies Research Page

techhead,

I understand that HPS is obviously more efficient than other light sources and that marijuana has a high co2 saturation point compared to other plant types (C3 vs C4, etc). I also realize that there is a light and co2 saturation point, but what I am trying to find out is at what point does photosynthesis slow as it reaches these saturation levels. See my photo attached. As you can see at the end of the graph on the right, saturation has occurred and photosynthesis has reached its limit (with environmental conditions). However, you'll notice at the 1/3 mark on the curve, efficiency begins to decline. This is what I am trying to discover for the marijuana plant. At what point does efficiency begin to significantly drop off, where additional light and co2 only impacts photosynthesis 5-10%. In my case, I am at 4,600 lumens / sq ft with a 400W HPS with a CO2 tank added and a light cooling tube allowing me to place the lamp 8-10" away from the canopy. I do not want to add another 250W HPS, jack up my temps and electricity bill if I am going to see a mere 10% increase in photosynthesis. You follow?

Thanks for your response. You are obviously very knowledgeable.


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Right on, I understand. Let me search for this, I know I've seen this graph that you're looking for.

Any word bud? Thanks again, I appreciate the information.

I can't find that damn graph with the numbers filled in! I've spent several hours online looking through research papers and nope, can't find it. Looked through some old text books and sure enough, there's the graph, but no numbers. I'll PM you if I ever find it. Sorry about that.

BTW, I run plants at about 6500-7000 f.c. HPS levels for budding which is around 900 umol/meter^2/sec equivalent. This should be past saturation for non-CO2 enhancement but regardless it does create higher quality buds in terms of resin, density and flavor over lower lighting levels. When it's cooler I run about 1500ppm CO2.

techhead,

Thank you again!! I greatly appreciate you going out of your way to look for me, you definitely know your stuff. Based upon the graph in your book and your knowledge, how well do you think I can do with 4,600 lumens / sq ft with proper co2 concentrations and a cooling light tube (allowing me to lower my light to 8-10")? My primary concern is size of yield followed by growth speed. Do you believe I will be losing out on significant potential given the fact that I will be using co2 and hydroponics? In your estimation, what % would I be gaining from reaching saturation, 5%, 15%, 25%, 50% more per plant (yield + growth speed)?


I really appreciate your help! You have been awesome. kiss-ass

jjng5

If your concern is yield then run your set up at 6000-8000 lumens/square foot. This will get you to the photosaturation point of your plants (the 5000 lumen figure is for sunlight only).

It's hard to give exact numbers as there are other factors that affect the rate of photosynthesis such as temperature and humidity. If the humidity is to low the stomas will close so they'll be no gas exchange. If the humidity is to high then it becomes harder for plants to transpire water so photosynthesis will slow (it has to do with vapor pressure on the leaf's surface). Same with temps; 75 degrees f will have a greater potential rate of photosynthesis than 55 or 95 degrees.

Blast your plants hard, with or without CO2, and you'll be pleased with the results. Yes, you might be able to get a greater biomass if you allow your light to spread over a greater surface area but for good quality buds you need the intensity.