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Let's talk about PAR

PLreef

PLreef

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PAR - Photosynthetcally Active Radiation
Well, think of PAR (Photosynthetcally Active Radiation) to be the light (radiation) that promotes (active) photosynthesis. The illustration above, from Wikipedia, shows the wavelengths of PAR. PAR, scientifically, delineates which light is available for photosynthesis. ( found this text on Google)

So I purchased a PAR meter a while back and only played around with it a bit. Is there a PAR standard for seedling, veg and flower?

As it stands right now I have a handful of newly flowering autos under a GROWERS CHOICE ROI E680 at 60%. Roughly 25" above the canopy. Center of the grow its reading at 745ųmols. Should I up the % and lower the light fixture to increase PAR? Maybe get the number closer to 1000?
What'd ya'll think?
 
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Bruce has been a wealth of helpful information. Not much of his older videos mention cannabis specifically, but the theories he presents do translate over to our beloved little stink plants.
 
I don't supplement co2 in my grow and I find that the tops of the colas start to show signs of stress when ppfd gets over 1000 umoles.
If you can produce an even canopy, I'd shoot for a 750 ppfd average for best growth in ambient air.
 
I copied this to help me figure out how to adjust the height of my new light (using the manufacturers numbers I don't have a par meter) this tells you how to get your DLI I've been told a plant can process up to 60 per day but above 40 will probably need co2.
hope this answers your question i get it but have to read this to do my math

PAR (Photosynthetic Active Radiation) - The spectrum of light plants utilize for photosynthesis

PPF (Photosynthetic Photon Flux) - The amount of light in the PAR spectrum that a light emits.

PPFD (Photosynthetic Photon Flux Density) - The amount of light in the PAR spectrum reaching the plant surface

PPF is measured in umol (micromoles) per second, and PPFD is measured in umol per meter squared per second (umol/m2/s)

DLI (Daily Light Integral) takes that reading and converts it to mol (moles) per day, so total light accumulation throughout an entire day

If you're curious to do the math yourself, or to understand how the math works:

PPFD x (3600 x photoperiod) / 1,000,000 = DLI

Where 3600 is the amount of seconds in one hour and 1,000,000 is the amount of micromoles per mole.

My recommendation would be that any time you go above 40 DLI, it's time to require CO2 supplementation
 
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As a couple others touched on and a rule of thumb. Under 1000 umols without co2. Anything above 1000 you will need co2 to compensate for the extra energy the plants will be able to absorb.
 
I love his stuff. In terms of knowledge his videos are a goldmine. That being said it can be kinda drab listening to him lol
 
Interesting discussion. I am running a Mars Hydro SP3000. At the recommended hanging height of 19.5 inches it puts out close to 1000 PPFD. If I run my light at this height in my current auto grow, I find the center plant gets excessively droopy, though no signs of light burn. I am keeping my lights on 18 hrs. So at 19.5 inches they would get a DLI of 65.

My plants seem happier with the light at 22 inches. Because intensity goes by the square of distance, this should be:

DLI= 65 x (19.5/22)^2 = 48. This is still 20% higher than recommended above and certainly explains why 19.5 inches is too close.
 
I arrange my plants to were the shorter ones are in the middle where the light is the strongest and the taller ones can be closer because the edges aren't as strong
 
So I turned the dimmer at 100% full power. Lights are at about 25" over the top of the canopy. PAR is readying at about 820 center of the grow. In a few days I'll lower the light by an inch or two till the PAR get to just under 1000.
 
PLreef Sounds like you might have the same voice in your head that keeps me awake when I don't have the answer even when there's not a question pending 🙃 I don't have a meter yet but I've done the research the math plants are responding great getting fatt but that damn voice wants the exact numbers lol
 
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