Best PAR to run at Canopy in Flower with CO2 1200ppm 1500ppm

I finally broke down and got a par-far meter from apogee after decades in the game and am looking for the best numbers to get the best quality and quantity. I run 1200ppm CO2 in winter with the burner and 1500 ppm in summer with tanks. In soil organic with HLG scorpion diablo 650 LED. Room is about 85 degrees F and 50-55 humidity. Week 4 right now.

I took some readings as the room is now and peak in middle of lights is just under 900 and outside edges are sitting around 600. The vast majority of the canopy is sitting between 700-800 PAR. Also is it better to use the PFD reading vs PAR as that number is a bit higher on my meter. I could easily lower my lights a few inches and bump the readings up 10-20% but dont really know what im lookng for. I see some people say they push almost 1500 and some people say 850 is best.

Any advice appreciated.

jaked3800 said:

I finally broke down and got a par-far meter from apogee after decades in the game and am looking for the best numbers to get the best quality and quantity. I run 1200ppm CO2 in winter with the burner and 1500 ppm in summer with tanks. In soil organic with HLG scorpion diablo 650 LED. Room is about 85 degrees F and 50-55 humidity. Week 4 right now.

I took some readings as the room is now and peak in middle of lights is just under 900 and outside edges are sitting around 600. The vast majority of the canopy is sitting between 700-800 PAR. Also is it better to use the PFD reading vs PAR as that number is a bit higher on my meter. I could easily lower my lights a few inches and bump the readings up 10-20% but dont really know what im lookng for. I see some people say they push almost 1500 and some people say 850 is best.

Any advice appreciated.

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You can leave things as they are right now since your plants are still young, they won't require as much light as they will in flowering even with CO2.

Great thing for you is the plants are growing, thus they will get closer to the lights themselves which will increase the amount of light energy they receive overtime. In flower phase 1000-1500 should be min with CO2.

YoZeitgeist said:

You can leave things as they are right now since your plants are still young, they won't require as much light as they will in flowering even with CO2.

Great thing for you is the plants are growing, thus they will get closer to the lights themselves which will increase the amount of light energy they receive overtime. In flower phase 1000-1500 should be min with CO2.

Click to expand...

I'm in week 4 of flower not veg...maybe ill drop the lights down a few inches to up the intensity.

jaked3800 said:

I'm in week 4 of flower not veg...maybe ill drop the lights down a few inches to up the intensity.

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Ooh then you definitely can give them much more light up to the maximum suggested and keep an eye out for bleaching, burning or stress every 24 hours. More light equals more bud and bigger, denser buds and that plus CO2 will increase their metabolic rate and will need more food in accordance to an increased metabolism.

if your going to push them harder id suggest upping the co2's ppm from 1200 to 1500 before increasing your ppfd and i would do it incrementally in small amounts with your meter. the thing about this is some strains can handle greater ppfd vs other strains so things will vary.

PAR is "photosynthetically active radiation" which is the light that plants can use
PPFD indicates how many PAR photons (light particles) are hitting a square meter in a second.

You would get a lot of value out of some of the Bugbee videos on YouTube about light and grow lighting.

The drop off in light levels is a characteristic of HLG board lights. It's good that you've got the PAR meter so you can adjust the lights/location of the plants.

The goal is to have an even amount of light on all parts of the canopy. With the hot spot under the center of the light, if you haven't topped or LST'd your plants, you'll have a dominant apical stem which you'll need to super crop away from the hot spot.

Re. ramping up sloooowly - if the plant is in the vegetative stage, it should be able to handle light at the light saturation point. You can raise light levels as quickly or as slowly as you want, assuming that the plant is healthy and that the grow environment is sound. In that case, the "limiting factor" is the grower.

That just happened to me. Earlier this week, some parts of my current grow were >1100 µmols. There were no signs of stress but I just didn't think it was wise to be so close to CO2-enhanced light levels so I turned it down to about 1k.

Per Bugbee, the magic numbers are 1200 PPM and 1200µmols. I think that's in the video he did with Dr. Grow It and it's in the Q & A, right at the end of the video.

The Chandra paper "Photosynthetic response of Cannabis sativa L. to variations in photosynthetic photon flux densities, temperature and CO2 conditions" has a lot of insight, especially this:



That's a graph of net photosynthesis and, plain as day, the curve starts to flatten out pretty early. That's net photosynthesis though and I'm not harvesting photosynthesis. I'm more concerned about yield and quality.

The eye opener for me was the paper "Cannabis Yield, Potency, and Leaf Photosynthesis Respond Differently to Increasing Light Levels in an Indoor Environment" that showed that, even though the net photosynthesis curve was rolling off, plant quality and yield and crop quality and yield increased.

I created this table from the data in that paper. In short, the increase in yield for each increase in 50µmols in flower, resulted in about a 5% (4%?) increase in yield. Instead of running lights at, say, 600 µmols, yield would increase by 20±% if the PPFD were to raised just to 800µmols. Like a good idea to me!