Found this thread referenced from another related so thought this info might add value. I get ~2GPW at 17 Watts/sq ft at 5-10 inches and no cal-mag. Copied from
Could Low Humidity Do This?:
All leaves on the plant should be "praying to the light", otherwise you have an issue. The initial pictures of this thread showed signs of too much light. The larger the trunk is on the plant, the more light it can take. Too much light (not heat from light) will cause the plant's "pump system" to overload and break down. New leaf growth will be curled and stunted. Larger new leaves at the top will become leathery as they shrivel to protect from the light. The first signs of too much light will be rust spots or cal-mag like deficiencies in older leaves. I think this is caused by deficiencies in other parts of the plant due to the system overload from the excess light. Magnesium is a "mobile" element so the plant can rob it from the leaves if it is needed elsewhere. If not corrected the rust spots will continue to spread into flowering and trichome density will be affected negatively. There might be other factors that contribute to the rust spots. I never had much success with adding cal-mag, if any. VPD might be a factor. My plants were continuously fed a large amount of low humidity air from the house but I never measured.
If you ever see the rust spots, back off on the light and you will see that the leaves will recover so that the areas around the rust spots will fill with dark green and the spots will cease to spread.
Broad spectrum LED's can easily overload a plant. I ran a 8 sq ft box with a 400 watt sodium for years and suffered from the rust problem but still produced decent bud. The box had very low head space so I had to keep my plants small. The trunks never got larger than 3/8" or so. I switched to Bridgelux LED at 200 watts and fried my plants real quick. When I finally caught on after trying just about everything I fine tuned and found out exactly how much light those small plants could take before getting rust spots. For that 8 sq ft canopy area the most they could take was 130-135 watts of BridgeLux 3500k strip LED's at 5-10 inches from the canopy.
Finely burned tips are not necessarily an indication of an issue. You can get rid of them by turning the light way down but that will affect the yield.
I've run PPM up to around 4000 before any signs of toxicity showed. My cheap Chinese meter had gone way off and I didn't know it. The leaves got very dark green, curled over, and got crispy dry. PPM had been high for a while but they showed no signs of ill health. When I added the last bit of nutes they turned overnight. When I got a good meter I was amazed that they could take that much nutes. I normally run 1300-1400 all the way through with RO water that comes out around 70-100.
You have to have a nute reservoir large enough so that changes in water level don't affect PPM and PH. The best you can do is run a float valve with continuous RO water supply. PPM will drop as the plants eat, PH will rise slightly as the PPM goes down, and you feed just a little at a time to keep PPM and PH in the desired range.
If you do see rust spots and it's not too bad, they might stop spreading as the plant becomes larger and can handle more light.
The best thing you can do for your efforts is to keep a log with pictures, PPM, Watts, PH, other changes... everything. Otherwise you will chase your tail and get frustrated. Keep it simple. Change one thing at a time. Be patient and see what those changes do.
If the plant is stressed, turn the light down until you figure out what's going on. Transplanted plants need low light to slow the metabolism so it can have time to adjust to the new environment and get the "bio engine" running again. You'll know when the plant is ready to go. Push it too hard and you'll end up with immune issues and root rot. Healthy plants can deal with a little bacteria in the water without getting sick.
When you fill a new tub, give it a few days for the bacteria flora to stabilize before you put transplanted plants in that are stressed. The tub doesn't have to be 100% sterile. Just remove all organic material, run water with a cup of bleach through the pump and everything for a day or so, flush that, then fill it, add nutes, and let it sit for a few days with pump and air stone running so that the good bacteria can stabilize the environment. Then put your plants in with low light until they start growing and look healthy. I've actually gone two full harvests without changing the nutes so don't think you have to kill yourself to flush and refill. Just keep healthy plants and the tub will stay healthy.
If you don't have a dial or app to adjust the light intensity you are doing it wrong.
What I'm learning now is that a sealed environment requires a crap ton of humidity removal. Those plants are like water pumps. And to keep humidity and temperature stable it requires a huge amount of air space just like the nute reservoir needs to be large enough to handle small changes. Otherwise humidity will go up and down as your equipment tries to deal with it.
The attached pics are of an LED burnt plant from 25watts/sq ft and a bud produced with 17watts/sq ft of BridgeLux 3500k LED strips and no cal-mag added.
I lied about what lights I run. The 4 in the middle of the room are HLG 600h, the 8 on the sides are hlg550s. It was only after purchasing the 600s did I realize they were too powerful for my room, and I have them hung about 33 inches from the tallest colas. The HLG 550s are hung about 26-27 inches away from the top cola. All lights are dimmed to 65%. this gives me roughly 650ppfd in a 2ft square from center of the light, and around 450ppfd out to the edges on all 3 trays. My current room temp at 80 degrees is my swing low, my swing high is around 83-84, I personally don't like it to get hotter than that. And for those of you who said that you can't grow good weed with LEDs. I'll note the tray I photoed here I averaged 1.25 lbs per plant with 6 plants in a 4x8 (and room in between each plant). Assuming I get the same numbers now. I'll pull 20lbs on 16 plants in a 4x15 area under 4 HLG 550s (I know I said 600s, but I raised them higher to keep the ppfd levels the same as the 550, ill be switching out the drivers next grow) 20lbs = 8960 grams. 480w at 65% = 312w x 4 = 1248w. 8960/1248 = 7.17 grams per watt.
