Hi Paul,
I’m surprised you can only get 1000 µmol/m2/s (which I’m going to refer to as ppfd) in a tent with that light, you must have it hung pretty high up. But if so, that’s good, because those HLG lights have a tight spread. I consider them to be high-bay lights; I have them in one of my rooms and have them connected to the ceiling, not even on a hanger, so I can have a remote hope of spreading the light around evenly. Every other one of my rooms has bar style lights which is nice because the plants can grow to within 4 inches of them without damage and I still have even light across the canopy.
700-800ppfd is a good recommendation for 12 hours of light for hungry plants (tomatoes, peppers, cannabis) grown without CO2 supplementation. 1500 ppfd is enough to cook almost any plant and if anyone gets results with these values they are an exception, not a rule. Technically I can get a plant to grow under 1500, but it’s not going to look as good as the one under 1300. If you are vegging with 18+ hours, you should be under 700ppfd even with CO2. Get used to calculating DLI; my lights are always set based on DLI.
As far as transpiration, like I alluded to in my last comment, there can be too much of a good thing. Traspiration is your way of getting the plant to suck nutrients out of the soil and dry the soil back which pulls oxygen into the soil. For one, if you are growing in deep water hydro you don’t ever dry back so you “need” even less transpiration. There is also an idea amount of fertlizer, too much being a way more common result of poor yields than too little. As a grower, I do my best to match transpiration to the amount of nutrient I feed. If I’m used to growing with 1000ppm of a nutrient at 70% humidity, and I decide to switch things up and drop to 50% humidity then my plant will be consuming far more water so I have to drop my nutrient to say 750ppm to avoid burning.
You can actually get very scientific about this: for instance if I want my plants to consume 5.5 grams of a nutrient mix per day, and the plant consumes 1 gallon of water per day at 70% humidity, then I would mix 5.5 grams per gallon in the irrigation water; if I then reduce my humidity to 50% and the plant consumes 1.3 gallons per day, I would reduce my nutrient to 5.5*1/1.3= 4.2 grams per gallon as a good estimate.
Put another way, you don’t need transpiration to increase the amount of food the plants consumes, you can also just increase the amount of food you put in the soil. You must balance the two factors. But as I’ve said to a hundred other growers and had a hundreds other growers say to me, less is often more when it comes to nutrients. Show me ten photos of glossy curled up burnt tipped plants being fed 3.0 EC and I’ll show you ten thousand photos of supple healthy green beautiful flawless leaves being fed at 1.8 EC.
There is plenty of transpiration occurring at 70% humidity provided the airflow is good. My pots can go from fully saturated to deeply dry in less than 24 hours.
Yield per plant is another relative thing. This is totally dependent on which plant, which media, how much media etc. I used to grow autos as a seed producer and in a deep water culture setup with unrestricted root growth I have grown autos with 2.5 pounds of flower on a single plant. One unfortunate thing about autos is that your only control over their “veg” time is the amount of space you give the roots; they basically go to flower as soon as the roots hit a wall. My average for autos in 2 gallons of coir given 2.2 square feet per plant and grown under 18+ hour light cycles is about 4-5 ounces.
As far as my recommendations go, it’s hard to go out on a limb and assume I know what your environment and pest/disease pressure is like as well as how your plants respond to the nutrient you are giving it. You kinda have to read the plants and ask them what they want. I also don’t disagree with the recommendations in the article at the top of this thread, my reason for chiming in was to reinforce putting things in perspective.
But if I was setting up a tent and had CO2 and had good airflow and was ontop of my deleafing/pruning then I’d shoot for around 1.2 kPa vpd beginning to end, only considering dropping at the end of flower if I was worried about bud rot or air flow. And if I was growing autos I’d have my lights dimmed to provide 700 to 750 umol for 18 hours which is a lot of light; I’d set my CO2 ppms to match the umols, so around 750ppm. If my dehus or ACs were struggling or I was growing in an area with poor insulation causing light/dark temp or humidity swings I’d grow with 24 hours of light and dim the light proportionally. The only thing I’d adjust throughout the cycle is the temp, starting at 82-85F, eventually landing at ~72F (adjusting the humidity to keep the vpd about the same). That’s just me tho, there are a hundred ways to fry an egg.
soo i’m hoping for some insight.