If you are interested in increasing or maximizing the yield and quality of your crop, higher light levels are the ticket. In ambient CO2, once the inputs in the grow environment have reached the sufficiency level, light is the only input that you can increase that will lead to an increase in crop yield or crop quality.
Note that I specify "ambient CO2" because CO2 is the first input to optimize but most growers can't because they're in tents. Given that you're at 1200PPM for CO2, the next input to maximize is light. Per my statement, once you hit the sufficiency level for all of the other inputs, there's no point in adding more.
If you're running 1200PPM, you should be able to take PPFD well on the way to 2k. Crop yields increase in an almost linear manner as light levels increase. There's plenty of research to support that, if you're interested in the gory details. An interesting data point is that Bugbee recommends 1200µmol at 1200PPM that advice is for commercial growers and he states that light levels really are dependent on how each much each grower wants to provide. One of his students, Mitch Westmoreland, has said that they have run cannabis at light levels up to 2000µmol and yield continues to increase even at that level. I run my grows in least 1k in ambient CO2 and have had excellent results.
VPD is not a function of which week. "optimal" VPD values are stated for each of the phases of plant growth because plant structure changes according to the phases of plant growth.
A VPD of 0.8 is considered optimal for a cannabis plant in seedling stage, it's 1.0 for veg, and 1.2-1.5 in flower. Those are "optimal" values.
What does "optimal" mean? I would explain it - when VPD is at those values, plants will transpire at a rate at which you can use normal concentration of nutrients and you can provide water at a standard rate. VPD is not a goal, necessarily. VPD is a measurement that indicates the rate at which your plants will be transpiring and, as a grower, you need to adjust your watering schedule and nutrient strength accordingly.
VPD came up in another thread here on RIU a few days ago. After reading the thread, I came up with the thoughts that I expressed in the above paragraph and I think it defines how VPD should be used by growers. As I stated in that other thread, as a new grower I saw VPD as a goal and it took a while to get my head around the idea that it's not a goal, rather it's a measurement used for managing the environment.
Ironically, about 24 hours ago, RH here in Southern California plummeted and the RH in my grow, which is in an unheated garage, dropped into the 30's. I've got 190+80+68+106=444 watts of light in the tent so there's "lots of warmth" in the tent and not much humidity. My AC Infinity humidifier is running 10/10 and yet my VPD has averaged 1.6. That's higher than I want and I'm seeing some tip burn. I added pure RO this morning (it's hydro) and I'll add another 1.2± gallons when I add water to the res tomorrow AM. The plants will be fine as long as I make sure they've got plenty of water in the res and EC will drop as I add more RO.
I think it's great that you're running CO2. I'd love to make that leap but no can do in a tent so the best I can do keep the other parts of the grow environment up to snuff.
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The best advice on temperature is from Mitch Westmoreland in the YT videos he dropped at about this time last year. He is/was a PhD candidate under Bruce Bugbee and he shared some of the research that he did for his thesis. His advice is to run up to 85° from seedling through about the second week in flower. The high temps increase growth rates and, by the second week in flower, the infrastructure of the plant will be built out. Once you hit the second week, drop your temperatures so that the temperature of the tops of the buds is <=78°. The reason to keep temps low is that elevated temps has a dramatic negative impact on the levels of secondary metabolites.
From one of his videos (title of the video at the bottom of the graphic).
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His video is excellent. It really is a "how to" on getting the most out of your grow.
And, yes, the tops of my buds are <78°.
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