A lot of plants “determine” their production of biomass and secondary metabolites by the 4th week of flower and I suspect this is similar in Cannabis. Anecdotally, my plants always seem “the hungriest” in the transition from veg to flower and then a few weeks passed flower initiation. I think I have an article or two about this on my computer but I will have to double check.
The ecophysiology behind this is that in natural settings plants need to invest a lot of energy and nutrients into flower production. To successfully flower and complete their reproductive life cycle they need a lot, making offspring is energetically expensive and they want to make as many seeds or flowers as they can while partitioning enough resources to each egg so that seeds can start life with energy reserves. Some plants only flower after reaching a critical point of nutrient concentrations. Cannabis isn’t one of those, but the need to secure enough nutrients to ensure seed survival is a pretty heavy selection pressure.
That being said, I have had good luck with a foliar spray composed of salicylic acid, epsom salt (sulfur and magnesium), calcium. Salicylic acid is a plant defense hormone that if applied foliar in the first 2-3 weeks of flower can increase teichome production and stimulate terpene synthase genes. Epsom salt is good because it adds extra sulfur, and more importantly magnesium. I dont do much calcium in the spray and I’m not sure I will keep it in the formulation for next time. My experience and the research I’ve seen leads me to believe that the important part of calmag is the mag and not the cal. I’ve only ever seen cal problems in hydro, but that could also be a result of the soil i use. I include sulfur as a supplement for terpene synthesis as it is an important component of some enzymes and modified proteins.
I have a hypothesis about the elevated magnesium cannabis seems to prefer and I am happy to share it, but I don’t want to take away from the main thread.
Lastly, the number 1 determiner of cannabinoid and terpene content is genetics in every study I have seen. We know cannabis reacts to environment, but it can’t do what it’s genetics doesn’t have.