...i agree that 3 weeks before the chop is fine to see some yellowing.
...keep in mind that what media and style you grow in has some impact on this timing, meaning i grow in coco so i feed with every watering up to the final 2 week flush so i control when my plants start to yellow to some degree by when i start decreasing the nitrogen in the daily feed.
...with soil it can be somewhat controlled as well but through different variables that i wouldn't feel qualified to opine on.
...anyway, seeing some yellowing at day 40/63 is no big whoop and nothing to be concerned about in my opinion.
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