Is 21% RH too low during flower? its been at about...

it's not optimal, but you'll still get a harvest... how much of an impact? don't trust anyone that gives you a precise answer with confidence. lower humidity in flower should cause more trichome production... as with everything there's probably a happy medium of stress vs. min/max strategies. that's just a guess. If leaves don't show signs of stress and remain praying, i wouldn't worry. You'll likely need to water slightly more, so if relying on habits, pay attention for unexpectedly quicker time between irrigations. rely on weight of pot as always. Good thing is that the low RH is great for avoiding bad microbial growth. almost assuredly avoid fungi/mold type issues.