How the plant tells time
Hormone
Phytochrome in the Pr form starts building from germination
Red light
Daylight and lamp light flip Pr to the inactive Pfr form
Dark pile-up
In a long night with no red flash, Pr can reach a trigger level
Reset
Even a brief light leak turns Pr back off. That is why dark must stay dark
Outdoor seasons
Long days
Late spring and early summer nights are too short for Pr to hit the mark
After June 22
Northern days shorten. When night reaches about 9 to 11 hours, many plants flip
Timing
That often lands in late July or August, depending on latitude
Five nights
About five days at the critical dark length starts the change from veg to flower
Indoor switch
Veg light
Continuous light or at least 18 hours keeps plants in leaves, stems, and roots
Flower light
12 hours on and 12 hours off lets Pr hit the mark every night
Same physics
Lamps do not change the rule. The plant still counts dark
Fast genes
Quicker varieties need fewer dark hours to flip. That trait is linked to finish speed
Tuning a known variety
Mono-crop
Once you know a variety's critical night, you can give a little more light than 12 hours if flower still stays strong
Mixed room
Tune to the slowest variety or the plants that need the longest night
More light
Extra photons help if they do not break the dark requirement
Age myth
A three-month plant and a six-month plant of the same genetics finish at the same potency when the buds are equally ripe