[Hippie3]

actually folks keep getting that bass-ackwards.
the light period is not the determinant,
the dark period is.

during darkness
cannabis plants secrete a certain hormone,
which is cumulative but is destroyed by exposure to bright light.
so
if the dark cycle reaches a certain length,
the hormone level reaches a critical threshold and
flowering is triggered.
however this does not occur until the seedlings reach a certain age,
which prevents flowering in early spring when nights are still long.