@@Zauberkraut, Well it may have greek roots, but in English, that is not the right word for a "photoperiod" plant that exhibits "photoperiodism."
A periodical in English is:
adjective
1) Periodic.
2) Published at regular intervals of more than one day.
3) Of or relating to a publication issued at such intervals.
"Photoperiodic" and the adverb "photoperiodically" can be used, but not photoperiodical. You just made that one up or used a word from your language assuming it was the same in English. You also seem to contradict yourself.. you mention english is a 2nd language but you say my greek is rusty, lol, which is it? Shit or get off the pot as you can't have it both ways.