Night length can be a trigger; temperature can be a trigger; water pattern can be a trigger; barometric pressure drop can be a trigger. It varies from species to species.
Most plants growing close to the Equator don't have a night length trigger, because there is so little variation.
Rain seasonality is a big trigger to a lot of plants from areas with dry winters and wet summers.
At least one orchid, Dendrobium crumenatum, is triggered to bloom by the drop in barometric pressure with thunderstorms.
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