In September, when temperatures can still exceed 30°C, plants begin to change their behavior. This change is not related to heat or rain, but to something considered more reliable. Lettuces begin to bolt, spinach takes off for good, and strawberry plants prepare for the following season. Plants don't respond to the thermometer, they respond to the clock.
The explanation for this phenomenon lies in the fact that plants measure nights and not days. The heat of one year is not the same as another, there are very hot Septembers and cool Septembers, years of drought and years of early rain. A plant that depended exclusively on temperature to decide when to flower would risk being wrong often.
Photoperiod is the name given to the relationship between the duration of day and night throughout the year. Plants can measure this relationship with great precision through pigments found in their leaves, which allows them to perceive that the season has changed regardless of variable weather conditions.
