I live in an apartment with south-facing windows. Even though the winter gets super cold here, there’s always lots of sun beaming in almost every day of the year. I keep the apartment at about the same temperature all year long. My African Violet has started blooming and my other plants have started growing like crazy. I water them once a week all year long as well. How do they know it’s Spring and time to bloom?
Is it nothing more than the longer hours of sunlight? I wouldn’t have thought that would be it because they get full sun all day long anyway. Would an extra couple hours do it?
Yup, many (if not most) plants monitor the season primarily by the length of sunlight. Some flower after the day length increases above some threshold, others flower after the day length decreases below a threshold in the summer or fall. You can artificially manipulate this with sufficiently bright lights, and trick your plants into flowering at the wrong time of year.
Some plants react to the amount of daylight with amazing precision. I have some hanging succulent plants that produce gorgeous flowers within a day or two of the summer solstice . . . and at no other time. Each bloom lasts just one day.
Same thing works with breeding horses. Here in a northern location (Minnesota) many mares don’t go into heat in the winter, or don’t conceive easily. Most mares come into season in May & June, leading to April & May foals. If you want to have a foal at a specific earlier date (and for many breeds, like race horses, an earlier date is preferable), you will have specific stalls with supplemental lighting installed, controlled by timers to make sure the mare in that stall gets enough light to trick her body into going into reproductive mode.
As I recall, it’s about 10-11 hours of daylight to trigger this. So you look up the hours of daylight for your location, and set the timers to turn on the lights before sunrise and after sunset for the needed number of additional hours to trigger reproduction. The amount of light is important too. I believe it’s somewhere between 1/3 to 1/2 of full summertime sunlight that’s needed. Sometimes, gray winter days don’t produce enough light to meet that, and the stall lights need to be on all day long.
Humans may have such triggers, too.
Human births seem to peak in the Fall, after the heavy farm work is done, and meaning that the baby will be 6 months old before the next spring planting comes. Or perhaps it’s just that nights are much longer in January & February, giving more time to conceive a baby for the Fall.