Photoperiodism is the process by which plants use the length of daylight and/or darkness to regulate flowering & certain other processes, regardless of the ambient temperature or weather.
Photoperiodism is dependent on a plant’s ability to sense light. Plants do not have eyes but they do have photoreceptors — these are cells that contain a light absorbing pigment. When these cells receive light, specific proteins are altered and cause changes in hormone production, gene expression & growth.
Short-Day, Long-Day, Day-Neutral:
bloom when the length of daylight (the photoperiod) drops below a particular critical threshold, typically in late summer or autumn, after the equinox.
Examples: chrysanthemum, rice, soybean, onion, violet, Christmas cactus and poinsettia.
The critical threshold varies among plant species, but is often around 12 hours.
flower when daylight lasts longer than their critical threshold, typically in spring or early summer, before the equinox.
Examples: aster, hibiscus, coneflower, lettuce, spinach, radish, sugar beet, and potato.
have a flowering process that is not regulated by day length at all. Instead these plants bloom when they are simply old enough too.
Examples: cucumber, corn, tomato, pea, and dandelion.
Tuber formation in potatoes, preparation for winter dormancy in trees, and many other physiological processes also are controlled by the length of daylight.
As the days grow shorter in autumn, many trees set their winter buds and begin to drop their leaves, even if it isn’t cold yet. Potato plants start forming tubers to survive through the summer when the days in spring begin to lengthen, even if it isn’t hot yet.
Why aren’t Short-day Plants called Long-night Plants?
Recent research has shown that most short-day plants are actually reacting to the duration of continuous darkness! For these plants, if the period of darkness is interrupted by a brief period of light, they won’t flower, even though the duration of light is short enough and the total duration of darkness is long enough. They need an uninterrupted period of darkness that is longer than their species-specific threshold. Some short-day plants cannot flower up North because by the time the days are short enough to induce flowering, there’s already been a killing frost.
Flowering for most long-day plants, on the other hand, seems to require long periods of light, and disrupting the dark period with a brief flash of light, or disrupting the light period with a brief shadow of darkness has no effect. Many long-day plants won’t flower in the tropics because the days never get long enough.
Plants with specific day length requirements can be tricked into blooming out of season by manipulating the light they receive or don’t receive.
Thus nurserymen can get short-day chrysanthemums to bloom in summer by keeping them in the dark for more than 12 hours each day. Poinsettias can be induced to bloom if they get 10 or more hours of darkness for a week or so. Indoor lighting and the light from street
lights can be enough to prevent short-day plants from blooming. (Moonlight isn’t bright enough, though.) Florists can get long-day plants to bloom early (say, for Easter or Valentine’s Day) by holding them under grow lights for a few hours longer than the normal late winter daylight.
Some plants are tricky in that they have photoperiod AND temperature requirements. For example, the Christmas cacti will form flower buds if the nighttime temperature is between 50 and 59 F, regardless of day length. If the temperature is between 59 and 70 F, however, they need 13 hours of uninterrupted darkness every 24 hours for eight weeks to form flower buds. If the temperature is above 70 F, they need 15 hours of darkness for eight weeks.