Definition:"Short day"A plant that requires a long period of darkness, is termed a "short day" (long night) plant. Short-day plants form flowers only when day length is less than about 12 hours. Many spring and fall flowering plants are short day plants, including kalanchoe, onion, viola, some chrysanthemums, poinsettias and Christmas cactus. If these are exposed to more than 12 hours of light per day, bloom formation does not occur.
Photoperiodism The response of an organism to changes in day length (photoperiod). Many plant responses are controlled by day length. (Short, Long, Neutral)
Chrysanthemum (some types) critical day length: 15 hrs.
Poinsettia critical day length: 10 hrs.
"Long day" plants, bloom only when they receive more than 12 hours of light. Many of our summer blooming flowers and garden vegetables are long day plants, such as asters, cone flowers, California poppies, (dill, 11 hrs), lettuce, (spinach, 13 hrs) foxglove, lettuce, petunia, sedum, and hibiscus.and potatoes. These all bloom when the days are long, during our temperate summers.
"Day neutral" Some plants simply begin flowering once they’ve reached a certain age. They form flowers regardless of day length. Tomatoes, cucumbers , pea, sunflower, dandelion and some strawberries are day-neutral.
Day-neutral strawberries called “ever-bearing” strawberries, are smaller and easier to manage than June-bearers.Scientists discovered that it was actually the hours of uninterrupted darkness that triggered flowering, rather than the hours of light. Experiments showed that even brief flashes of light during the dark period of the cycle could interfere with flower development. Despite this new understanding of photoperiodism, the terms long- and short-day, which refer to hours of light rather than darkness, are still commonly used