Many living organisms rely on the amount of sunlight or darkness they receive to detect seasonal changes. Photoperiodism is how an organism responds to the photoperiod, or length of the day. Many plants do not detect the photoperiod and are thus non-photoperiod responsive. The most common type of photoperiodism is in plants that flower according to the duration of the day.
Critical day length is the amount of either sunlight or darkness required for a plant to change its physiology. For some plants, it is the amount of darkness being measured, not the amount of light. Thus, long nights and short days cause a reaction in the plant. These are called short day plants. Long day plants react when the amount of sunlight increases to the critical amount required to change.
The photoperiod at the equator changes very little with the seasons. Plants that evolved close to the equator are day neutral. They have little need to detect the change in sunlight or darkness, so they don't. Farther from the equator, the amount of daylight and darkness varies considerably; it may be as long as 24 hours above the Arctic Circle. Plants that have adapted to that climate respond quickly to changes in day length since the short summer provides little time for plants to grow, flower and produce seed for the next generation.
Short Day Plants
Short day plants are those that flower only when the day length is shorter than a critical amount. This may occur when the amount of sunlight decreases during the summer and into the fall, continuing until winter when the length of day increases to the critical photoperiod once again. Plants that flower in response to a short daily photoperiod are actually responding to the length of the night. When the darkness is artificially interrupted but the shortened daylight hours remain the same, these plants stop flowering.
Long Day Plants
Long day plants respond to more daylight hours by flowering. Generally, the critical threshold is crossed in the spring as the days lengthen, causing the plants to begin flowering. In the late summer or fall, as the days shorten again, the plants stop flowering. Unlike short day plants that actually measure the length of darkness, it is unknown whether long day plants respond to periods of darkness or sunlight.
Day Neutral Plants
Some day neutral plants will flower sooner, and others later, if the photoperiod is changed artificially. Most day neutral plants have developed means other than photoperiodism for making physiological changes in response to the environment, such as quickly flowering after a significant rainfall like the desert hyacinth.