Photoperiod is the duration of daylight during a given day. Photoperiodism is the ability of any organism to detect and respond to changes in daylength. In plants this phenomenon is expressed mostly in flowering. Plants are classified into three categories depending on the number of hours of light they require before flower initiation.
Short-Day Plants - plants that flower only when the daylengths are less than 12 hours.
Common examples of short day plants are poinsettias, chrysanthemum, Christmas cactus, some varieties of strawberries.
Long-Day Plants - plants that flower only when the daylengths are longer than 12 hours. Among the plants that fall under this classification are radishes, lettuce, spinach, potatoes, sedum, and beets.
Day-Neutral Plants - plants that do not have a particular response to varying daylength. They flower regardless a long as the rest of the growing conditions are favorable. Examples of day-neutral plants are tomato, corn, cucumber, and grapes.
To know the tendencies of the different horticultural crops is important in achieving desired results for the home gardener as well as the commercial grower.
My backyard observation of photoperiodism.
Last spring I planted cilantro and parsley. During this period the daylengths were on the rise. I observed that bolting occurred early on during the growth the plants. For these plants which are grown particularly for their leaves, bolting or flowering is anti-productive. Once flowering begins, growth is diverted to the reproductive parts of the plants and the vegetative growth ceases. I tried to deter flowering by adding more nitrogen fertilizer but that did not work.
Early this fall, I planted another batch of cilantro and parsley, again. The same cultural management was applied. To my surprise, the plants continue to grow lots of leaves, and there is no sign of flowering at all.
With these observations, I can say that both cilantro and Italian parsley are long-day plants. They will not flower until the days get longer than 12 hours.
Commercial Application of Photoperiodism
Photoperiod or daylength can be artificially altered in order to get a certain response from photoperiodic plants. Poinsettias for example are short-day plants which require 14 hours of continuous darkness for at least one week before they begin to flower in which the leaves change colors. Under natural conditions, this would happen only in late December. Why then do we see bright colored poinsettias for sale even before Thanksgiving? It is because all the poinsettias you see were grown in greenhouses where the lengths of days can be shortened by merely pushing a button. For the plants in an artificially-lit greenhouse, turning off the lights means that the night has begun.
The cilantro and the parsley that we buy from the grocery store in the summer, on the other hand, are most likely grown in greenhouses where the dark period is long. Remember, long-day plants flower when the days are long. Herbs like cilantro and parsley are grown for their leaves. One can trick these plants to keep producing, by artificially shortening the daylength.