Pollinators such as birds and bees are more active in summer than winter, so plants that rely on certain birds and bees are better off if they wait until the long days of summer to open up their flowers. The photoperiod is the number of hours of daylight in a 24-hour period. In the northern hemisphere, winter days have less sunlight than summer days, so they have a shorter photoperiod. The leaf is the site of light perception, and only a small amount of leaf surface needs to receive light to maintain the plant’s internal clock.
The idea that day length could influence flowering arose in the 1920s, when researchers in Maryland were trying to breed large-leaved tobacco plants of the variety Maryland Mammoth. Though the researchers manipulated temperature, moisture, and soil nutrients, they couldn’t get the plants to flower. In the winter the researchers moved the plants into a greenhouse where they wouldn’t freeze and at last, they flowered.
The researchers learned by manipulating light that the tobacco plants would flower only when there were less than 14 hours of daylight. This earned Maryland Mammoth the label, "short-day plant," because it needed a day shorter than some critical length to flower. Other short-day plants include chrysanthemums, poinsettias, Easter lilies, and some soybeans.
The opposite of a short-day plant is a long-day plant. In order to flower, these plants need light periods that are longer than a certain time. Examples of long-day plants include spinach, radishes, lettuce, and irises.
Some plants aren’t picky about their photoperiod and will flower no matter how long they get sunlight—these are called day-neutral plants, and include tomatoes, rice and dandelions. Other plants are extremely picky and require certain conditions in addition to photoperiod. Some plants that grow in northern climates need a specific photoperiod and prolonged cold temperatures to stimulate flowering. This process, called vernalization, is required of cabbage, celery, magnolias and beets.
After a few more decades of manipulating plant flowering periods, researchers realized that flowering responds to the length of the dark period (night), not the length of light. For example, one short-day plant they used was cocklebur (Xanthium strumarium), which required days that were shorter than 16 hours to flower. If researchers interrupted the light period by putting the plant in total darkness for a brief time, the plant still flowered. However, if they interrupted the dark period by briefly turning on the lights, the plant didn’t flower. The researchers concluded that the short-day plants are actually long-night plants because they need a period of uninterrupted darkness that is longer than a certain length.
The length of the night necessary for the plant to flower is called the critical period. For the Maryland Mammoth, the critical night period is ten hours because it needed a daylength shorter than 14 hours and there are 24 hours in a day.