When Flowers Bloom
In nature, plants are triggered to flower by any number of factors. It might be warm weather. It might be the beginning of the rainy season. Or it might be the amount of available light.
Some plants, such as the kalanchoe plant or poinsettia plant, time their blooms to the amount of sunlight they receive. When the hours of sunlight declines, they are triggered to bloom. These are called "short day" plants. Others, such as spinach, are triggered to bloom only after the days stretch out to a certain length. These are called "long-day plants."
This trait in plants is called photoperiodism. It simply means the plant's reproductive cycle is timed to the amount of light available.
The term "photoperiodism" was coined to describe a plant's ability to flower in response to changes in the photoperiod: the relative lengths of day and night. Because flowers produce seeds, flowering is crucially important for the plant to complete its life cycle. Although people had long known that plants such as tulips flower in the spring and chrysanthemums flower in the fall, until the early 1900s little was known about what actually caused flowering.
Beginning in 1910, Wightman Garner and Henry Allard conducted experiments to test the effect of day length on flowering. They discovered that plants such as barley flowered when the day length was longer. These plants, which they named long-day plants (LDPs), flower mainly in the summer as the days are getting longer. Others, such as soybeans, flower when the day length is shorter than a certain critical length. These short-day plants (SDPs) flower in the fall as the days are getting shorter. Still others are not sensitive to the photoperiod and are called day-neutral plants.
Most home growers will never have to worry about photoperiods, but there are some cases where it matters. For example, poinsettias don't naturally bloom around Christmas. Instead, they are forced to bloom by keeping them in the dark for a certain number of hours each day in the winter to trick them into blooming for the holiday. The same is true for kalanchoes: they can be forced into bloom any time of the year simply by keeping them in darkness for 14 hours a day. That's why you can find flowering kalanchoe any time of the year, but they'll only bloom in the fall or early spring on their own.
Photoperiodism and the Distribution of Plants
Photoperiodism is responsible for the distribution of many plants worldwide. For example, ragweed (a SDP) is not found in northern Maine because the plant flowers only when the day length is shorter than 14.5 hours. In northern Maine, days do not shorten to this length until August. This is so late in the growing season that the first frost arrives before the resulting seeds are mature enough to resist the low temperatures, and so the species cannot survive there. By contrast, spinach (a LDP) is not found in the tropics because there the days are never long enough to stimulate the flowering process.