It can be discouraging when your lettuce bolts or you can’t get your mum to bloom. There’s a reason for both, and it’s all about day length.
To understand plant flowering, you need to understand photoperiodism, which is the amount of light and darkness a plant is exposed to. The amount of uninterrupted darkness determines the formation of flowers on most types of plants, according to Oregon State University Extension Service horticulture experts.
Botanists used to think the length of daylight a plant is exposed to determines whether it would form flowers. But experiments proved otherwise. The length of darkness a plant experiences plays the most crucial role.
A plant that requires a long period of darkness is termed a short-day (long-night) plant. Short-day plants form flowers only when daylight length is less than about 12 hours. Many spring- and fall-flowering plants are short-day plants, including chrysanthemums, poinsettias and Christmas cactus. If these are exposed to more than 12 hours of light daily, bloom formation does not occur.
Other plants require only a short night to flower. These are termed long-day plants and bloom only when they receive more than 12 hours of light. Many summer-blooming flowers and garden vegetables are long-day plants, such as asters, coneflowers, California poppies, lettuce, spinach and potatoes. These all bloom during the summer when the days are longer.
Some plants form flowers regardless of day length. Botanists call these day-neutral plants. Tomatoes, corn, cucumbers and some strawberries are day-neutral. Some plants, such as petunias, defy categorization. They flower regardless of day length but flower earlier and more profusely with long days.
Horticulturists and home gardeners manipulate the day and night lengths—indoors with lights—to get plants to bloom at times other than they would naturally.
For example, chrysanthemums, which are short-day plants, naturally set flowers and bloom with the long nights of spring or fall. But by covering the chrysanthemums for at least 12 hours a day for several weeks over the late spring and early summer, you can simulate the light and darkness pattern of spring or fall, thereby stimulating summer blooming.
You can also bring a long-day plant into bud formation and eventual bloom early before our day lengths surpass 12 hours. To do so, put the plant under grow lights for a few hours a day beyond the natural day length for a few weeks. Adding supplemental day length to stimulate early blooming is a common practice in the nursery and fresh flower industry, especially this time of year, for Valentine’s Day and Easter flowers.