by Lynn Byczinski
Day length is a huge factor influencing the growth and blooming of flowering plants, so it's important for flower and bedding plant growers to understand its effects.
The first thing to know is that the term day length is a misnomer. Scientific research has confirmed that it's actually the length of the dark periods, not the length of daylight periods, that controls plant growth. But this fact was discovered long after day length became a widely used term in horticulture, and the name has stuck. Understanding the importance of dark periods can come in handy for the grower, though, because it can be used to induce plants to bloom outside their normal season. More on that later.
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Many plant species have day-length triggers that determine when they grow vegetatively and when they bloom. Plants that flower when the days are longer than 12 hours are referred to as long-day plants, and those that flower when days are less than 12 hours are called short-day plants. Those that don't respond to day length are called day-neutral.
Summer flowers are often long-day species, so they won't bloom in winter even if you were to plant them in a tropical greenhouse. Rudbeckia Indian Summer is a good example. It grows vegetatively when days are short and then when days reach a certain length, it sends up tall flower stems. Plant it in spring, and you'll get a great crop in summer. But if you plant it in summer, hoping for a fall crop, you will be disappointed. The plants will grow nicely, as the days get shorter, but they won't flower — or if they do, it will be on short stems. They simply need longer days to produce long-stemmed flowers.
Many other cut-flower varieties are either long-day or short-day plants. Most sunflower varieties, for example, are long-day species that will grow vegetatively during the short days of spring, then bloom when the days reach a specific length. If you want to grow sunflowers in early spring or late fall, when days are short, you need to plant day-length neutral varieties.
Greenhouse growers use supplemental lighting to get around the day-length requirements of flowers. They may turn on lights shortly before sunset to extend the length of the light period. Or, they may turn on the lights during the middle of the night for a short period of time, a procedure called night interruption lighting. This brings us back to the fact that it's the dark period that's important to plants: The short days of winter have long nights. If a grower breaks up those long nights by turning on lights in the middle of the night, some plants will act as though the night is short (and therefore the day is long) and behave just as they would in the middle of summer. Many bedding plant and cut-flower greenhouses use night interruption lighting to force flowers to bloom in winter.
With day length playing such a big role in flower production, it's smart to get acquainted with the day length at your location during each month. Get a sunrise/sunset calculator and mark your calendar with the dates when you have 10 hours of daylight, 11 hours, 13 hours, and so on. Over time, you will begin to note correlations between day length and flowering plant lifecycles.