How Day Length Affects Cut-Flower Blooming & Growth

Light is a huge factor influencing the growth and blooming of flowering plants, so it's essential for flower and bedding plant growers to understand its effects.

Regarding the relationship of light to blooming and growth of flowering plants, the first thing to recognize is that the emphasis on day length is somewhat misguided. Scientific research has confirmed that it is the length of darkness rather than daylight that acts as the primary driver of plant growth and blossoming. But because this fact was discovered long after day length became widely used in horticulture, the misperception has stuck around. Botanists and many horticulturists now prefer to use the term photoperiodism instead, to describe the response of plants to the relative lengths of light and dark periods to which they're exposed.

Terminology aside, simply recognizing the importance of dark periods can come in handy for the grower, because it can be used to induce plants to bloom outside their normal season. (More on that below.)

Long-Day Plants, Short-Day Plants, Day-Neutral Plants

Many plant species have day-length triggers that determine when they grow vegetatively and when they initiate 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 long 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.

Night Interruption (NI) and Day Extension (DE) Lighting

Greenhouse and other protected-culture growers use supplemental lighting to get around the day-length requirements of flowering crops. To create a long-day (LD) photoperiod, you can use one of two methods:

1. You may turn on lights shortly before sunset to extend the length of the light period. This is called day-extension (DE) photoperiod lighting.
2. Or, you may turn on the lights during the middle of the night for a short period of time, a procedure called night interruption (NI) lighting.

Both bring 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 shortens the night by turning lights on toward dusk (DE), or breaks up those long nights by turning on lights in the middle of the night (NI), 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 NI or DE lighting to force flowers to bloom in the off seasons.

Light-Dark Rhythms at Your Latitude

Naturally, there are many other variables that play a role in the transition between vegetative growth and reproduction cycles in flowering plants, but light is predominant for many if not most crops.

With day length playing such a big role in flower production, it's smart to get acquainted with the day–night cycles at your location during each month. Get a sunrise/sunset calculator or add one to your calendar with ticklers to remind you on 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, which will enable you to find ways to enhance the health and productivity of your flowering crops and plants.