How Day Length Affects Cut-Flower Blooming & Growth
A Hensley's Seed Co. guide, drawing on advice from Lynn Byczynski, founder of Growing for Market
Lynn Byczynski grew organic vegetables and cut flowers for market before starting a magazine to help market gardeners across the country swap experience and know-how. The first issue of Growing for Market came out in January 1992, and GFM went on to become a respected source of practical, realistic advice on growing and selling produce and flowers.
Darkness & Light
Light has an enormous effect on how flowering plants grow and bloom, so flower and bedding plant growers need to understand it.
The first thing to know about light and flowering is that the focus on day length is a bit off. Research has shown that the length of darkness rather than daylight is what mainly drives plant growth and flowering. Since that was discovered long after "day length" became standard horticultural language, the old idea has lingered. Botanists and many horticulturists now use the term photoperiodism for how plants respond to the relative lengths of light and dark.
Whatever you call it, knowing that the dark period matters is useful, because you can use it to make plants bloom out of season. (More below.)
Long-Day Plants, Short-Day Plants, Day-Neutral Plants
Many plants use day length to decide when to grow leaves and when to start flowering. Plants that bloom once days grow longer than a certain critical length are called long-day plants, and those that bloom once days get shorter than their critical length are called short-day plants. Plants that don't respond to day length are called day-neutral.
Many summer flowers are long-day plants, so they won't bloom in winter even in a warm greenhouse. Rudbeckia 'Indian Summer' is a classic case. It makes leafy growth while days are short, then sends up tall flower stems once days get long enough. Sow it in spring and you'll have a fine summer crop. Sow it in summer hoping for fall flowers and you'll be let down: the plants grow well as days shorten but don't bloom, or bloom only on short stems. They need long days for long-stemmed flowers.
Many other Many respond to either long or short days. Some sunflowers, for instance, react to day length and may not bloom well when days are short. For sunflowers in early spring or late fall, when days are short, choose day-neutral varieties.
Night Interruption (NI) and Day Extension (DE) Lighting
Greenhouse and other protected-culture growers use extra lighting to work around the day-length needs of flowering crops. To give plants a long-day (LD) photoperiod, there are two approaches:
- Switch lights on a little before sunset to stretch out the light period, known as day-extension (DE) photoperiod lighting.
- Or switch lights on briefly in the middle of the night, known as night interruption (NI) lighting.
Both come back to the dark period being what counts. Winter's short days mean long nights. When a grower shortens the night by adding light around dusk (DE) or splits a long night with a burst of light (NI), some plants respond as if nights were short and days long, just as in midsummer. Many bedding plant and cut-flower greenhouses use NI or DE lighting to bring crops into bloom out of season.
Light-Dark Rhythms at Your Latitude
Of course, plenty of other factors affect when flowering plants shift from leafy growth to flowering, but light is the dominant one for many, if not most, crops.
Since day length matters so much for flowers, it pays to learn how day and night lengths change each month where you are. Use a sunrise/sunset calculator or set calendar reminders for the dates when you reach 10, 11, 13 hours of daylight and so on.
Over time you'll start to see links between day length and your plants' growth and bloom, and you can use them to grow healthier, more productive flower crops.
