Photoperiod or day length is the duration of light in a given 24-hour period. Photoperiod naturally varies with latitude and time of year, and many plants, including annuals and perennials, have fine-tuned their flowering to coincide with these seasonal changes in day length. As a result, we can classify plants as being short day, long day or day neutral species.
Short day plants flower when the day length is less than a critical length (often considered to be 12 hours of light or less). Long day plants flower when the day length is greater than a critical length (often considered to be 14 hours or greater). Day neutral plants flower regardless of the day length.
Plants can further be described as having a facultative or obligate photoperiod response. Plants with a facultative response flower faster under a particular photoperiod but will eventually flower under all photoperiods. For example, a facultative short day plant will flower faster under short days but will eventually flower under long days as well (Figure 1). Plants with an obligate response absolutely require a particular photoperiod for flowering. For example, an obligate short day plant will only flower under short days – and never under long days.
Photoperiod manipulation can be achieved in the greenhouse with relative ease. When the day length is long but a short day photoperiod is desired, blackcloth fabric may be pulled over plants in the evening and opened in the morning to provide plants with a short day. This is the same technique used in fall mum production to promote earlier flowering of chrysanthemum when days are naturally long. Growers may utilize existing blackout systems used in fall mum production to provide short days for other crops.
Conversely, long days can be provided when the day length is naturally short by using night break lighting with incandescent or high pressure sodium lamps. This is the same technique used in lighting chrysanthemum crops to keep them vegetative until flowering is desired. Night break lighting generally occurs from 10:00 PM to 2:00 AM and is the most efficient method of providing long days. Growers may also choose to light crops for several hours at the end of each day from 5:00 to 10:00 PM.
Reprinted from "Light and flowering of bedding plants" by Beth Fausey, OSU Extension ABE Center in the February 17, 2006 edition of the Michigan State University Greenhouse Alert newsletter.
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