Light and flowering of bedding plants

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.  

Dr. Ryan Warner, assistant professor of horticulture at Michigan State University, has conducted numerous studies investigating the flowering requirements of bedding plants both at MSU and at the University of Minnesota with Dr. John Erwin. Ryan and his colleagues have found that light, specifically photoperiod and irradiance, impact flowering of a number of bedding plant species. He recently shared this research with growers in Northwest Ohio at the Toledo Area Flower and Vegetable Grower’s Winter Conference. Highlights from his work are summarized in this article.


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. A short list of bedding plants exhibiting these responses is provided in Table 1.

Providing the desired photoperiod

By understanding how the photoperiod varies in Northwest Ohio throughout the year, growers in this region can manipulate the greenhouse environment to create a desired photoperiod. In Northwest Ohio, the photoperiod ranges from a minimum of 10 hours in December to a maximum of 16 hours in July. Short day lengths of 12 hours or less naturally occur from mid-October through late February in Northwest Ohio while long day lengths of 14 hours or greater naturally occur from early April through the first of September.

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.

Irradiance responses

In addition to photoperiod, flowering of bedding plants can be influenced by greenhouse light conditions, specifically irradiance. Irradiance is the amount of light reaching a plant at any given moment in time. It is an instantaneous value and is measured in units called micromoles over a square meter of greenhouse space (µmol·m-2·s-1). Growers can use a number of hand-held light meters to measure irradiance in the greenhouse. Depending on the meter used, measurements may be available in footcandles or µmol·m-2·s-1. One µmol·m-2·s-1 of sunlight approximately equals 5 footcandles of light.

Bedding plants may have a facultative irradiance response where high light levels promote faster flowering or an irradiance indifferent response where high light levels do not affect flowering. Species exhibiting a facultative irradiance response flower faster because the juvenile stage of development is shortened under high light levels. As a result, plants form fewer leaves before initiating flowers than those grown under lower light levels. For example, Salvia farinacea has a facultative irradiance response. Plants generally form 24 leaves before flowering under long days yet form only 18 leaves before flowering when grown with an additional 150 µmol·m-2·s-1 (approximately 750 footcandles) of light.

It is important to note that temperature influences the rate of development, and plant temperature of species in both response groups can increase under high light levels. Plants exposed to direct sunlight can be 5 to 7ºF warmer than the surrounding air temperature. Although elevated plant temperature due to high light conditions does contribute to faster flowering, it does not influence the number of leaves formed below the flower.

Useful examples

How can we use information on photoperiod and light responses to improve efficiency of bedding plant production? In addition to classifying light responses of numerous bedding plant species, Ryan wanted to know if growers could significantly reduce production time by placing bedding plant plug trays under a specific photoperiod to induce flowering, then transfer seedlings to the greenhouse to be grown with other crops under natural light conditions. Ryan found that some bedding plants can be induced to flower as young seedlings in the plug tray. Here are a few examples:

Cosmos. Cosmos bipinnatus is classified as a facultative short day plant and flowers more quickly under short days than long days. For example, cosmos grown under continuous long days in a 68°F greenhouse flower in 65 days while those grown under continuous short days flower in 29 days!

Ryan found that he could significantly reduce production time under long days from 65 to 42 days to flower by first exposing cosmos seedlings with one leaf pair to a minimum of 5 short days before transferring them to the greenhouse to be grown under naturally long days. He could continue to reduce production time under long days if seedlings were held under short days for up to 15 days before growing them on under long days. Using this technique, production time was reduced from 65 to 29 days to flower. Note that plants held continuously under short days also flowered in 29 days.

By holding cosmos plugs under short days for two weeks before transplanting, a grower could conceivably flower two crops of cosmos in the time it would take to flower one crop under long days alone! *An interesting note: Ryan also found that growing cosmos plugs for at least 10 days under long days before transfer to short days generally increased branching and flower number.

Rudbeckia. In contrast to Cosmos, Rudbeckia hirta ‘Indian Summer’ is an obligate long day plant and must be exposed to long day lengths to flower. Plants grown in a 68ºF greenhouse under long days generally flower in 12 weeks while those grown under short days do not flower. Unlike Cosmos, Rudbeckia ‘Indian Summer’ has a lengthy juvenile period (about 4 weeks) before plants can be fully induced to flower in the plug tray. Ryan found than ‘Indian Summer’ seedlings with two leaves had to first be grown under long days for at least 25 to 30 days for complete flowering to occur under naturally short day lengths. Increasing the number of long days plants were exposed to also increased flower number. Ryan also noted that inducing Rudbeckia plants under long days and transferring them to short days resulted in extremely compact plants.

Table 1. Photoperiod responses of numerous bedding plant species.

Obligate short day plants
Hyancinth bean
African marigold
Facultative short day plants
Globe Amaranth (Gomphrena)
Moonflower (Ipomea)
Morning Glory
Signet marigold
Obligate long day plants
Bachelor’s buttons (Centaurea)
China aster (Callistephus)
Monkey flower (Mimulus)
Petunia ‘Purple Wave’
Sweet pea (Lathyrus)
Facultative long day plants
Pansy (Viola)
Petunia (Grandiflora types)
Day neutral plants

Table 2. Irradiance responses of numerous bedding plants.

Facultative irradiance plants
Flowering tobacco
Irradiance indifferent plants
Ageratum Stock
Amaranthus Thunbergia
Celosia Verbascum
Dianthus Zinnuia

Want to learn more about lighting greenhouse crops?

Check out each of these references originally published as a series of articles in Greenhouse Grower Magazine:

Lighting Up Profits

Available from Ball Publishing, or 888-888-0013.

Fundamentals of flowering

A series of articles on bedding plant responses to photoperiod and irradiance,

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