How plants use day length to decide when to flower (Photoperiodism)

Photoperiodism is the response of an organism to the length of day or night. In many plants it is the mechanism by which plants determine the seasonal time of year and whether they should initiate flowering.

The process is controlled by the protein phytochrome which exists in two forms, phytochrome red and phytochrome far red. In sunlight phytochrome red is converted to its active form phytochrome far red. At night phytochrome far red converts back to phytochrome red. On long summer days more phytochrome red will be converted to phtochrome far red than on shorter days.

Plants use this mechanism to control the initiation of flowering. However different plants respond differently and you may have heard garden presenters talk of long and short day plants.

In long day plants flower initiation will begin only after the plant has experienced a critical number of daylight hours.

In short day plants (or more correctly long night species) flowering is initiated when the uninterrupted hours of darkness reaches a critical length.

There are also day neutral species that do not require a specific day length to initiate flowering and where maturity of the plant is more important.

In nature long day plants will typically flower in the summer months whilst short day plants will flower in the spring or autumn.

Within long day species and short day species there are also two groups. The obligate (qualitative) plants must experience a critical day length (or night length) before they will flower whilst in the facultative (quantitative) group plants will flower under any day length but flower earlier with the appropriate photoperiod.

Just to add another layer of complexity there are also two additional groups; long-short-day plants (LSDP) and short-long-day plants (SLDP). LSDPs will flower after a series of long days followed by short days. SLDPs flower after a series of short days followed by long days. Asters are an example of a long-short day plant. Long days promote shoot elongation and short days initiate flowering.

Critical day length

There is no single threshold for the critical day length that will initiate flowering. Armitage and Lushman provide individual details of these for many common cut flowers and highlight that in some cases the critical day length will vary between varieties of the same species.

Warner (2006) helpfully sets out the following ‘rule of thumb’ that might be useful “The “critical” day lengths vary by crop but, generally, short-day plants will flower when the day is less than 11 hours (night length >13 hours), and long-day plants will flower when the day is longer than 14 to 16 hours (night length < 8 to 10 hours).”

Photosynthetic lighting versus Photoperiodic lighting

If using artificial lights it is important to recognise that there is a distinction between photosynthetic lighting (which increases the intensity of the light available for photosynthesis and growth) and photoperiodic lighting (day length control). It is purely the latter which is the subject of this article. By choosing the correct day length you can boost growth by offering the plant higher intensity lights (photosynthetic lighting) without triggering the flowering.

Why do plants need this mechanism?

The response to day length provides plants with a mechanism by which they can recognise the time of year, not by temperature which can be very variable, but by day light hours.

It allows all the individuals of a particular species to coordinate their flowering and come in to flower at the same time. Cross pollination and therefore fertilisation will be much more successful if all the flowers of a species are out at the same time rather than randomly distributed throughout the year.

The ability to time flowering may also allow the plant to align its flowering with the life cycle of a particular pollinating insect.

What does this mean for the cut flower grower?

Many commercial growers will use this feature of plant growth to force plants to flower at unnatural times of year by artificially adjusting the day length. This is particularly useful if you are growing plants for a particular show or event when you need the flowers to be at their prime during a particular week. Similarly growers of bedding plants will adjust day length so that the plants are in flower early to coincide with the peak spring selling period. Plants in flower walk off the shelves much faster than those that are solely in leaf.

In early spring we often use fluorescent lights to bring on our seedlings indoors before it is warm enough to put them out in the unheated greenhouses and polytunnels. We want to get our plants to a decent size much earlier in the season so that they are large robust plants that will flower in the garden throughout the summer. It is important to understand the critical day length for the species that you are growing so that you don’t leave the lights on too long or too little and inadvertently trigger flowering too early. Some plants are inhibited from flowering by receiving too many daylight hours.

The large commercial growers who grow a huge volume of a single flower type can control their environment and photoperiod very precisely to the needs of that particular crop. For the many small cut flower growers across the UK the challenge of creating the correct day length regime across multiple varieties can be more challenging.

The distinction between photosynthetic lighting and photoperiodic lighting is important here. To enhance growth increase the intensity of your lights not the length of time you leave the lights on.

Some examples

I will refer you to Armitage and Lushman for the detailed requirements for each cut flower. However to illustrate the photoperiodic response I will quote a number of examples from their very comprehensive reference book:

Anemone coronaria – “it appears that short days accelerate flowering and long days result in early termination of flowering and hastening of dormancy”

Antirrhinum – “essentially a quantitative long day plant meaning that it is capable of flowering under short days but flowers much earlier and at a lower leaf number in long days”

Cosmos bipinnatus – “a quantitative short day plant … although plants flower more rapidly under short days than under long days, they eventually flower under all photoperiods … the optimum photoperiod for flowering is less than 14 hours … under long days flower buds appear sporadically”

Ranunculus – “the highest percentage of tubers flower when placed under short day treatments (12 hours or less. Although long day treatments (>12 hours) accelerate flowering and flower quality, yields may not be as high”

In conclusion

The whole area of flower initiation is complex and fascinating and it is not possible to do it justice in such a short article.  Although I have focused on day length here in some other species plant maturity or temperature may be more important.  Botany as a subject has captivated me for years and continues to do so.

Further reading

“Specialty cut flowers” by Armitage and Lushman (ISBN 0-88192-579-9)

Photoperiod and bedding plants, Center for Agriculture, Food and The Environment, UMass Amherst

https://ag.umass.edu/greenhouse-floriculture/fact-sheets/photoperiod-bedding-plants

Supplemental Lighting on Bedding Plants –Making it Work for You by Ryan M.Warner
Michigan State University

http://flor.hrt.msu.edu/assets/PdfAttachments/SupplementalLightingonBeddingPlants.pdf

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