How to Manage Photoperiod for Better Crop Production

Photoperiodism is the way a plant reads the length of the day and night and uses it to time flowering. For photoperiod-sensitive crops it is one of the cheapest levers a grower has, because a few hours of low-intensity light, or a few hours of blackout, can advance or delay flowering. This article explains what the plant is actually measuring, how the response groups differ, and how to light for it without wasting energy.

What Is Photoperiod?

Photoperiod is the number of hours of light in a 24-hour cycle. Many plants have a flowering response that tracks it, and that response is called photoperiodism. It is really the uninterrupted period of darkness that controls the flowering response, not the hours of light, which is why a short burst of light in the middle of the night can change what a plant does even though the day itself is no longer. Day length is the cue that times flowering, while the spectrum a crop grows under shapes how it grows, which we cover in plant morphology and spectrum. Spectrum matters for photoperiod too, but in a different way, covered in the luminaire section below.

Plants read the night using light-sensitive pigments, phytochrome chief among them, which is why a source rich in red and far-red light is the one that works for photoperiod control. The mechanism is covered in the red and far-red section of the morphology article.

The natural photoperiod changes through the year, and the swing widens with latitude. Near the equator the difference is small. In Toronto, sunrise to sunset runs from just over 15 hours in June to just under 9 hours in December, and the plant perceives a little more than that at each end because civil twilight is bright enough to count. Purdue Extension puts the range for New Orleans at about 11 to 14.5 hours and for Minneapolis at about 9.5 to 16, which is why the same crop needs a different lighting calendar in different parts of North America.

Short Day, Long Day and Day Neutral Plants

First, recognize the flowering response of your crop. Plants fall into three photoperiodic response groups:

• Short day plants flower when the night is long. For most floriculture crops a short day means a night of at least 12 hours, so a day of 12 hours or less.
• Long day plants flower when the night is short. For many long day crops that means a night of less than 10 hours, so a day of about 14 hours or more.
• Day neutral plants flower on age or size regardless of day length. Tomatoes, cucumbers and many modern bedding plant cultivars are day neutral, so photoperiodic lighting does not change when they flower.

There is no single dividing line at 12 hours. The day length that triggers a response is called the critical day length and it varies by species. Potted chrysanthemums, for example, have a critical day length of about 14 hours and flower when the day is 14 hours or shorter, according to Purdue Extension’s flower induction guide. Those two figures above are general production guidance, not a boundary line, and the critical day length of the crop in front of you is the number that matters.

Within the short day and long day groups the response is either obligate or facultative. An obligate plant will not flower at all outside its inductive photoperiod. A facultative plant flowers faster under its inductive photoperiod but will flower eventually regardless. The examples below are taken from Purdue’s response group table:

• Obligate short day: hyacinth bean (Dolichos lablab) and fairy primrose (Primula malacoides).
• Facultative short day: chrysanthemum (Dendranthema × grandiflorum), cosmos (Cosmos bipinnatus), most zinnia cultivars (Zinnia elegans), morning glory (Pharbitis nil), celosia and dahlia.
• Obligate long day: fuchsia (Fuchsia × hybrida), strawflower (Acroclinium roseum, though the strawflower sold as Bracteantha bracteata is day neutral), rudbeckia and dill.
• Facultative long day: snapdragon (Antirrhinum majus), mealy sage (Salvia farinacea), China aster (Callistephus chinensis) and pansy. Petunia is split, with some cultivars facultative and others obligate long day.

Two cautions on those lists. Cultivars within a species can differ, so one zinnia is facultative long day while its neighbours are facultative short day, and a plant sold under a common name can be a different species with a different response. Cannabis is a short day crop, and the cultivars that have been measured respond quantitatively rather than absolutely. Hemp cultivars measured by Zhang and colleagues had critical day lengths from 13 hours 45 minutes to more than 15 hours 30 minutes depending on cultivar, four fibre cultivars flowered under 18 and 24 hour days and are likely day neutral, and the one drug-type cultivar they cite from earlier work had a critical photoperiod of 15 to 16 hours, as reported in Frontiers in Plant Science in 2021. Confirm the response of any new cultivar with the breeder or young-plant supplier rather than assuming it, and validate the schedule on a small block before running it across production.

Why Manage Photoperiod in Your Crops?

Managing photoperiod lets you control the timing of your crop rather than waiting on the season. It allows you to:

• promote flowering in long day plants when the natural day is short
• hold short day plants vegetative to bulk up roots and shoots before flowering
• schedule flowering to hit a market date

The main application is when the natural day is short. For spring-finishing crops such as bedding plants in northern North America that usually means lights running from January to early April. Once the days are sufficiently long, typically beginning in April, there is no value in photoperiodic lighting, as Erik Runkle at Michigan State University puts it.

How to Use Photoperiodic Lighting in Your Crops

To create a long day you need to deliver at least 2 micromoles per square metre per second, measured at plant level, which Purdue gives as 10 foot-candles. That is enough for the plant to perceive but far below what photosynthesis needs. Because foot-candles are weighted to human vision, the relationship between the two units shifts with the spectrum of the luminaire, so measure in micromoles where you can, and check the darkest part of the growing area rather than one spot under a fixture. There are two standard regimes, and Purdue reports them as generally similar in effect:

Day extension lighting turns the lights on before sunset and keeps them on until the desired day length is met. If the natural day is 9 hours and the crop needs 14, Purdue’s worked example runs the lights for about 5.5 hours from sunset, the extra half hour being insurance against dusk.

Night interruption lighting uses light to break the dark period in the middle of the night, commonly from 10 pm to 2 am, which is the window Purdue gives because plants are most sensitive to light then. Four hours is typical, and some crops respond to two or three. By interrupting the dark period the plant perceives a short night, which it reads as a long day.

Getting the most from photoperiodic lighting means knowing how each crop responds and adjusting the photoperiod through the year to control growth. The details on both regimes above come from Purdue Extension’s guide to managing photoperiod in the greenhouse.

One practical consequence follows from the dark period being what the plant measures. If the goal is a short day, stray light during the dark hours can undo it, so check curtain closure, aisle and corridor lighting, exit signage and any work light that runs at night. If the goal is a long day, that same sensitivity is what lets a few hours of very low-intensity light do the job.

Cyclic Lighting to Cut the Energy Cost

The plant does not need continuous light across the whole night interruption. Cyclic, or intermittent, lighting runs the luminaires for a few minutes at a time through the four-hour window, for example five minutes on in every twenty. Purdue’s guidance is that plants need at least 10 foot-candles for a minimum of five minutes every half hour, and that this can reduce the energy used to create a long day by up to 83 percent. The trade-off is in the same sentence of the source: in some crops, flowering is delayed compared with continuous four-hour lighting. Confirm the response on your crop before you build a schedule around it. Purdue also describes boom-mounted high-pressure sodium lamps as a way to deliver intermittent light, programmed so plants receive light at least once every 20 minutes, at a higher suggested intensity of at least 50 foot-candles.

Which Luminaires Work for Night Interruption

Because the intensity needed is so low, the spectrum of the luminaire matters more than its output. Incandescent bulbs were the traditional choice because they emit an effective spectrum for the job, but they are very energy inefficient and are being phased out of production. Compact fluorescent lamps are more efficient and work on many crops, but they are low in far-red, which is required for rapid flowering of some long day crops such as pansy and petunia.

Early studies show LEDs that emit both red and far-red light are similarly effective to incandescent lamps for day extension and night interruption, which makes a red and far-red source the sensible replacement as incandescents disappear. In a Michigan State University trial, petunia and snapdragon flowered earliest under a night interruption with an intermediate red to far-red ratio, while rudbeckia and fuchsia flowered under every treatment except the one establishing the highest phytochrome photoequilibrium, which those plants read as a short day, as Craig and Runkle reported in Environmental and Experimental Botany in 2016. Blue light on its own was not perceived as a long day by any of the seven crops tested at photoperiodic intensity, and white sources emitting little far-red created long days for the short day crops but only some of the long day crops, as Meng and Runkle reported in Scientia Horticulturae in 2015. Specify the spectrum for the response you want rather than assuming any low-intensity source will do.

If a photosynthetic fixture such as high-pressure sodium is used to extend the day at full output, it also raises the daily light integral. That is not a fault, but it means the lighting is doing two jobs, and the energy cost should be judged against both.

Controlling Growth in Short Day Plants

Short day plants such as chrysanthemums and cannabis initiate flowering as nights get longer, which in natural conditions is late summer and fall. Growers use blackout curtains to create short days at other times of year, so short day crops can be grown year round indoors if photoperiod is controlled. Photoperiodic lighting does the opposite job: long days hold short day crops back from flowering, which growers use to build root and shoot development before flowering is allowed to start. An obligate short day crop will not flower at all under long days, while a facultative one is delayed rather than held indefinitely.

The timing matters because a seedling that receives short days too soon after germination can be induced to flower early and finish as a small, poor-quality plant. Facultative short day bedding plants such as celosia are prone to this in early spring, when young seedlings receive natural short days before 21 March. Where the aim is to prevent that premature flowering, the remedy is long days rather than blackout. Placing the seedlings under long days for two to three weeks after germination, which in practice means night interruption lighting, holds them vegetative until they are large enough to carry a crop, as Erwin, Warner and Mattson at the University of Minnesota described in 2004. Blackout remains the right tool when the aim is to induce flowering on schedule.

Blackout brings its own risk. Purdue reports that plants repeatedly exposed to night temperatures above 75 degrees Fahrenheit, about 24 degrees Celsius, under black cloth can develop heat delay, a physiological disorder that delays flowering and is most often reported in poinsettia and chrysanthemum. Pulling the cloth near or after sunset, once the greenhouse has cooled, and using a cloth with a reflective outer layer both help.

The Difference Between Photoperiodic and Supplemental Light

Photoperiodic lighting promotes flowering in long day crops or holds short day crops vegetative. At about 2 micromoles per square metre per second it is bright enough for the plant to perceive but contributes almost nothing to photosynthesis, so it is not a way to grow a heavier crop. Changing when a crop flowers does change how long it grows for, so it can still affect the size and timing of what you harvest. It can also change plant form, since a low red to far-red ratio promotes stem elongation in many species, and Runkle notes that photoperiodic lighting can increase plant height even though it does not increase growth.

Supplemental lighting contributes to the amount of photosynthetically active radiation received during the day, measured over the day as daily light integral. Growers use it to raise the daily light integral in winter at northern latitudes, when natural light levels are low enough to hold back growth and quality. The rate of photosynthesis rises with light intensity up to the light saturation point, so plant growth usually rises with it. Supplemental lighting run for longer than the natural day also has a photoperiodic effect, which is why a high-pressure sodium day extension does both jobs at once. For supplemental lighting, the right fixture depends on the crop and the facility, and that is what a light plan works out.

Frequently Asked Questions

What is Photoperiodism in Plants?

Photoperiodism is a plant’s ability to measure the length of the day and night and use it to time developmental changes, most importantly flowering. The plant is actually measuring the uninterrupted dark period, which is why lighting for a few hours in the middle of the night can change the response even though the total day length barely moves.

What is the Difference Between Short Day and Long Day Plants?

Short day plants flower when nights are long, generally a night of 12 hours or more, and include chrysanthemum, poinsettia and cannabis. Long day plants flower when nights are short, generally a night under 10 hours, and include snapdragon, rudbeckia and fuchsia. Day neutral plants such as tomato and cucumber flower regardless of day length. Those are general guidelines, and the exact threshold, the critical day length, varies by species and cultivar.

How Much Light is Needed for Photoperiodic Lighting?

At least 2 micromoles per square metre per second, measured at plant level, using a source that emits red and far-red light. That is a tiny fraction of the intensity used for photosynthetic supplemental lighting, so a photoperiodic system uses far less energy than supplemental lighting over the same area.

What is Night Interruption Lighting?

Night interruption lighting breaks the dark period with low-intensity light, commonly for four hours from 10 pm to 2 am, so the plant perceives a short night and responds as it would to a long day. It is the standard way to promote flowering in long day crops or hold short day crops back during winter and early spring.

Does Photoperiodic Lighting Increase Yield?

Not directly. At about 2 micromoles per square metre per second it is far below the intensity that drives photosynthesis, so it adds almost no photosynthetic light. Its job is controlling when the crop flowers, which affects scheduling and how long the crop grows for. To grow a heavier crop you need supplemental lighting that raises the daily light integral, which is a different fixture doing a different job.

Sources

• Currey, C. J., Lopez, R. G. and Runkle, E. S. Managing Photoperiod in the Greenhouse. Purdue Extension HO-253-W, 2013. Source for the latitude ranges, the intensity and timing of day extension and night interruption, cyclic and boom lighting, lamp types, and heat delay under blackout.
• Currey, C. J., Lopez, R. G. and Mattson, N. S. Flower Induction of Annuals. Purdue Extension HO-249-W. Source for the response group table, the species examples and critical day length.
• Erwin, J., Warner, R. and Mattson, N. Day Length Effects on Bedding Plant Flowering. Minnesota Commercial Flower Growers Bulletin 53(1), April 2004. Source for the celosia seedling guidance.
• Runkle, E. S. Light and Flowering of Bedding Plants. Michigan State University Extension, 2006.
• Runkle, E. S. Technically Speaking: Horticultural Lighting Applications. Greenhouse Product News, Michigan State University Floriculture. Source for the April cut-off and the height effect.
• Craig, D. S. and Runkle, E. S. An intermediate phytochrome photoequilibria from night-interruption lighting optimally promotes flowering of several long-day plants. Environmental and Experimental Botany 121, 132 to 138, 2016.
• Meng, Q. and Runkle, E. S. Low-intensity blue light in night-interruption lighting does not influence flowering of herbaceous ornamentals. Scientia Horticulturae 186, 230 to 238, 2015.
• Zhang, M., Anderson, S. L., Brym, Z. T. and Pearson, B. J. Photoperiodic Flowering Response of Essential Oil, Grain, and Fiber Hemp (Cannabis sativa L.) Cultivars. Frontiers in Plant Science 12, 2021.