How to manage photoperiod to increase crop yields

Photoperiod control is a basic prerequisite for high yields when cultivating indoors and in greenhouses. We'll teach you how changes in day and night length affect plants and how to use that when growing indoors, outdoors, or in greenhouses. We'll also reveal several advanced techniques used by professional and commercial growers.

The alternation of day and night is an important factor for life on Earth. Both plants and animals respond to changes in the length of daylight and darkness (photoperiod) with physiological or behavioral reactions. In animals, day length affects, for example, fur and feather coloration, migration, or the start of mating or hibernation periods. For many plant species, changes in photoperiod are a controlling factor that governs their flowering.

Photoperiod

We call the photoperiod the number of hours of daylight and darkness during a single day. Natural photoperiod changes throughout the year depending on geographic location. At the equator, photoperiod changes during the year are minimal and the day is split roughly into 12 hours of light and 12 hours of darkness year-round. In the Northern Hemisphere, the longest day of the year (the summer solstice) occurs around June 21. After that date days shorten (i.e., nights lengthen) until December 21 (the winter solstice). In the Southern Hemisphere, the situation is reversed (i.e., the longest day is December 21 and the shortest is June 21).

How do plants know when the sun sets?

Many flowering species (angiosperms) are equipped with photoreceptor proteins such as phytochrome, which allow plants to perceive seasonal changes in photoperiod length. Phytochrome exists in plants in two forms: the active form Pfr and the inactive form Pr. Red light emitted by the sun during the day converts inactive Pr into active Pfr. As sunset approaches, far-red light increases, which converts Pfr back into Pr. At the same time, Pfr can be converted to Pr by a process known as "dark reversion," which occurs during extended periods of darkness. The phytochrome conversion system allows plants to recognize when it is night.

By manipulating the ratio of red to far-red light, flowering can be influenced. It has been found that plants flower later if exposed to larger amounts of red light. Experiments have also shown that flowering can be inhibited by illuminating plants with red light in the middle of the night. For short-day plants, this means they will not flower if exposed to light for a few minutes during the night. Long-day plants, on the other hand, may flower even if illuminated with red light during the night.

Short-day and long-day plants

Manipulating photoperiod is one of the fundamental tools growers use to control plant growth and maximize crop yield, especially in indoor cultivation (in tents or enclosed rooms), but also in greenhouses and outdoors. By darkening or supplementing with artificial lighting, growers can accelerate or delay flowering, extend the vegetative growth period, or keep plants in a vegetative state long-term for cloning purposes.

We distinguish short-day plants, which flower when days shorten and nights lengthen, and long-day plants, whose flowering is accelerated by lengthening days and shortening nights. A third category is day-neutral plants, whose flowering is not influenced by photoperiod. Short-day and long-day plants are further classified as facultative or obligate depending on their response to photoperiod.

• Obligate short-day plants flower only when nights are long (more than 12 hours) and days are short. Examples include hyacinth bean, plants grown in controlled environments, poinsettia, and chrysanthemum.
• Facultative short-day plants flower earlier when the photoperiod is short but will flower regardless of day length. Examples include Erigeron (fleabane) or plantain.
• Obligate long-day plants flower only when days are long and nights are short (less than 12 hours). Examples include fuchsia, China aster, and strawflower.
• Facultative long-day plants flower earlier under long days but will flower regardless of day length. Examples include sage or calendula.

Methods for controlling photoperiod

Darkening (blackout)

The goal of darkening is to shorten the photoperiod to the desired length, which is particularly useful for short-day species grown outdoors or in greenhouses. By darkening, short-day plants can be induced to flower earlier than natural light conditions would allow. Whole plants can be covered (with some difficulty), but it is most practical to darken plants in greenhouses adapted for that purpose.

Supplemental lighting (day extension)

The use of grow lights to supplement natural light is most commonly employed by greenhouse or outdoor growers. Artificial lighting can be used to extend day length (photoperiod), which is useful both for short-day plants (delaying flowering) and for long-day plants (accelerating flowering). Supplemental lighting can also be used to interrupt the night, preventing plants from flowering.

Artificial lighting

When plants are grown entirely under artificial lighting in a grow box or grow room, the photoperiod is completely under the grower's control. Most growers keep lights on 16–18 hours during the vegetative phase and 12 hours during flowering. This assumes they are cultivating short-day or photoperiod-neutral plants. It used to be necessary, when switching to flowering, to replace a blue-growth lamp with a bloom lamp of a different wavelength; however, modern LED fixtures simulate daylight much better than outdated sodium technology.

Want to learn more about lighting for plant cultivation and other tips for indoor growing? Don't hesitate to visit our blog.