How to Control Photoperiod to Increase Crop Yield

Controlling photoperiod is a basic prerequisite for high yields when growing indoors and in greenhouses. We will show you how changes in the length of day and night affect plants and how to use this in indoor, outdoor or greenhouse cultivation. In addition, we will reveal several advanced techniques used by professional and commercial growers.

The alternation of day and night is an important factor for life on Earth. Plants and animals respond to changes in the length of daylight and darkness (photoperiod) with physiological or behavioural reactions. In animals, day length affects, for example, coat and feather colouration, migration, or the onset of mating season or hibernation. For many plant species, changes in photoperiod are a controlling factor that governs flowering.

Photoperiod

We use the term photoperiod to describe the number of hours of daylight and darkness during one day. The natural photoperiod changes throughout the year depending on geographical location. At the equator, photoperiod changes during the year are minimal and the day is divided year-round into approximately 12 hours of light and 12 hours of darkness. In the Northern Hemisphere, the longest day of the year (the summer solstice) occurs approximately on 21 June. After this date, the days become shorter (i.e. the nights longer) until 21 December (the winter solstice). In the Southern Hemisphere, this situation is reversed (i.e. the longest day is 21 December and the shortest day is 21 June).

How do plants know that the sun is setting?

Many flowering species (angiosperms) are equipped with photoreceptor proteins, such as phytochrome, thanks to which plants can perceive seasonal changes in photoperiod length. Phytochrome occurs in plants in two forms, the active Pfr form and the inactive Pr form. Red light emitted by the sun during the day converts inactive Pr into active Pfr. As sunset approaches, far-red light begins to increase, converting Pfr back to Pr. At the same time, Pfr can be converted to Pr in a process known as “dark reversion”, which occurs during a long period of darkness. The phytochrome conversion system allows plants to recognise whether it is night.

By manipulating the ratio of red and far-red light, it is possible to influence plant flowering. It has been found that plants flower later if they are exposed to more red light. Experiments have also shown that flowering can be inhibited by illuminating plants with red light in the middle of the night. In the case of short-day plants, this means that they will not flower if they are exposed to light for a few minutes during the night. Long-day plants, on the other hand, can flower even when illuminated with red light during the night.

Short-day and long-day plant species

Manipulating photoperiod is one of the basic tools growers use to control plant growth and maximise crop yields, especially in indoor growing (in tents or enclosed rooms), but also in greenhouses and outdoors. By using blackout or supplementary lighting, growers can speed up or delay flowering, extend the vegetative growth period, or keep plants in vegetative growth for a long time for cloning purposes.

We distinguish between short-day plants, which flower when days shorten and nights lengthen, and long-day plants, in which the onset of flowering is accelerated by longer days and shorter nights. The third category consists of neutral plants, whose flowering is not affected by photoperiod. Long-day and short-day plants are further divided into facultative and obligate types, according to their response to photoperiod.

• Obligate short-day plants flower only when nights are long (more than 12 hours) and days are short. These include hyacinth bean, hemp, poinsettia and chrysanthemum.
• Facultative short-day plants flower earlier if the photoperiod is short, but they will flower regardless of day length. These include mimulus or plantain.
• Obligate long-day plants flower only when days are long and nights are short (less than 12 hours). These include fuchsia, China aster and strawflower.
• Facultative long-day plants flower earlier during long days, but they will flower regardless of day length. These include sage or marigold.

Methods for controlling photoperiod

Blackout

The aim of blackout is to shorten the photoperiod to the required length, which is especially useful for short-day species grown outdoors or in a greenhouse. Blackout makes it possible for short-day plants to flower earlier than natural light would allow. It is also possible to cover entire plants (with some difficulty), but the most suitable method is to blackout plants in specially adapted greenhouses.

Supplementary lighting

The use of grow lights to supplement natural light is most commonly used by growers in greenhouses or outdoors. Artificial lighting can be used to extend the length of the day (the lighting period), which is useful both for short-day plants (delaying flowering) and for long-day plants (accelerating flowering). Supplementary lighting can also be used to interrupt the night, which causes plants not to flower.

Artificial lighting

When growing plants entirely under artificial lighting in a grow box or grow room, the photoperiod is entirely in the grower's hands. Most growers keep the lights on for 16-18 hours during the vegetative phase and for 12 hours during flowering. This, however, assumes that they are growing short-day or photoperiod-neutral plants. In the past, it was necessary when switching to flowering to replace the discharge lamp with blue light intended for growth with another wavelength for flowering; however, modern LED luminaires simulate daylight much better than outdated sodium technology.

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