Introduction to flowering with far-red light and Phytochromes

Plants have evolved over thousands of years and utilise the natural light spectrum in different ways. The most common way is photosynthesis where the plant uses light and converts it into growth. However there are other functions of light as well, mainly to trigger plants and provide guidance. The receptors that are responsible for this are Phytochromes and they mainly respond to light in the 600 to 800nm area. Many people refer to this light as far-red, although it is far red and some parts of other red. By Jille Kuipers

What does far-red do to plants?

Far-red light, which is barely visible to the human eye, plays a significant role in plant growth and development. It triggers elongation, causing plants to stretch and grow taller. Far-red light also influences the size of leaves, the length of stems, and the overall shape of plants.

What is far-red light used for?

In horticulture, far-red light is primarily used to influence plant development and the formation of plant organs. It is widely applied in greenhouses to control flowering timing and regulate plant height. Far-red light is often used as supplemental lighting since, on its own, it does not strongly support photosynthesis or plant growth. Instead, it works in conjunction with other light spectrums, such as red light, to enhance specific plant responses.

In several studies with HortiPower LEDs the effect of far-red on flowering is visible. It is also found in other public studies such as above from Park, Y., and Runkle, E., from Michigan State University. 

What is the difference between red and far red?

Red light, typically visible to the human eye, has a peak wavelength of around 630 nm. Far-red light, which is mostly invisible to humans, peaks at around 730 nm. Both red and far-red light play critical roles in plant development, particularly in regulating the phytochrome system that controls flowering, stem elongation, and other growth processes.

Does far-red light inhibit flowering?

Far-red light can inhibit flowering in short-day plants like chrysanthemums and poinsettias by interrupting the critical dark period required for these plants to bloom. When far-red light is applied during the middle of the night, it can “break” the nighttime length, effectively altering the photoperiod and delaying flowering. This technique is often used by growers to control flowering schedules.

Is infrared the same as far-red?

No, far-red light and infrared light are not the same. Infrared light spans a much broader wavelength range, typically from 700 nm to 1 mm. Far-red light, which peaks at 730 nm, overlaps with the lower end of the infrared spectrum but is classified separately due to its unique effects on plants.

What light is best for flowering?

For promoting flowering, red LEDs are the most effective, as they strongly influence the phytochrome system, which regulates flowering processes. Far-red light combined with deep red light has the greatest impact on phytochromes, making this combination particularly effective for controlling and stimulating flowering in plants.

Are far-red LEDs included in regular plant lights?

Most standard plant lights are designed to emit light in the photosynthetically active radiation (PAR) range, which spans 400–700 nm. Since far-red light falls outside of this range, it is typically not included in general plant lights. However, far-red light is often used as a supplemental light source with flexible spectrum HortiPower fixtures or separate far-red LEDs. These allow growers to selectively use far-red light for specific purposes, such as flowering or stem elongation, without affecting the overall efficiency of the lighting system.

What are Phytochromes?

Phytochromes are light-sensitive pigments in plants that regulate processes such as flowering, seed germination, leaf development, and seasonal adaptations. These processes, collectively known as photoperiodism, depend on the plant’s ability to perceive and respond to day length and light quality. Phytochromes also help plants adapt to shade by promoting elongation in response to far-red light.

What are the types of flowering plants?

Flowering plants are categorized based on their response to day and night lengths:

• Short-day plants Flower when nights are long (e.g., chrysanthemums, poinsettias)
• Long day plants Flower when nights are short (e.g., spinach, lettuce)
• Day-neutral plants Flower regardless of day or night length (e.g., tomatoes, cucumbers)

Short-day plants typically flower in autumn or winter when nights are longest. Conversely, long-day plants flower in late spring or summer when nights are shortest. Tropical plants, often day-neutral, are adapted to consistent 12-hour day/night cycles.

The length of darkness matters to flowering plants

Flowering responses in plants are primarily influenced by the uninterrupted length of darkness, rather than the duration of daylight. Interrupting the dark period with light, especially far-red light, can delay flowering in short-day plants or accelerate flowering in long-day plants. This principle is widely used in controlled horticultural settings to manipulate flowering schedules. For example:

• Poinsettias: Growers ensure these plants bloom in time for Christmas by carefully managing their dark periods.
• Chrysanthemums: Using far-red light at night can delay flowering, allowing the plant to grow taller and stronger before blooming.

How greenhouse growers can use far-red

Greenhouse growers often use far-red light to manipulate flowering and plant height. Research with HortiPower grow lights has shown that even small amounts of far-red light (above 10 micromoles) in the 600–800 nm spectrum can trigger significant plant responses. Far-red light works in conjunction with red light to optimize photoperiod control and plant elongation. For instance, applying far-red light at the end of the day can mimic a natural sunset, promoting stretch and growth.

Fruit development and light

Fruit development occurs after flowering, so controlling the timing of flowering can directly impact fruit yields. For crops like dragon fruit, managing flowering schedules ensures that fruiting aligns with favorable environmental conditions. This prevents premature fruiting during seasons when survival and growth might be compromised.

Phytochromes and the biological clock

Phytochromes also help the plant develop an internal biological clock. The approximate twenty-four-hour cycle is known as the "circadian rhythm" and is calibrated by the sun. The sense of time is important for plants as it helps them to readjust the position of leaves and flowers in the evening and night so that they can be in an optimal position during the next sunrise. During the night the plants will also relax their leaves and reduce the surface area for photosynthesis. Sunflowers can predict the direction of the sun and adjust their heads to make sure they are oriented in the best possible way. Other plants such as orchids use the circadian rhythm as a way to release scent during the night or day, depending on the animals that pollinate them. 

Temperature also provides additional information to the plant, but when plants get conflicting information it throws them off balance and they are creating new leaves when they shouldn’t or dropping leaves when they shouldn’t. 

Plants require periods of darkness to rest and reset their biological processes. Moonlight and other low-intensity light sources do not disrupt this rest because they lack significant red light. However, artificial lighting at night can interfere with flowering and growth. Continuous lighting is not recommended, as it can cause stress, disrupt circadian rhythms, and increase susceptibility to environmental stressors.

Benefits of understanding far-red in the horticulture sector

Understanding the role of far-red light enables growers to:

• Stretch plants to produce taller, more marketable specimens.
• Manipulate flowering timing for optimal harvest periods.
• Enhance fruit yield and quality by regulating flowering and development.
• Maximize plant growth and nutrient efficiency at different growth stages.

What products are made for Phytochromes impact?

Specialized products, such as the Bloomer 1 and the Nurser 3 FR use only far-red and deep-red light to influence plant development. These lights do not emit blue or green wavelengths, making them ideal for specific applications like mimicking sunset conditions or breaking the nighttime length. With low power consumption (5–20 watts), these lights offer energy-efficient solutions for growers to improve plant control and optimize crop performance.

This article was first published 25th July 2022, and revised on 25 February 2025. 

The original article contained some inaccurate information regarding short-day plants. We thank Peter H for alerting us to this issue and the article has been corrected.