Red and far-red light: the optimal R/FR light ratio for plants

How do red and far-red light affect plant flowering? In this article, you'll discover how the R/FR light ratio works and how to use it optimally for maximum growth and flowering.

Red and far-red light play an important role in plant growth and flowering. Especially during the flowering phase, the ratio between these light colors (R/FR ratio) determines how and when a plant will flower.

In this article, we explain how red and far-red light work, what phytochromes do, and how to apply this knowledge to grow lighting.

🌈 Light and the plant spectrum

Plants react to different parts of the light spectrum. The most important light colors for plant development are:

• UV light (340 – 400 nm) – affects photomorphogenesis
• Blue light (400 – 500 nm) – stimulates growth and structure
• Red light (600 – 700 nm) – important for photosynthesis and flowering
• Far-red light (700 – 800 nm) – affects flowering and elongation

👉 Also read more about the light spectrum for plants.

🌱 What are photoreceptors?

Plants use special pigments (photoreceptors) to perceive and process light. These control important growth processes.

• Cryptochromes – react to blue and UV light
• Phototropins – direct growth towards light (phototropism)
• Phytochromes – react to red and far-red light

👉 Phytochromes, in particular, play a key role in plant flowering.

🔴 Phytochromes and the R/FR light ratio

Phytochromes exist in two forms:

• B1 – absorbs red light
• B2 – absorbs far-red light

During the day, with a lot of red light, B2 is especially active. During the night, this changes back to B1. This interaction determines how a plant reacts to day and night.

👉 The ratio between red and far-red light (R/FR) has a direct influence on:

• Flowering induction
• Plant elongation
• Germination
• Chlorophyll production

🌸 Effect on the flowering phase

Plants flower in response to light signals, a process known as photoperiodism.

The ratio between red and far-red light determines how quickly and how strongly a plant reacts:

• More far-red light → faster flowering and taller plants
• More red light → more compact growth

👉 The right balance is essential for optimal results.

🌗 Long-day and short-day plants

Plants react differently to the length of day and night:

• Short-day plants (SDP) – flower during long nights
• Long-day plants (LDP) – flower during long days
• Day-neutral plants – react less to day length

For short-day plants, a long period of darkness is essential to achieve the correct phytochrome balance.

🧬 What is florigen?

Florigen is the "flowering hormone" of plants. This protein is produced when the correct light and dark ratio has been reached.

Once florigen becomes active, it triggers the plant to flower. This process is controlled by genetic signals within the plant.

💡 Red and far-red light in grow lights

Modern LED grow lights are specially developed to provide the correct ratio between red and far-red light.

This allows you to control and optimize plant flowering, even indoors.

✅ Conclusion

• Red and far-red light determine plant flowering and growth
• The R/FR ratio is crucial for the correct growth behavior
• Phytochromes control these processes
• Photoperiodism determines when a plant flowers
• LED grow lights enable optimal control