The impact of red and blue light on plant growth: How to improve greenhouse lighting?

Red and blue light are the core spectrums of plant growth lamps, and they play a significant role in plant photosynthesis and morphological building. To maximize photosynthetic efficiency and encourage good plant development while optimizing greenhouse lighting, it is essential to fairly change the ratio of red and blue light depending on the needs of different crops.

 

 

Red and blue light are key spectra of plant photosynthesis, which influence all phases of a plant, from seed germination to maturity.

 

1) Red light's (600–700 nm) function

Promote photosynthesis: Chlorophyll a and b significantly absorb red light, enhancing the efficiency of photosynthesis and raising biomass accumulation.

Red light can induce auxin synthesis, increase stem elongation, and support seedling development by stimulating cell elongation.

Red light controls phytochrome, promoting flowering and fruit development and increasing crop yields.

Too high a proportion of red light could lead the plant to grow too tall, thin the stems, and lower resistance.

 

2)  Blue light's (400–500nm) function

Blue light is particularly absorbed by carotenoids and chlorophyll, enhancing photosynthesis capacity and rendering leaves thicker and greener.

Blue light increases stomatal conductance, improves plant absorption of CO₂, activates cryptochrome and photosensitive pigments, and raises water use efficiency using stomatal opening and shutting.

Blue light can reduce excessive elongation, shorten and compact plants, and strengthen mechanical and wind resistance.

 

A percentage of blue light that is too high could cause restricted development and lower photosynthesis.

 

 

1) Different plants and various growth phases have different needs for the red-to-blue light ratio:

Crop Type	Growth Stage	Red-Blue Light Ratio (R: B)	Light Duration (h/day)	Light Intensity (PPFD, µmol/m²/s)
Leafy Greens (Lettuce, Cabbage, etc.)	Vegetative Stage	4:1 - 5:1	14-16	150-300
Fruit and Vegetables (Tomato, Pepper, etc.)	Vegetative Stage	5:1 - 6:1	14-16	250-400
Fruit and Vegetables (Tomato, Pepper, etc.)	Reproductive Stage	6:1 - 8:1	12-14	400-600
Strawberry	Full Growth Cycle	5:1 - 7:1	12-14	200-400
Cannabis	Vegetative Stage	5:1 - 6:1	18-20	300-500
Cannabis	Flowering Stage	6:1 - 8:1	12-12	600-900

2) Optimisation in concert with other spectrums

🌿Far-red light falling between 700 and 750 nm

Promote floral bud differentiation and improve flowering rate and fruit quality.

Adding an appropriate amount in the early stage of plant growth can increase plant elongation, but excessive addition may lead to leggy growth.

Applicable to high-value-added crops such as tomatoes, cucumbers, and hemp.

 

🌿Green light ranging 500–600 nm

Though the green light has a minor direct impact on photosynthesis, it can pass through the canopy, encourage photosynthesis of lower leaves, and maximize general light energy use.

Adding a green light to densely planted crops (such as lettuce and strawberries) can improve growth uniformity.

 

🌿Ultraviolet light (UV, 280-400nm)

A proper amount of UV can increase plant resistance, promote the production of secondary metabolites (such as flavonoids and carotenoids), and improve crop quality (such as the THC and CBD of cannabis). Excessive UV may hinder growth and should be used with caution.

 

 

 

1) Lighting time and photoperiod management

The photoperiod of plants controls their growth rhythm and flowering timing. The setting of the lighting time should be based on the type and growing stage of the crop.

 

(1) Photoperiod type

💡Long-day plants (>12 hours of light)

Like tomatoes, cucumbers, lettuce, etc., they demand a longer, lighter duration to develop and blossom appropriately.

Lighting solution: Provide 14–16 hours of light daily. If you lack enough natural light, use LED light.

 

💡Short-day plants (< twelve hours of light)

Only a short-day environment-that of cannabis, strawberries, chrysanthemums, etc.-can stimulate flowering.

Control the light cycle to ten to twelve hours to prevent too much light from influencing flowering.

 

💡Neutral plants-not light cycle sensitive

For instance, whilst light intensity still influences yield, maize and cucumbers are not responsive to light duration.

Lighting solution: Give enough light intensity without especially regulating the light cycle.

(2) Suggestions on real light-filling time

Greenhouse Type	Supplementary Lighting Method	Recommended Light Spectrum
Glass Greenhouse	Supplemental lighting during morning and evening	Primarily red and blue light, with appropriate far-red light
Plastic Greenhouse	Full-day supplemental lighting	A combination of white light + red-blue light to simulate natural light
Vertical Farming	Fully artificial lighting	Adjustable spectrum LED, optimized red-blue ratio

2) Optimal spectrum of lighting

Apart from the fundamental arrangement of red and blue light, the impacts of other spectrums should be considered holistically to maximize light energy use and crop quality.

 

(1) Suggestive spectrum mix

🌍RB: 4:1~8:1 red and blue light dominance

Good for most vegetables, fruits, and green crops.

🌍Red and blue light plus far-red light ( RB: 6:1 + FR: 10%)

Suitable for encouraging cannabis flowering or fruit enlargement (including strawberries and tomatoes).

🌍Red-blue light combined with green light (RB: 5:1 + G: 10–20%)

It is relevant for dense planting conditions (like vertical farming) to increase lower leaf illumination.

🌍Full spectrum-red-blue plus green plus far-red plus UV

Relevant to upscale gardening, including strawberries, cannabis, medicinal herbs, etc., to enhance quality and active components.

(2)Two stages of spectrum optimisation

Crop Type	Growth Stage	Daily Supplementary Lighting Duration (hours)
Leafy Greens (Lettuce, Spinach)	Vegetative Stage	12-16
Tomato, Pepper	Vegetative Stage	14-16
Tomato, Pepper	Flowering & Fruiting Stage	12-14
Strawberry	Full Growth Cycle	10-12
Cannabis	Vegetative Stage	18-20
Cannabis	Flowering Stage	12

3)Management of Light Intensity (PPFD)
Various crops and growing phases have varying needs for light intensity. Measuring the impact of additional plant lighting depends critically on PPFD, or photosynthetic photon flux density.

Crop Type	Growth Stage	Recommended PPFD (µmol/m²/s)
Leafy Greens (Lettuce, Spinach)	Vegetative Stage	150-300
Tomato, Pepper	Vegetative Stage	250-400
Tomato, Pepper	Flowering & Fruiting Stage	400-600
Strawberry	Full Growth Cycle	200-400
Cannabis	Vegetative Stage	300-500
Cannabis	Flowering Stage	600-900

4) Intelligent control and monitoring

Usually mixed with clever control systems, modern greenhouses help to save energy usage and increase the efficiency of supplementary lighting.

🔍Monitoring daylight strength, the light sensor automatically changes the output of additional LED lights.

🔍Based on daily light integral, it precisely supplements the necessary light.

🔍Set the light cycle with an intelligent timer to guarantee consistent additional lighting.

🔍The temperature and humidity are controlled with light intensity to prevent overheating or transpiration imbalance.