Red Light on Plants: A Crucial Light Color Frequency for Boosting Plant Growth


Farming has been at the center of attention recently due to the rapid increase in the human population. While conventional farming methods still work, they might not be sustainable. New and more efficient techniques such as indoor farming look promising. But this method requires artificial lighting. Shining red light on plants has proven effective, and we have covered how the light benefits crops in detail below. Take a look!

Different “Kinds” of Light

The colors of light get defined by their wavelengths. There is the visible light that forms the color spectrum containing the seven rainbow colors. Longer wavelength light appears red while shorter wavelength light appears violet.

The visible light spectrum

The visible light spectrum

There’s also the invisible light we cannot see, but the body can feel. Light wavelengths longer than red light (infrared) warm the body. On the other hand, wavelengths shorter than violet light (ultraviolet light) cause sunburns.

Plants use these different kinds of light to perform several functions, such as:

  • Photosynthesis
  • Deciding when to flower
  • Determining how large to make their leaves
  • Deciding which direction to grow and how long to grow their stems

However, these functions require different kinds of light. Therefore, farmers need to identify which light type causes specific responses.

Effects of Color Lights on Plants

Light has multiple colors, and each affects plants in the following ways.

The Primary Colors

The white light spectrum consists of seven primary colors: red, blue, green, yellow, orange, indigo, and violet. Also known as the primary colors of the rainbow, they become visible when a ray of white light gets split by a prism.

Light dispersion using a prism

Light dispersion using a prism

An object usually adopts the color it cannot absorb. Plants cannot absorb the color green, so they evolved to be green. This phenomenon explains why we have green plants.

Red Light

Plants are sensitive to red light because they have a red light photoreceptor known as phytochrome. This receptor is a blue-green pigment in plant cells.

Red light makes plants grow large and tall with multiple branches. A large quantity of natural red light can increase meta-tooling production, a hormone that prevents chlorophyll breakdown. Therefore, the plant will stay green during autumn to boost photosynthetic efficiency.

Reddish light on an indoor farm

Reddish light on an indoor farm

This light also affects flowering and seed formation. Plants compare the frequency of red and far-red light to decide if it’s time to begin flowering. You can extend the non-flowering period by exposing the plant to red light.

Lastly, this color can affect the concentration of the special flavor-influencing oils.

Blue Light

Plants sense blue light using a photoreceptor called cryptochrome. The high availability of bluish light during winter and autumn slows the effect of auxin, a hormone responsible for stem and root growth.

The hormone is also responsible for apical dominance (central stem dominance over other stems). Therefore, blue light causes a short plant height with multiple branches.

Bluish light above leafy veggies in an indoor farm

Bluish light above leafy veggies in an indoor farm

Blue light also makes plants open their stomata widely to increase metabolism, which boosts plant growth and development.

Additionally, it makes leaves grow towards the light source and avoids leaf multiplication around fruits. Therefore, a blue light shortage can cause a 20% harvest loss.

Green Light and Other Colors

As stated earlier, plants are hardly sensitive to green light because they lack green light receptors. Therefore, plants grown under green light will be weak and remain young for a long time.

Orange and yellow are closer to red light, while indigo and violet are closer to blue light. Plants react to these colors in the same way they do to red and blue, respectively.

Reddish and bluish lights in an indoor farm

Reddish and bluish lights in an indoor farm

“Invisible” Light

Invisible light is usually in the form of far-red and UV. Seeds use the red to far-red light relationship to determine if it’s time to germinate or not. Also, they use this relationship to determine the number of plants nearby.

Plants take in large amounts of red light while reflecting far-red. Therefore, an area with several plants will have a low red light intensity and high concentrations of far-red radiation.

In such a scenario, seeds will avoid germinating while plants above the surface will shoot faster to emerge above the rest in competition for light.

A comparison between far-red and UV light on the light spectrum

A comparison between far-red and UV light on the light spectrum

On the other hand, plants perceive UV using the blue light cryptochrome photoreceptor. High UV levels increase the concentration of Anthocyanin, a purple substance that protects plants from UV radiation and micro-organisms.

However, the impact of excess UV light on photosynthesis is negative. It interrupts the process and damages a plant’s cell surface membranes & DNA.

Red Light or Blue Light for Plants?

As you can see in the section above, red and blue light are the most crucial for photosynthetic activity (plant growth and development). If using artificial light for indoor plants, here’s how to use the two light colors.

When to Use Red Supplemental Light

Solar radiation has a high ratio of red light. Therefore, sunlight is ideal for stimulating fruiting, chlorophyll B production, flowering, and increasing the photosynthetic capacity.

However, plants experience lower sunlight levels during winter because there are darker days with shorter daylight hours. You can provide artificial red light plus a full spectrum beam to mimic the solar spectrum to stimulate growth.

Vegetable farming indoors

Vegetable farming indoors

Studies on artificial red light have shown it increases cabbage and tomato biomass. However, overhead artificial red lights alone can cause uneven growth. Therefore, you should combine it with the full spectrum beam.

Artificial Red/Blue Light Manipulation

Commercial growers usually manipulate red and blue light to maximize yields. They mix red, purple, and blue light, setting them to specific time schedules to speed up plant growth. However, this optimization is not necessary for houseplants.

A combination of red and blue light in a commercial lettuce farm

A combination of red and blue light in a commercial lettuce farm

The Best Way to Use Red and Blue Light

If running red and blue lights, you should install them in the optimal ratio for the specific plants and their growth phase. A high red to blue ratio is ideal for fruiting/flowering and increasing plant mass. The inverse is better for leafy veggies or plants requiring stronger stems.

Therefore, you should have customizable controls for red and blue light intensities to optimize plant growth on your indoor farm. These controls will enable high-quality, high-yield farming while reducing production time and costs.

Using Light to Make Better Plants

Solar radiation from natural light works well for plants. But it contains some light frequencies that provide no benefits. Therefore, you can use artificial light with the required frequencies to make plants grow better.

Why Intensity Matters

Giving plants the light color they need is one thing. Providing it in the required light intensity is another. Outdoor plants naturally get light from the sun, which comes in as high-intensity light. Therefore, you can’t place any blue or red light bulb in a grow room. The light must provide enough photons to support and enhance photosynthetic efficiency.

Light measurement in an indoor farm

Light measurement in an indoor farm

Thus, you should consider a variable called PPFD (Photosynthetic Photon Flux Density) to determine the best light for your indoor farm. This unit (expressed in µmol/m2/s) shows the PAR (amount of usable light) reaching your plant from a grow light.

Flowering plants require 300-600 µmol/m2/s in the vegetative phase and 800-1000 µmol/m2/s when flowering. You can get such high light intensities from HID and LED lights.

Blue and Red HID Lights

HID is an acronym for High-Intensity Discharge, and as the name suggests, such lamps have a high-intensity output. However, HID lights do not project the solar spectrum in full.

A metal halide HID lamp.

A metal halide HID lamp.

Therefore, you need to switch between Metal Halide (MH) and High-Pressure Sodium (HPS) lights to get the blue and red light, respectively.

A high-pressure sodium lamp on a farm

A high-pressure sodium lamp on a farm

Blue and Red LED Lights

LED technology easily matches the intensity of HID lights and their photosynthetic efficiency while giving the following advantages.

  • Consuming half the energy
  • Emitting lower heat levels
  • Lasting longer (over 50,000 hours with minimal decline in light quality)
  • Having a less complicated installation process

On top of that, you can use an RGB LED light fixture that switches between red and blue. Therefore, LED grow lights are more practical, cheaper (in the long run), and easier to use.

The Benefits of Full-Spectrum LED Lights

Although plants mostly use blue and red light, they require other light colors in the Photosynthetically Active Radiation (PAR) range, albeit in low quantities. For instance, green light penetrates canopies better.

Full spectrum LED grow lights.

Thus, it is better to have a full spectrum LED light that delivers the required light colors to reduce wastage and maximize light use efficiency.


In conclusion, red light is crucial for the growth of plants, and the best option is to use LED lights (blue & red or full spectrum) for your indoor farm. If you need these LEDs or have questions/comments about the article, contact us for more details.

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