What if the secret to bigger, better flowers was hidden in the color of light your plants receive? As you know plants need light to grow, produce energy, and eventually flower. But they don’t just depend on light to grow—they depend on the right kind of light. Different types of light have unique effects on plants, from boosting photosynthesis to triggering flowering. Red and far-red wavelengths are especially important for flowering plants, influencing their growth phases and final yields.
In this article, we’ll explain the science behind these light spectrums and show you how to optimize them for indoor cultivation.
Understanding the light spectrum plants use
If you currently have an indoor grow room and are using a high-intensity grow light such as a metal halide, high-pressure sodium or LED, you will probably notice that the colour of the light produced is a clear white, or a slightly off-white.
What we don’t see is what wavelengths are actually at work in harmony, which is what creates the white spectrum we are all used to seeing when we enter our grow rooms each day.
Below is a breakdown of the different spectrums and the nanometres of wavelength they represent
● Ultraviolet light (UV): 200–400 nm
● Blue light: 400–500 nm (important for leaf and root growth)
● Green light: 500–600 nm
● Red light: 600–700 nm (crucial for photosynthesis and flowering)
● Far-red light: 700–800 nm (triggers flowering and affects plant structure)
Red from 600 to 700 nm
Red light is highly absorbable by plants’ chlorophyll pigments promoting photosynthesis. This type of light also affects hormones like auxins, which control how plants stretch and develop flowers.
However, red light must be combined with blue light during the vegetative period. Red light alone is less efficient for photosynthesis compared to a mix of red and blue light. Adding blue light helps balance energy absorption and leaf structure development.
Far-red (the start of infra-red) from 700 to 800 nm
We have many uses in our daily lives with far-red light, ranging from televisions, music systems, and air conditioners, to the more technical devices such as infrared cameras and night vision goggles. It is the far red bandwidth of light that promotes size, structure, and flowering.
How do plants use the available red light?
The way plants can absorb wavelengths of light is through chlorophyll. As red light and blue light are the most absorbent and used spectrums, they are easily absorbed and converted to energy for the growing or flowering phase.
Phytochromes: The Sensors for Red and Far-Red Light
While chlorophyll powers photosynthesis, another key player in plants’ response to light is phytochrome, a light-sensitive protein that detects red (600–700 nm) and far-red (700–800 nm) light. Phytochromes act as receptors, triggering important processes based on the type of light they perceive.
Here’s how phytochromes work:
● Red Light Activation: When phytochromes absorb red light, they shift to an “active” state, promoting flowering, seed germination, and other growth-related responses.
● Far-Red Light Reset: Far-red light reverses this activation, helping plants measure day length and control their internal clocks.
● Beyond Flowering: Phytochromes also regulate shade avoidance (by elongating stems) and synchronize metabolic activity with day-night cycles (circadian rhythms).
By understanding how phytochromes interact with red and far-red light, we can further understand how the plant responds to each type of light and optimize lighting conditions to control flowering and improve crop performance, so let’s put it clear:
How do plants respond to red light?
To achieve the ideal growing environment indoors, you will want to have a balance of Blue light from 400 to 500 nm and a mix of Red light (from 600 to 700 nm) and infrared light (from 800 to 1200 nm). Plants when they are triggered by darkness periods will respond to the red light spectrum in the following ways.
● Red light will promote photosynthetic efficiency,
● Red light is highly absorbable by plants,
● Red light triggers flowering and far red light controls activity,
● Fruiting and flowering are dependent on red light and far red light,
● Red light affects hormones and plant morphology
● Internodal spacing is affected by both types of red light.
Which grow light is the best for flowering plants indoors?
Now that you understand which light spectrum benefits plants the most during the flowering period, it is a good idea to know which horticultural grow light is best for indoor use. Below are the 3 most common grow lights on the market, the spectrum and nano metres of each and their relationship to fruit and flower production.
Metal halide (MH)
This type of HID lighting is often used for car parks, sports stadiums, and street lamps. They emit a white light that runs between 385 nm and 674 nm. For the seedling and growth stage of plants, metal halides are great, however, should be combined with an LED or HPS for the optimal spectrum.
High pressure sodium (HPS)
High-pressure sodium grow lights have been a popular choice amongst indoor growers for decades, due to the fact they are cheaper compared to high-end LEDs, and they also produce excellent results under the right circumstances. HPS grow lights will produce a spectrum between yellows 570-590 (nm), oranges 590-630 (nm), and reds 630-750 (nm).
LED grow light
Whilst HPS and MH lights are universal and emit one spectrum, LED grow lights work differently. The diodes used in the lights can be focused and engineered to focus on what plants really need for both the growth and flowering period. LED lights usually range between 400 nm to 780 nm of light (depending on the manufacturer) and are highly popular amongst indoor producers.
Our final thoughts
When it comes to flowering plants, it is important to provide your plants with a mixed lighting spectrum and to focus more on the 600-800 nm spectrum once the darkness periods are the longest they can be (12/12). Red light plays a major role in how a plant stretches, flowers, and ripens, so our advice is to use an indoor grow light such as MH and HPS combined, or an LED that is dominant in the blue and red light bandwidth.
