The Perfect Light Spectrum for Flowering Plants

Did you know that plants can “taste” light?

In fact, when it comes to light, plants are picky eaters, and their taste preferences will change from one growth phase to another.

Giving your flowering plants the light that they enjoy will maximize their yield.

So, choosing the right spectrum is as important as choosing the right intensity, if not more. In this article, we’ll tell you everything you need to know about the ideal light spectrum for flowering plants.

The Best Light Spectrum for Flowering Plants

Here are the characteristics of the best light spectrum for heavy-flowering and fruiting plants.

1. Rich in Red Light

Light that plants can use for photosynthesis is called PAR, short for photosynthetically active radiation.

light spectrum for plants
PAR RANGE

Red light is at the end of the PAR range. It’s considered as a low-energy light because of its relatively long wavelength and low frequency.

red light range

Despite that, red light is used more efficiently by plants to drive photosynthesis than high-energy light.

Red light is extremely important during the flowering phase of your plants.

Multiple studies have shown that red light plays an essential role in regulating the number and quality of flowers, and the duration of the flowering phase.

Long story short, the spectrum of your grow lamp should be richest in red light (but not exclusively red).

The spectrums of grow lamps that produce most of their light in the red spectral range are often described as “warm” and have a low Kelvin rating.

For example, many growers use HPS lamps during the flowering phase of their plants because they produce a lot of red light. They have a warm spectrum and a kelvin rating of 2000K (relatively low).

Kelvin ratings are mostly used with HID and fluorescent lamps and can give you a good assessment of the light spectrum.

2. Has Enough Blue Light

The wavelength of blue light is shorter than that of red light, so it has higher frequency and energy.

blue light range

Blue light is equally important for your flowering plants as red light, but it’s needed in smaller quantities.

For leaf and stem growth, blue light is more beneficial for plants than red light. For this reason, during the vegetative phase, your plants need an abundance of blue light. And strong vegetative growth is a prerequisite for a productive flowering phase.

Lamps that generate most of their light in the blue spectral range have “cool spectrums” with high Kelvin rating. You can use cool-spectrum grow lights during the vegetative phase, then switch to ones with warmer spectrums when your plants flower. (Learn more about growing plants with lights)

Yet, blue light is also important during the flowering phase, because it regulates important processes during photosynthesis, such as stomatal opening.

Studies showed that plants grown under red light only grew a smaller number of flowers than plants grown under a combination of red and blue light.

Depending on the plant species, increasing the ratio of red to blue light can increase the number of flowers and the yield.

For instance, one study showed that pepper plants grow the highest of number of flowers when light is 95% red + 5% blue.

Red to blue light ratio for flowering pepper plants

In another new study, tomato plants achieved optimal growth when the ratio of red to blue light was 10 to 1.

3. Contains Some Green Light

The importance of green light is often overlooked by indoor growers.

It’s true that green light is not as beneficial for plants as red or blue light, but that doesn’t mean it’s not useful at all.

There’s evidence showing that green light plays an important role in leaf growth and stem elongation.

One study done on orchids revealed that plants grown under a combination of red, blue, and green light showed a 60% increase in mass compared to ones grown under red and blue light only.

Green light also has better penetration ability than red or blue light. This feature can help the lower leaves of your indoor plants remain photosynthetically active.

Some grow lamp manufacturers add green light to the spectrum of their devices to make plants more visible under artificial light. This is because green light can balance the dazzling effect of red and blue light.

It is important to note that green light may have a negative effect on THC content in cannabis plants. If you are growing cannabis plants, choose a grow lamp that emits small amounts of green light.

4. Includes Wavelengths Beyond PAR (UV and IR)

We said earlier that only PAR (light between 400 to 700 nm) can drive photosynthesis. But there are frequencies outside that range that are useful for plants.

Ultraviolet (UV) light is one of those frequencies. UV light can cause damage to living cells because it has a very high energy.

uv light spectrum

In nature, the damage that UV causes to plant cells can make them produce several useful chemicals. The process is kind of similar to how UV light can make human skin produce vitamin D.

In cannabis plants, UV light increases the production of cannabinoids, including THC.

Although UV light is harmful, grow lamps usually emit minimal amounts of it, so no need to be worried. But, if you spend a lot of time under UV-emitting lights in your indoor garden, make sure to take the necessary precautions. For example, you can wear protective glasses and clothes.

Besides UV, infrared (IR) light can also be useful for your plants. IR light can increase plant development and speed up the flowering phase.

IR light spectrum

You can’t see IR light, but you can sense it as heat. Since IR light is hot, you don’t want your grow lamp to emit significant amounts of it.

Old grow lights, such as HPS lamps, generate a lot of IR radiation that can damage (burn) plants if not properly eliminated.

Most modern LED lights, however, emit tiny amounts of IR light.

Comparing the Spectrums of Popular Grow Lights Used for Flowering Plants

Let’s look at the light spectrums of three different grow lamps used for flowering.

· HPS Lamp Spectrum

Here’s the spectrum of an HPS lamp:

light spectrum of HPS lamp for flowering plants

First thing you notice about the spectrum is the relatively low quantity of blue light. For this reason, the lamp is not suitable for the vegetative phase.

You can also see the substantial volume of yellow light the lamp emits. Most of this light is wasted because the plant cannot use it efficiently.

The spectrum is rich in red light (from 620 nm to 700 nm). This feature makes the HPS lamp suitable for the flowering phase.

The lamp also emits a significant amount of IR light (700 nm and above), which can heat the growing environment.

· Black Dog LEDs spectrum

Black Dog LEDs are worth mentioning because they have one of the most optimized light spectrums for flowering plants. Take a look:

Black Dog LED spectrum for flowering plants

The spectrum is rich in red and blue light. The volume of red light is significantly higher than blue light.

Black Dog LEDs also emit enough green light (500 to 560 nm) and a small quantity of UV light (375 to 400 nm).

Modern grow lamps, like Black Dog LEDs, are hundred times better than an HPS lamp.

· HLG 600 V2 RSpec Quantum Board

Quantum boards (QBs) are among the most popular LED grow lights in the indoor gardening community.

Here’s the spectrum of HLG 600 V2 RSpec QB that is designed for flowering plants:

HLG Rspec light specturm

It’s quite different from the spectrum of Black Dog LEDs, but it’s also perfect for the flowering phase.

The spectrum of HLG 600 V2 RSpec QB is highly rich in red light. It also has a great volume of blue light.

The device emits a good amount of green light but generates zero UV light. This may negatively affect THC levels in cannabis plants.

All in all, the spectrum of HLG 600 V2 RSpec QB is much more optimized than that of an HPS lamp and highly suitable for flowering plants.

Conclusion

Today you learned all about choosing the ideal light spectrum for your flowering plants.

In summary, remember that your flowering plants need:

  1. A lot of red right
  2. A good quantity of blue light
  3. Some green light
  4. Minimal amounts of UV and IR light

You’ll find a lot of grow lights on the market that promise you with the best results. However, always make sure to check the light spectrum before you purchase any grow lamp.

Finally, enjoy growing your favorite plants indoors and don’t forget to share your thoughts below!

Blogger, Gardener | + posts

I'm Jad, a biologist, blogger, and experienced indoor gardener. I am knowledgeable in plant biology, particularly in plant cultivation and propagation. I founded HerbsWithin.com in 2019 to share my knowledge in indoor gardening with passionate home growers.

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