Choosing the optimal lighting for cannabis cultivation can be challenging. There are many factors to consider, like light intensity, distribution, and spectrum  Spectrum can be especially challenging because there is a lot of conflicting information on the internet and forums. This article is going to take a research-based approach to understanding green light.

The Key Points:

Green light…

  • Is absorbed by plants and used during photosynthesis
  • Penetrates the plant canopy, allowing light to reach the lower branches of the plant
  • Regulates seed germination and flowering in some species
  • Provides the most benefit when used in combination with red and blue light

Vegetative Growth

Green light is radiation with wavelengths between 520 and 560 nm and it affects photosynthesis, plant height, and flowering. Plants reflect green light and this is why they appear green to our eyes. As a result, some growers think that plants don’t use green wavelengths, but they actually do!

In fact, only around 5 – 10% of green light is reflected from leaves and the rest (90 – 95 %) is absorbed or transmitted to lower leaves [1].

Green wavelengths get used in photosynthesis. Chlorophyll pigments absorb small amounts of green wavelengths. Light that doesn’t get absorbed is transmitted to leaves that are shaded out from direct light. This means that leaves at the bottom of the canopy get more green light than leaves at the top. A high proportion of green wavelengths compared to other colors tells lower leaves that they are being shaded out, so they are able to react accordingly. Lower leaves may react by opening or closing their stomata or growing longer stems that help the leaves reach brighter light [1, 2, 3].

green light

Flowering

When it comes to growing cannabis, many cultivators are interested in the quality of light used for the flowering stage. In many plants, flowering is regulated by two main photoreceptors: cryptochrome and phytochrome. Both photoreceptors primarily respond to blue light but can also respond to green, although to a lesser extent. Green can accelerate the start of flowering in several species (although cannabis has yet to be tested) [1, 4, 5]. However, once flowering has begun, it’s important to provide plants with a “full spectrum” light that has high amounts of blue and red light, and moderate amounts of green, in order for photosynthesis to be optimized.

 

Seed Germination

Green light mediates seed germination in some species. Seeds use green wavelengths to decide whether the environment is good for germination. Shade environments are enriched in green relative to red and blue light, so a plant can tell if it is shady or sunny. A seed that senses a shaded environment may stay dormant to avoid poor growing conditions [1]. Some examples of plant species where researchers have documented this response are: ryegrass (a grass that grows in tufts) and Chondrilla (a plant related to dandelion) [1, 6].

Although green wavelengths generally tell plants NOT to germinate, there are some exceptions! Surprisingly, green wavelengths can stimulate seed germination in some species like Aeschynomene, Tephrosia, Solidago, Cyrtopodium, and Atriplex [1, 6, 7].

Of course, light is not the only factor affecting seed germination – it’s a combination of many factors, such as soil moisture, soil type, temperature, photoperiod, and light quality.

The Ideal Amount of Green Light

When combined with red and blue light, green can really enhances plant growth [1, 8]. However, too much green light (more than 50% of the total light) can actually reduce plant growth [8]. Based on the most current research, the ideal ratio of green, red, and blue light is thought to be around 1:2:1 for green:blue:red [9]. When choosing a horticultural light, choose one that that has high amounts of blue and red light and moderate amounts of green and other colors of light.