How Full Spectrum LEDs Impact Plant Size, Appearance and Potency

Cultivating cannabis indoors allows for complete control over the plant’s environment, from germination to flower. With the advance of LED lights, growers can harness photomorphogenesis to get even more control over the most minute details of a plant’s existence, fine-tuning everything from the plant’s lifecycle to its biology.

Photomorphogenesis is a plant’s response to various light spectrums, independent of photosynthesis, including growth, development and differentiation. Light wavelengths between 430 and 660nm are typically used for photosynthesis, but plants are able to “see” and respond to a much wider section of the spectrum, ranging from 280nm to 730nm — so, in some ways, plants have much better “eyesight” than humans.

What Makes Photomorphogenesis Possible?

Beyond photosynthesis, plants undergo several other processes at the cellular level. While chloroplasts are the primary structure driving nutrient development, phytochromes and cryptochromes are specialized light receptors that respond to different spectra. Depending on the type of light absorbed, these receptors influence plant behavior by triggering hormonal responses.

Understanding each of these structures’ roles in plant development can offer some insight on how to influence final results.

  • Phytochrome exists in two forms (Pr and Pfr) and absorbs mostly red and far-red light. These two forms constantly interact with each other to find equilibrium. The plant undergoes certain changes based on what form of this protein is dominant.
  • Cryptochromes and phototropins are important UV-A, UV-B and blue-light receptors. They also play a role in regulating plants’ circadian rhythms.

Which Wavelengths Elicit What Results?

In order to fully grasp the power of photomorphogenesis, it’s important to understand what wavelengths result in physical changes that can be moderated within a grow facility. With a full-spectrum LED fixture — and a willingness to experiment — growers are able to create the ideal combination of light spectrum for their crop.

  • UV-A and UV-B Light (280-400nm) – UV-A and UV-B offer benefits in moderation. In plants, these wavelengths are known to activate defense mechanisms through triggering a cryptochrome chemical reaction. Plants can produce up to 15 different defense proteins, which in turn increase the anti-oxidant content within the plant. Some research indicates that UV light may prevent plant elongation and increase the production of color and fragrances. Notably, UV-B has been shown to encourage THC  and total cannabinoid content in cannabis.
  • Blue Light (400-500nm) – Blue light has shown to have a variety of effects on plant morphology, including the suppression of stem elongation, development of thicker stocks and larger rooting zones, chloroplast movement, absorption of carbon dioxide and has an effect on the plant’s circadian rhythm. Some studies indicate cannabis plants grown under blue light with a short photoperiod resulted in higher cannabinoid concentrations.
  • Green Light (500-600nm) – While less research has been conducted on the effects of green light on plant photomorphogenesis, some researchers have discovered that it is able to deeply penetrate the plant canopy — however, THC production is negatively affected by green light.
  • Red and Far-Red Light (600-700nm) – Exposure to red and far-red light stimulates seed germination, leaf unrolling, chlorophyll formulation and stem elongation. It also plays a role in the promotion or suppression of flowering, thereby triggering or preventing plant reproduction. An important response is “shade avoidance,” a reaction triggered by an abundance of far-red light and hastens flowering. Understanding the ratio of red to far-red light will help create or prevent the avoidance response.

How Can Growers Apply This Knowledge in Their Grow?

Now that you have an overview of what photomorphogenesis is, it’s time to try it out yourself. It might require some trial-and-error to find the best light combination to support your personal priorities, but using the light spectrum strategically can reap numerous benefits. Your plants will thank you, too.  

What does this look like in practice? Some growers might have space restrictions and rely on slightly more blue light to keep plants short and wide, rather than tall and spindly. Others might utilize red and far-red light to speed up flowering or encourage more bud development. Those looking to break some potency records can experiment with UV-B light to boost THC development.

Choosing the right grow light doesn’t only ensure a healthy crop but can also achieve desired morphological responses. While high pressure sodium and metal halide lamps may provide the right light for photosynthesis, these lights fail to allow the user to tailor the wide range of light wavelengths that can have an impact on overall plant health.  When deciding what fixtures to buy, explore LED options that include a full-spectrum or spectral tuning capabilities.