The wavelengths of electromagnetic radiation span from the very short wavelengths of cosmic and gamma rays, to the very long waves that are used for radio broadcasting. The wavelengths plants use for photosynthesis fall within a very small portion of the total electromagnetic spectrum called the visible and near visible light spectrum.
The wavelengths of the visible and near visible light spectrum is measured in nanometers, 100 nm – 1000 nm, which refers to the length of the wave. At the low end of the spectrum is short wave ultraviolet light, 100 nm. At the high end of the spectrum is long wave infrared, 1000 nm. In the middle of the spectrum are the wavelengths that plants use for photosynthesis, 400 nm – 700 nm.
This range of 400 nm – 700 nm is referred to as PAR. PAR is an abbreviation of photosynthetically active radiation.
Across the range of PAR, there are different wavelengths of light. We see these wavelengths of PAR as colours like this:
Wavelength patterns are not the same for every light. For example, a hospital light emits a bluish hue because it outputs more blue wavelengths than red, while a street lamp emits an orange hue because it emits more red wavelengths than blue.
Horticultural lights output unique wavelength patterns that can target a plant’s photoreceptors, stimulating optimal growth, and also influence if they direct their energies towards root growth or top growth.
The chlorophylls in cannabis that power photosynthesis capture light at specific wavelengths. The chart below shows the wavelengths where chlorophyl gets captured. Maximum absorption of chlorophylls a and b occurs between 430 – 453 nm (blue) and 642 – 662 nm (red). Horticultural lights often feature high output of these specific wavelengths to target the chlorophylls.
Chlorophyl light absorption peaks at 430 – 453 nm and 642 – 662 nm. To cater to the chlorophylls, some horticultural lights output almost all their power on these wavelengths. Their partial output pattern is represented in the chart below. The light output by these fixtures looks purple due to very high output of red and blue wavelengths, combined with very low output of green wavelengths.
Although partial spectrum lights can be used to successfully grow cannabis, evidence suggests better results are achieved with full spectrum light that includes red, blue and green light.
Green light has important special properties. Green light can penetrate the surface of the leaf, helping deliver energy to leaves under the canopy, and to the inner portion the leaves. Without green light, shaded leaves get deprived of energy.
Cannabis grows best under full spectrum lighting that includes green light, with extra output between 430 – 453 nm (blue), and 642 – 662 nm (red) wavelengths that target chlorophylls.
For production plants in their bloom phase, a full spectrum light that targets the chlorophylls and also has intense output on the red end of the spectrum works best.
When there is more intense red light, plants respond with greater top growth and stem elongation. These are the desired plant behaviours during budding.
Production plants in vegetative growth also grow best with a full spectrum pattern, but with extra intensity on the blue end of the spectrum. Blue wavelengths suppress the top growth promoting effect of the red wavelengths, allowing plants to focus greater energy on lateral root growth and branching.
When a blue biased wavelength pattern is used for vegetative growth, you can gain shorter, bushier plants with more roots.
A blue bias is also helpful for nursery plants because seedlings and mother plants should not be encouraged to grow tall. Root development and lateral branching are the most important objectives for them.
Nursery lights should be full spectrum with a heavy bias away from red.
It’s not absolutely necessary to have separate wavelengths for both production growth stages. Any full spectrum light will work for both stages of growth provided it can deliver suitable intensity for the size of plant. Custom wavelength patterns are a performance optimization, not a requirement of life. If budget does not permit separate fixtures for vegetative growth and flowering growth, it’s best to use the full spectrum bloom wavelength pattern with the red bias for both growth stages. The effect of strong red light to boost budding is the most important benefit to be gained from wavelength optimization.