Understanding how cannabis reacts to grow light radiation

Hi all,

Today I'm going to tackle some important high level concepts for those of you who really want to know how to maximize cannabis growth and plant yields.

The focus will be on understanding what plants need from our sun (and the grow light you hang over them). And more important - the tools available so you can measure this yourself.


The sun is the main source of energy for the development of life on our planet. The sun's energy is emitted in the form of electromagnetic radiation and strikes in different wavelengths onto the earth.

The human eye usually perceives light in the wavelength range of 380-780 nanometres (nm).  Plants absorb the portion of light in the wavelength range of 400-700 nm. The photo-synthetically active radiation is abbreviated as "PAR". We can argue that plants also are affected by wavelengths beyond 700nm - infrared. Most scientists believe that they can recognize the shadow of surrounding plants upon their own leaves which stimulates a growth response to the crowding. There is a bit less evidence that plants use the wavelengths below the 400nm level because this is ultraviolet light and damages cellular growth. However scientists speculate that this damage also creates a stress response which in turn causes the plant to grow (and flower) to reproduce. But for now we'll focus on 400-700nm spectrums.

Plant growth is driven by three processes which are responsive to light:

► Photosynthesis (metabolism)
► Photomorphogenesis (form development)
► Photoperiodism (daylength reaction)


The most important of these processes is the photosynthesis: the basis for plant growth and development. More simply, it is a process that all plants use, to collect the energy from the sunlight. The plants store the collected energy as carbohydrates, so that the sunlight basically serves as food for the plant. The light is absorbed with the aid of the pigment chlorophyll.

The two most important chlorophylls are chlorophyll A and chlorophyll B. Chlorophyll A absorbs the light in the blue and red wavelengths. Green and far-red light however, are little or not absorbed. Chlorophyll B uses a similar range, with absorption peaks closer to the blue end of the spectrum. So right there if we are custom designing a light spectrum we want to hit blue and red.

Absorption Spectrum Chlorophyll A, B and Beta-Carotene

The "action spectrum" is the sensitivity curve of the light on plant's photosynthesis. In order to make accurate statements about the light absorption of different pigments, scientists undertook a complex measurement process using a spectrophotometer where each wavelength was tested for the specific absorption rate. The result of the activity of main pigments and auxiliary pigments is shown graphically in the action spectrum.

Action Spectrum

Comparing the action spectrum with the corresponding absorption spectrum of chlorophyll you will note that they do not match. In fact, the absorption spectrum leads to the conclusion that photosynthesis is primarily driven by blue and red light and we believe this is true in cannabis photosynthetic response - depending on the phase of plant growth.

Young plants such as newly rooted clones prefer the Action Spectrum. In fact too much light intensity on the red wavelengths is harmful to young cannabis plants.

On the other hand as plants grow in the 20-24 hour vegetative stage they move from only being able to handle the Action Spectrum, to very much being driven by the Absorption Spectrum. And this makes sense because this is when plants are growing like crazy, absorbing the light to create chlorophyll A & B.

Now what is really fascinating is that when it is time to turn the lights down to a 12 hour day - which will induce the reproductive or flower phase - then we've found that the plants are firmly desirous of the spectrum weighted to chlorophyll A and in fact prefer much more red wavelengths. The belief is that cannabis, like all plants in their reproductive phase, know they are dying. They produce flowers as a means to pass on their genetic heritage. So stressing cannabis is important just as it is when making wine with grapes. Red light seems to trigger a response in plants that they need to stretch to out compete their neighbors. And to produce the largest flowers possible. Now the question becomes how much light is enough? And how much is too much?  

More on this in our next post about how to understand and measure not just PAR but also the full spectrum of your lights.