There are many theories online regarding the benefits of using Far Red for growing Cannabis. Most of them are wrong.
Fortunately controlled studies have been published recently on the effects of Far red on cannabis yield and quality. Let's take a look at the arguments for using Far red and the new evidence published by scientists in the last year.
Far Red effect on the morphology or shape of the plant
In nature Far red is relatively high in the sun spectrum when the plant is in shade. When plants detect a high percentage Far red their response is to grow taller to get out of the shade and into full sunlight i.e. If there is a relatively high proportion of Far red in the grow light spectrum the plant will 'stretch' more and become taller and more spread out.
This is not ideal for indoor growers who are limited in space. It is also a waste of the plants resources into growing stems instead of leaves and flowers.
Bud development is also impacted. Buds grown under high far red (25%) will be less dense than buds grown under a light source with a relatively low proportion of far red (3%). 3% far red is around the normal proportion of far red emitted from a white LED.
Far Red increases yield due to the Emerson effect
The Emerson effect is 'the increase in the rate of photosynthesis after chloroplasts are exposed to light of wavelength 670 nm (deep red light) and 700 nm (far red light)'. In simple terms adding far red to a light emitting deep red will increase growth by more than the sum of the deep red and far red photons in isolation.
This has been tested in a controlled experiment where the percentage far red is increased and the yield compared. There was a slight increase of about 6% yield reported when the PAR intensity was the same but the proportion of Far Red increased.
However Far Red LEDs are more expensive than deep red to buy and they are less efficient. It can easily be argued increasing the PAR output by 6% would have the same effect and less likely to reduce harvest quality as will be shown.
Far Red does not improve cannabis harvest potency
We all want to grow the most potent weed full of flavour and with the best aroma. However all the evidence is that adding more far red during flowering will reduce the levels of cannabinoids, flavonoids and terpenes.
Looking at the same study the controlled experiments recorded a reduction in Cannabinoid concentration when the proportion of Far Red is increased.
This result was even more pronounced in a previous study with a light source with low far red of 3% was compared to a high far red of 25%. The reduction in the potency of the flowers harvested was very significant.
There is no evidence Far Red improves potency and convincing evidence it reduces potency instead.
Far Red does not improve cannabis harvest flavour or taste
The terpene levels in the flower will largely determine the intensity of the smell. In both controlled experiments the terpene concentrations reduced as the % Far red increased.
This chart shows the change in concentration of the terpene Myrcene, known for its relaxing, sedating effects. All of the other terpenes tested also reduced with increased far red including pinene, limonene, bergamotene and farnesene.
In the other experiment the total monoterpenes reduced very significantly with a big difference in the % far red.
Far Red does not promote flowering in Cannabis plants
This is a common misconception. A high proportion of far red in the grow light spectrum can cause long day plants to increase flower production. However medicinal herbs such as cannabis are short day plants and increasing far red does not effect the timing or rate of flowering.
Summary of the effects of far red on cannabis harvest and quality
There is some evidence that adding some far red to the grow light spectrum will increase yield very slightly. However the corresponding reduction in cannabinoids and terpenes means a much lower quality of harvest in terms of potency and aroma.
