Overview of Far Reds Impact on Cannabis Production

Professor DeBacco


Red Light (600-700nm)
Red light is at least as effective as other colors of light at promoting plant growth.
While red light may not be the most efficient color for general illumination for people, (high lumens) it is among the best colors of light to stimulate plant growth due to its role in photosynthesis.
If you only grow plants under red light (with no blue) they will be elongated
Red LED’s are the most efficient of the colors

Far Red (700-800nm) Impacts on Plants
Far red influences the size of leaves, the length of stems, and ultimately the height of plants.
Far red may make some plants flower earlier
To measure you need a spectroradiometer which is a light measurement tool that is able to measure both the wavelength and amplitude of the light emitted from a light source.
However, this comes at an increased cost due to internal complexity

Lights Ability to Penetrate Into a Leaf
Notice that both green and far-red light penetrate deep into the leaf.

Blue and Red are absorbed mainly be the chlorophyll located in greater concentrations on the upper leaf surface.
However, the deep penetration suggests that structures other than chlorophyll are responsible for the utilization of these wavelengths.



Plants Response to Shade
Reductions in the red to far-red ratio (R/FR ratio; 660 nm / 740 nm proportion) are the most prominent.
The left y-axis is for full sun data, and the right y-axis is for both moderate and deep foliar shade data.

Far Red Over and Under Leaf Comparison
Above and below leaf light spectrum comparison
Notice how the under leaf still receives about 50% of the total far red compared to the minimum % of the other colors.

Red and Far-Red Ratios
The ratio of red and far red changes in full sun vs shade conditions
Full sun has equal red and far red
Under plant shade most of the red is used, but the far red will reach the shaded plant.
This will induce shade avoidance but changing morphology and can induce early flowering.



Far Red Plant Shade Impact
Petunia Seedlings Grown Under Different Light
B = blue
R = red
FR = far red
Umol/m2/sec is the subscript number
High vs low light intensity of red and far red while keeping blue intensity constant



Interaction of R:FR and PPFD
Transplanted seedlings from previous slide to a greenhouse
All grown under the same conditions (including light) after transplanting
How does the seedling environment impact the flower cycle?
Plants flower faster under high intensity light and also when grown under far red light



Impact on Yield by Adding Far Red Light
Far red is in the 700-800nm range so it is outside of the PPFD range of measurements which only takes into account the 400-700nm range.
Lettuce is a leafy green that shows great response to far red light.
Reduced PPFD in far red trials attempts to provide all plants with a consistent PFD of 350 μmol/m2/sec


Added Far Red Increases Cell Expansion

Far Reds Impact on Cannabis Production
Far red can cause cannabis plant stretching so it is advised to be of greater importance later in the growing cycle.
Far red exposure may also induce earlier flowering times in cannabis.
When growing outdoors the high 20% far red is counter acted by the very high 27% blue spectrum.
Measuring light with a PFD meter will take into account UV and Far red light, but this should only be done with LED lights and not HPS lights due to the strong infrared spectrum these also produce.



Special Thanks To…
Combination of research data from…
Dr. Bruce Bugbee of Utah State University and Founder of Apogee Instruments Inc.
Shane Torpey MICRO founder
Professor Erik Runkle of Michigan State University



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