What is Far red in the grow light spectrum
Far red is on the extreme of the range of wavelengths that can be used by plants. It is next to red in the spectrum and is dimly visible as a deep red. Far red wavelengths are from 700nm to 750nm and not typically included in grow light spectrum.
Far red wavelengths are outside of the PAR spectrum of wavelengths and not included in PAR measurements. However Far red is used by plants to photosynthesise when delivered with wavelengths in the PAR spectrum. This is known as the Emmerson enhancement effect. When simultaneously exposed to light of both wavelengths, the rate of photosynthesis is far higher than the sum of the red light and far red light photosynthesis rates.
There is a move to use a new measure for the wavelengths used by plants to the ePAR spectrum. ePAR measurement includes wavelengths from 400nm to 750nm and it may become the standard measurement for grow lighting in the future. Lin to a related article and video here: The New 400-750 nm ePAR Range Explained
Far reds penetrates leaves
Far red is detected by plants by the photoreceptor phytochrome system. Far red penetrates deep into the leaves and into the lower canopy. Far red will reach cells lower in the leaf and can increase the total amount of photosynthesis in the plant.
Far red causes shade avoidance response in plants
Far red is at a higher proportion of the suns spectrum under a plant or trees canopy. Plants can detect a high proportion of far red compared to red in the light it is receiving. This will cause 'shade avoidance' response and the plant will elongate its stems and leaves. It does so to reach outside the shaded area to brighter light and better conditions for growth.
Increasing Far Red causes cell expansion and stretching
If the proportion of far red to red is high then cell expansion in the plant increases. This means the leaves and stems will increase in length and plants will grow taller. This is not a desirable effect for indoor growers because we want short and dense plants due to limited space. Also energy is wasted growing taller plants instead of using the energy to produce more flowers or fruit.
However if you are growing leafy greens such as lettuce or spinach this is a good light recipe as the leaf area will increase and your crop weight will increase for a relatively low increase in energy input.
Far red effect on flowering
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 rate of flowering.
Should Far red LEDs be included in grow lights?
It is generally considered that exceeding about 5% far red in a grow light spectrum will cause plants to stretch. Most grow light sources already emit a small percentage of their output in FAR red.
For this reason we do not recommend far red LEDs are added to grow lights.
However future studies may show benefits to using far red at particular stages of growth.
In theory, far red could be used in early veg to stimulate leaf growth to generate a full canopy to capture all of the light. Far red could also be used in the latter stages of flowering to contribute to the final yield.
We will keep an eye out for developments in this area and keep your posted.