Plants and Far-Red Light

What us humans call "light" is actually just a part of the electromagnetic radiation spectrum that our eyes can sense. If electromagnetic radiation bounces off a surface and our eyes can "perceive" it then we call it light. However, other parts of the electromagnetic spectrum are sometimes referred to as light too.

Light that we call PAR

The spectrum of light that plants use for photosynthesis and to grow with is called PAR (Photosynthetically Active Radiation). It is defined as being light with a wavelength between 400 nanometres (blue light) and 700 nanometres (red light).

However, this is a bit of an oversimplification. Yes, the majority of plant growth is driven by light in the PAR range, but plants are also affected by light which is below 400 nanometres (nm) called ultraviolet, and also by far-red light at about 730nm.

Ultraviolet Light

Many growers find out, sooner or later, that ultraviolet light (particularly UVB at around 280-300nm) stimulates terpene, resin and flavonoid production. The Solacure Flower Power is a lamp which has the spectrum designed to do just that:

But what about far-red light?

Far-red light is quite significant to plants. It typically isn't absorbed much by plant leaves, and it doesn't drive any photosynthesis because it is mostly reflected off leaves. However, if a plant "senses" a greater ratio of far-red to normal light then it "knows" that obstructions or other plants are close to it. This causes the plant to react. It knows that it is much less productive to grow laterally outwards where there are obstructions or another plant's leaves already there. The obstructions or other plant's leaves will will just end up shading its own leaves. Instead, it puts its energy into growing vertically to reach for where the light is. In other words, it stretches upwards, increasing internodal length. This is called the "shade avoidance" effect.

Most indoor growers prefer their plants to stay short with as little distance between the internodes as possible. The internodes are where the fruits and flowers will eventually spring from. The light from a grow light only penetrates so far down a canopy. It is best for as many internodes to be in the intense light, close to the top, as possible. Therefore, growers like the distance between the internodes to be as short as possible. So, during veg, giving your plants far-red light is not really a good thing.

Notice that in the Spectrum Graph above, there is a lump at around 740nm. This is what we call Far-Red Light.

Far Red Light is not always a bad thing!

When it comes to the flowering/fruiting stage, however, matters become different. One of the ways that a plant "senses" that half the time it is the light-period, and the other half is the dark-period is by the balance of 2 hormone-like pigments called Pfr and Pr.

During the lights-on period of 12/12, a pigment called phytochrome is converted from one form (Pr) into another (Pfr). The peak wavelength for this is around 665nm.

However, during the 12-hour dark period, Pfr is converted back into Pr. Far red light with a peak wavelength at about 730nm activates this conversion more quickly. If far-red light is given to a plant (for about 15-30 mins) at the beginning of the dark period it will react as though it has had a longer dark period.

The advantages of far-red light

This has a couple of effects, first of all, flowering/fruiting will be triggered more quickly; if it usually takes 12-14 days of 12/12 for a particular plant to start flowering/fruiting then by adding a short period of far-red to the beginning of the dark period this might be reduced down to 7-10 days. The first couple of weeks of the changeover in photoperiod from 18/6 to 12/12 is generally when plants will begin to stretch. Unfortunately, using far-red light will generally exacerbate the amount of stretch a bit. It is a trade-off. It is perhaps best to not use far red for the first 2 weeks after the changeover to 12/12 to avoid unnecessary stretch.

Once far red light is being used for 15 minutes or so after the main light has gone out, the plant thinks the dark-period is longer, it is usually possible to give the plants a longer lights-on period. It could be changed to 13 hours on and 11 hours off (with the first 30 mins of the dark period being far-red light only). The implications for this is that the length of the flowering/fruiting period can be shortened by 2-3 days.

The Emerson Effect

Back in 1957, a chap called Robert Emerson discovered that the rate of photosynthesis in plants that only received light below 680nm was actually quite low.

He then discovered that light at 700nm and slightly above only produced something called ATP (Adenosine TriPhosphate, which is a compound that stores energy both in plants and animal muscle).

However, when plants receive both light at 680nm and 700nm, the rate of photosynthesis jumped up to greater than the sum of the parts. He concluded that there were 2 seperate photosystems at work and that both needed to be activated for effective photosynthesis.

Photosystem 2 (sometimes called P680) uses the smaller wavelengths of light to split water into ions and to produce electrons.

Photosystem 1 (sometimes called P700) then takes these electrons and uses the longer wavelength of light to produce the compounds that store energy (ATP which is eventually turned into something called NADPH).

The way this happens is a bit long winded, and the above explanation is very simplified. But the take-home message is that plants need light at the end of the PAR region (700nm) and a bit beyond in order to perform effective photosynthesis. Just giving them light at or below 680nm is nowhere near as effective. Far red light can help with this.

LED Grow Lights that Include Far Red Light

Manufacturers who include far red light in the output spectrum also claim that far-red light stimulates greater essential oil production which improves the quality of the final product.

Virtually all HPS grow lights naturally produce far red light in their output. However, there are also a couple of LED grow lights now where the manufacturers have deliberately included far red light in the spectrum too: