The Best Light for Plants

Light is two main things for plants:
It is energy and it is information.

Learn how to use the right spectrum of light to get huge growth, abundant flowers, and massive fruit.

Plants are far, far more adaptable than we often give them credit.  And, they have to be.  They are stuck where their seed germinated.  Unlike an animal, they don’t have the luxury of being able to move themselves to a more desirable locale. They have to make the most of wherever they are.   So, how do they know how to adapt? The answer, to a great extent, is light.  Plants, incredibly, through light know what season it is, what time of day it is, whether there are other plants around them, and whether it is time to make babies (flowers and seeds, that is).  

How to Use Light to Trigger Blooming and Fruiting
What light is best for plant blooming?

The general rule is to provide a lot of red light, particularly around 660nm in wavelength, as this is the peak absorbance of phytochrome, a light detecting molecule.¹  This molecule helps the plant determine what time of year it is and whether it is time to bloom.  During the day, phytochrome absorbs light and changes shape.²  During the night the molecule slowly changes back to its original shape.  The amount of phytochrome that has been able to change  back tells the plant how long the night is and, coupled with other facts, what time of year it is.

If your plant species naturally blooms in the spring (long night species) then give your plants lots of red light and keep your lights on for 12 hours or more.  For fall blooming plants (short night species) have your day time shorter than 12 hours and you can also use another trick.  Time exposed to darkness isn’t the only thing that will turn phytochrome back into its normal state – so will infrared light.  Exposing your plants to infrared light around 730nm will, to simplify a little, cause them to think that the night is longer than it really is, which is great if they naturally bloom when the night is long.  This should be done with care, however, since infrared light can cause plants to stretch and, since infrared is not photosynthetically active light, it can lower the efficiency of a grow light.

How to Use Light to Shape Your Plant
What light is best for creating compact, highly branched plants?
What light is best for vegetative growth?

For many applications, the ideal plant shape is compact and highly branched.  This shape has many advantages both aesthetically and in terms of producing more flowers and fruits.  The opposite of this plant shape – highly stretched with few branches – is often the result of a plant’s shade avoidance response.

If a plant thinks that another plant is shading them, they will try to stretch towards the direction of light to get above the other plant.  This response is strongest not just under low light conditions, but especially when the plant detects light that has been filtered through another plant’s leaves.  Light filtered through leaves is green and, less intuitively, high in infrared light.  This is why plants grown under high infrared light (particularly around 730nm) and, in some cases, green will grow as if they stretching to grow above what they think is shading them.³

Direct sunlight, on the other hand, is high in red and blue light.  When you shine red light on plants, their cells expand.  That can result in larger leaves and, in some cases, longer stems.⁴ One interpretation is that the plant is trying to maximize their surface area in this direct light.

Conversely, blue light does not increase cell size.  That means that stems will be shorter and leaves smaller.  Blue light also results in more branching.  Why this is, is not fully understood.  Perhaps the smaller leaves allow more light to hit potential branching sites, activating them.  Or, it could be that with less energy directed to creating stretched stems, more can go to side growth.  It should be noted, though, that blue light can inhibit flowering.⁵  Hence, why many growers use blue heavy lights during vegetative growth and heavily red during flowering.

How to Use Light to Make Your Plants Grow Faster
What light is best to make plants grow faster?

There are three main types of experiments that are done to conclude what light is best to maximize plant growth: one at the molecular level, one at the leaf level, and one at the whole plant level over time.  Below we will give an overview of each, as all are relevant to understand, particularly when wading through the myriad of grow light options and information out there.

Molecule Experiments on Photosynthesis – Absorption Spectra

Chlorophyll a is the main molecule involved in absorbing light energy.  It’s the molecule that directly transfers the absorbed light energy to the chain of reactions that lead to the chemical storing of energy in the plant as sugar.  It’s not alone, though, there are dozens of other “accessory pigments” that also absorb energy from light and then pass that energy on to chlorophyll a, the most notable is chlorophyll b.

One can isolate these photosynthetic molecules (either alone or in the complex of molecules they normally are connected to), shine full spectrum light on them, and see what light the molecules are most prone to absorb.  No surprise, these measurements are known as absorption spectra. The advantages of this method is that you can directly measure what light is most important to the photosynthetic molecules.  The downside is that you’re not truly measuring how the molecules absorb light when they are within an actual leaf.  And, you’re not measuring how time affects the plant over time.

To the right is a diagram showing why red and blue light is so important to a plant – they are highly absorbed by chlorophyll a and b.

Leaf Experiments on Photosynthesis – Action Spectra

95% of plant dry matter comes from carbon dioxide in the air, a truly stunning thought – trees are made of air…This truth about plants consuming and eventually being made of the carbon in our atmosphere makes a simple experiment possible to test the effects of different wavelengths of light. Professor Keith McCree in the 1970’s put cut pieces of leaf from 22 crop plants into small chambers and shined low levels of light of various wavelengths on the leaves.  He then measured how much carbon dioxide was being absorbed as a proxy for how much photosynthesis was taking place.  His results became known as the McCree curve and his results have become the most referenced journal article on photosynthesis ever.  Since the results of these experiments are about causing an action in a plant leaf (the absorption of carbon dioxide) the measurement is called an action spectra.⁶

Long Term Plant Growth Experiments

The experiments mentioned so far to determine optimum spectra for plant growth, measuring light absorption and carbon dioxide absorption, have an obvious limitation – they don’t speak to the effects of light on a whole plant and over a significant portion of time.  That’s where plant growth chamber experiments come in.  The idea is simple: grow plants over time under different spectra of light and then measure some important or interesting aspect of the plants at the end – dry weight, number of flowers, height, etc.  Because of the simplicity of the experiment and the myriad of species and possible light combinations one can try, there have been scores of these kinds of experiments.  A summary of the major findings are as follows:

• As the absorption spectra and action spectra results suggested, red and blue are particularly important in
  

  NASA Indoor Plant Growth Experiment (Wikimedia Commons)

  spurring plant growth.

• Red light causes very vigorous growth.
• Red light alone causes malformed, bleached, stretched, swollen growth.  Blue light supplementation corrects these issues – stretching is reduced, chlorophyll production and efficiency goes up, and stomatas open to release water through the leaves (this is good and called transpiration).^7,8
• Green light, while obviously the most reflected color off of plants, is also the type of light that can penetrate deepest into leaves and canopies.  If you have a crop with a thick canopy or leaves, green light can boost whole-plant photosynthesis.  Some crops, however, receive green light as a signal to stretch or to slow growth (particularly under low light level conditions).⁹

References:

1. Phytochromes — multi-functional light sensors
2. Distinct classes of red/far-red photochemistry within the phytochrome superfamily
3. Phytochrome-mediated Responses Implications for Controlled Environment Research Facilities
4. Light-stimulated cell expansion in bean (Phaseolus vulgaris L.) leaves. II. Quantity and quality of light required
5. Photobiological Interactions of Blue Light and Photosynthetic Photon Flux: Effects of Monochromatic and Broad-Spectrum Light Sources
6. The action spectrum, absorptance and quantum yield of photosynthesis in crop plants
7. The Multisensory Guard Cell. Stomatal Responses to Blue Light and Abscisic Acid
8. Plant Productivity in Response to LED Lighting
9. Green Light Drives Leaf Photosynthesis More Efficiently than Red Light in Strong White Light: Revisiting the Enigmatic Question of Why Leaves are Green

Excellent resources on photosynthesis and/or photomorphogenesis

• Photobiological Sciences Online
• Manipulating Light for Horticulture
• The effects of light-emitting diode lighting on greenhouse plant growth and quality
• Photomorphogenesis Youtube playlist