- Created: 23-09-21
- Last Login: 23-09-21
Plants Only Use the Visible Light Spectrum for Photosynthesis
It's important to know that plants only use visible light (the colors of light that we see every day) for photosynthesis. However, as the chart below demonstrates, the complete spectrum of light is far greater than just the visible light spectrum. On the outer edge of the visible light spectrum is Ultraviolet (UV) light and Infrared Radiation (IR). UV light is the invisible light emitted by the sun and other sources that will cause sunburns when we don't wear sunblock. IR light can only be seen with special equipment, like night-vision goggles. Even further out from the visible light spectrum are light waves that we don't traditionally think of as light. These include X rays, Microwaves and even Radio Waves.
One of the most important things to understand is that scientists have demonstrated over and over again that plants only absorb visible light for photosynthesis. Plants do react to other forms of light like UV, but that reaction is typically negative. I'm told that marijuana growers actually use UV light to induce the production of psychoactive chemicals like THC, which seem to be produced in part as a defense mechanism against the damaging effects of UV light to the plant.
What is PAR?
PAR stands for "photosynthetically available radiation." PAR is made up only of visible light, because this is the only light that plants use for photosynthesis.
For decades, many indoor growers have used Lumens to measure a grow light's efficacy, but the industry is getting smarter and turning to PAR. Lumens are used to measure the brightness of a lamp to the human eye. But plants and people see light differently. Humans see yellow and green more brightly than other colors. Therefore, yellow and green lamps may have higher Lumen values than red and blue lights that put out just as much actual light, and which plants are likely to respond better to.
PAR measures all light from the visible light spectrum equally, and does not measure light outside of the visible light spectrum, which does not help the plant photosynthesis. So, for plants, the PAR value of a light is currently the best basic measurement of a grow light's brightness. Accurate PAR meters are quite expensive and generally cost $500 or more. Inaccurate PAR meters can be purchased for much less, but there's really no point to owning an inaccurate PAR meter.
The best way to get PAR values for your 400W LED grow light, assuming you don't want to purchase your own PAR meter, is to check with your reputable grow light manufacturer or provider for the PAR rating of their lights.
How Much PAR do My Plants Need to Grow?
The amount of PAR your plants require depends on what you are growing, as well as how far away from your plants the light is. Generally speaking, leafy greens like lettuce only need a PAR value of ~200, whereas tomatoes and other plants that flower and produce fruit require 400-500 or more PAR. Unless you place your 600W LED grow light right on top of your produce, you will need an even higher PAR rating from your grow light, to take into account the distance between your plant and the light source.
In the example below, you can see a very powerful grow light that puts out nearly 1,900 PAR (measured in umol) 8 inches from the source. Very few lights put out this much PAR, and they are typically quite expensive. This light will emit 1,900 umol every second. But at 23 inches from the source, the strength of the light is reduced to 890 umol. The PAR value is reduced further and further as you get further from the light source. When we get to 6 feet away from the light source, our PAR value is down to ~100umol, which means we would have trouble growing even lettuce well. So, always make sure you understand not just the PAR emitted from the light, but that every 8 inches or so away from your light, the PAR value will be reduced by ? or more.
There are many inexpensive grow lights on the market that make big claims, but they will ultimately leave their owners disappointed. This issue is especially rampant on the internet. Remember to check the PAR value of any light you purchase. Also, remember to take into account how far your light will be from your plant to ensure there is enough photosynthetically available radiation (PAR) for your plant to flourish.
Leafy Greens require 200 PAR for proper growth
Tomatoes, cucumbers and other flowering/fruiting vegetables require 400-500 PAR
Fruiting Trees should be given 600 PAR or more
What is the Temperature of Light I Should Use?
Interestingly, 'Kelvin temperature' is the metric used to describe the visual color that a light emits. As you can see in the chart below, 'warmer' light temperatures that have a red color have a lower Kelvin rating. On the other end of the spectrum are 'cooler' temperature lights which have a blue color and higher Kelvin rating.
Different temperatures of light have different impacts on plants. Generally, higher temperatures (blue) light encourages photosynthesis which leads to bushy plants that don't feel inclined to elongate and reach for more light. This is great if you want to grow in a compact space. Lower temperature (red) light reduces photosynthesis and signals to plants that that it's time to flower and produce fruit. Plants put under a red light will also be more inclined to stretch and grow taller, as opposed to growing bushier and more compact.
IGWorks focusses on providing full spectrum lights with a natural color temperature of between 4500K-6500K as these are most pleasing to the eye. They also allow plants to grow bushy and compact, without hindering the ability of plants to flower and fruit.
What Color of Light Should I Use?
LED lights can come in almost any color. Plants respond most to red and blue light. Interestingly, plants generally respond less well to green light. In fact, the reason that plants appear to be green is that they tend to reflect green light, while they absorb other parts of the light spectrum more readily. This is why a large scale or industrial grower of plants will often use a combination of red and blue lights to photosynthesize their plants. They don't want to waste electricity producing green and even yellow light, which plants use less effectively.
However, for those of us growing produce in our living spaces, it's probably worth the extra pennies it costs to produce a nice full-spectrum color that will be more natural and pleasing to the eyes. Full-spectrum grow lights will often come with a chart, which shows the distribution of blue, green, yellow and red light that is emitted. See the example below
Choosing the right grow light spectrum for your commercial operation can be a challenge. Many 800W LED grow light suppliers have conflicting information on the topic due to bad marketing or simply a lack of knowledge in plant and light research.
In this article, our light spectrum experts break down what light spectrum is, how plants respond to light, and how light spectrum influences plant growth.
What is Grow Light Spectrum?
Light spectrum is the range of wavelengths produced by a light source. When discussing light spectrum, the term 'light' refers to the visible wavelengths of the electromagnetic spectrum that humans can see from 380–740 nanometers (nm). Ultraviolet (100–400 nm), far-red (700–850 nm), and infra-red (700–106 nm) wavelengths are referred to as radiation.
As growers, we're most interested in the wavelengths that are relevant to plants. Plants detect wavelengths that include ultraviolet radiation (260–380 nm) and the visible portion of the spectrum (380–740 nm) which includes PAR (400–700 nm), and far-red radiation (700–850 nm).
When considering light spectrum for horticultural applications, greenhouse and indoor environments will differ. With indoor environments your grow light's spectrum will account for the total light spectrum that your crop receives. Whereas in a greenhouse you must consider that your plants are receiving a combination of folding grow light series and solar spectrum.
Either way, the amount of each waveband that your crop receives will have significant effects on growth. Let's learn more about how this works.
Plants use light for photosynthesis and photomorphogenesis. Photosynthesis is the process by which plants and other organisms convert light energy into chemical energy. Photomorphogenesis refers to how plants modify their growth in response to light spectrum.
One example of photomorphogenesis is a plant bending toward a light source. Light also affects plants' developmental stages, such as germination and flowering.
The light that plants predominately use for photosynthesis ranges from 400–700 nm. This range is referred to as Photosynthetically Active Radiation (PAR) and includes red, blue and green wavebands.
Photomorphogenesis occurs in a wider range from approximately 260–780 nm and includes UV and far-red radiation.
Although results are dependent on other factors, there are general rules of thumb that you can follow when using light spectrum to elicit different plant responses.
Outlined below is an overview of how each waveband is used for horticultural purposes so that you can trial light spectrum strategies in your own growth environment and with your chosen crop varieties.
Blue light has distinct effects on plant growth and flowering. In general, blue light can increase overall plant quality in many leafy green and ornamental crops.
A minimal amount of blue light is required to sustain normal plant development. In terms of adjustable spectrum lighting strategies, if we were to equate red light to the engine of your car, then blue light would be the steering wheel.
When combined with other light spectrum wavebands, blue light promotes plant compactness, root development, and the production of secondary metabolites. Blue light can be utilized as a growth regulator, which can reduce your need for chemical plant growth regulators (PGRs). Blue light can also increase chlorophyll accumulation and stomatal opening (facilitating gas exchange), which can improve overall plant health.
One example of blue light influencing secondary plant metabolite production is how blue wavebands promote anthocyanin development in leaves and flowers. Increased anthocyanin levels result in more pronounced color.
Blue light also promotes other secondary metabolic compounds associated with improved flavor, aroma and taste. For example, blue light treatments have been shown to improve terpene retention in some varieties of cannabis.
Higher intensities of blue light (>30 μmol·m-2·s-1) can inhibit or promote flowering in daylength-sensitive crops. Blue light does not regulate flowering at low light intensities (