LED Grow Lights are becoming very popular and they are a good choice if you are buying a new grow light system or upgrading your old florescent fixture. This post about LED grow light myths will save you time and money.
As with any new technology there are many myths about LED grow lights. Some are started because of a lack of knowledge by the general public, but many are started by manufacturers who are trying to sell their product. Some of them prefer to keep us in the dark so they can make outrageous claims, but the better companies don’t do this. We need to do our part and become educated consumers so that we can properly evaluate both the message being broadcast and the product itself.
Don’t go shopping for LED grow lights until you read all of this post.
LED grow light myths, showing a red and blue LED
Myth #1: Watts Indicate Brightness
With incandescent and florescent lights, watts were a good measure of the brightness of a light. A 100 watt bulb was always brighter than a 60 watt bulb. Not so with LED. Lower wattage can produce more light.
The watts rating on a LED grow light tells you how much electricity it will use, and therefore the ongoing cost to run the light, but it tells you very little about how bright the light is, or how suitable the light is for growing plants.
For a more detailed discussion on watts see: LED Grow Lights – The Myth About Watts
Myth #2: You Can Use a Simple Watts Per Area Rule
How many watts do you need per square foot of growing area? Consumers want to know, and manufacturers are quite willing to give you a rule such as, seedlings need 15 watts per sq foot. You can find similar rules for other types of plants, but none of them mean very much.
As explained above, watts do not equate to the amount of light. But even more important, watts tell you nothing about the quality of light (i.e. the wavelengths of light). What you really want to know is the PPFD (photosynthetic photon flux density) for a given spot under the grow light.
As a general guide you can use these values:
- 100-300 PPFD for seedlings
- 200-600 PPFD for vegetative growth
- 600-1,000 PPFD for flowering
- 800-2,000 PPFD for sunlight (depends on elevation, location etc)
- 600 – 1,600 PPFD for full shade
Plants can be damaged with more than 800 PPFD.
Myth #3: PAR is a Measure of Light Intensity
You will have trouble finding a PPFD value for most lights. LED shop lights will not provide this value because they are not being sold specifically for plant growth. Many LED grow lights will not give you this value because they want to sell you on watts and give you that value instead – don’t buy from these companies.
The other reason you will have trouble finding a PPFD value is that many people equate PPFD to PAR. They provide PPFD values but call them PAR values. They just don’t understand what PAR means – it is a measure of light quality, not intensity.
If the product does not advertise a PPFD value, but does show you a PAR value – you can usually assume they are the same thing. The units should be μmol/m2/s.
Myth #4: PAR Measures The Light Plants Need
LED PAR spectra, by Fluence Bioengineering
The term PAR (Photosynthetically Active Radiation), when properly used, describes the light spectra that plants use, between 400 and 700 nm. Since plants use more blue and red light these colors get weighted higher than yellow and green.
PAR is a way to measure the quality of light from the plants perspective. It does not measure quantity.
PAR ignores the light plants use below 400 nm and above 700 nm.
Myth #5: LEDs Are 100% Efficient
A common misconception of LED lights is that they are 100% efficient at turning electricity into light. Granted they are more efficient than older technology like incandescent and florescent lights, but they are not 100% efficient.
Myth #6: LEDs Don’t Produce Heat
In theory LED lights could convert all of the electricity into light, but that only works in story books. In real life, an LED converts 20% or more of the electricity into heat.
A light fixture containing 100 individual LED bulbs creates a lot of heat. The lights are designed so that most of this heat comes out the back of the fixture, directing it away from the plant. Larger units also contain fans that blow the heat away. This is important since heat shortens the life of LED bulbs.
Myth #7: Higher Watt Bulbs are Better
LED bulbs – the single units that give off the light, are available in various watt ratings. 1, 3, 5, 10 watt bulbs are common. This leads to another myth. It is common to see the claim that a 3 watt unit does not produce as much light as a 5 watt unit – so the 5 must be better. It is not that simple.
Most bulbs are not run at 100% efficiency. Higher wattage bulbs tend to be run at lower efficiency levels since they produce too much heat at higher efficiency. So a 5 watt bulb may be giving the same amount of light as a 3 watt bulb.
The wattage of the bulb does not tell you very much.
A DIY grow light using COB LED lights, spaced farther apart then on commercial units.
Higher watt bulbs are newer technology and generally cost more. They may also have a shorter life. Given the current technology, your best bang for the buck is a 3 watt bulb. It is a good compromise between efficiency, reliability and cost.
A newer technology called COB LED (chip-on-board LED), is more efficient, has a longer life, but is more expensive. At the moment, I think the technology is too new and still has issues. One potential benefit of this technology is that it allows the manufacture to make longer light tracks, similar to a traditional 4 ft florescent fixture. In that configuration it would cover a larger area for home use. Manufacturers have not taken advantage of this feature, maybe because of higher shipping costs for a larger unit, but there are some DIY systems worth looking into, such as the one pictured here, created by Ichabod Crane on International Canagraphic Magazine.
Myth #8: Mimicking Sunlight is Best
Plants have evolved under the sun, so we assume sunlight is what plants want. It is not. Much of the yellow and green light in sunlight is not used by plants.
Promoting a light for plant growth because it has the “same light spectra as the sun” clearly shows that the company does not understand grow lights.
Myth #9: White Light is Better Than Burple
“Burple” is the industry name for the light produced by many LED grow lights. Since most of these lights contain a lot 
of blue and red bulbs the result is a burple.
Traditionally we have always grown plants under white light, and outside they grow under sunlight which is a yellow-white. It is natural to think white light is better for growing plants – its not.
The best light is one that produces the wavelengths of light that plants need in the relative amounts plants want. They use more blue and red, and less yellow and green. It does not have to look white.
Myth #10: Intensity Drops by the Inverse Square Rule
As light moves away from the source, the light spreads out, and the intensity at any given point is reduced. This follows the inverse square rule, whereby if the distance doubles, the intensity is reduced to 1/4. If you move a plant from 1 ft under the light to 2 ft, it will receive 1/4 as much light.
This rule works for point sources of light, but most LED fixtures contain many LED bulbs, so they are not a point source of light. Therefore the rule does not apply to LED lights.
The other complication is that in the real world the rule only works well right below the light source. As you move out to the sides, the rule is also not valid.
Since it is important to know how much light you get at any point under the fixture, the manufacturer should provide you with that information, as seen in the diagram below
Myth #11: Coverage Area Specifications Are True
What is the growing area under an LED light? This is an important question since it determines how many plants you can grow and it varies from lamp to lamp.
Manufacturers try to help you by providing a “coverage area value” and say something like, the coverage area is 8 sq feet. That sounds great, but this number means absolutely nothing. If you raise a light up higher it will cover more area, so unless they also provide the height of the light and the light intensity values across that whole area, the coverage area number is of no value.
Lets have a close look at this. The diagram below shows the coverage area for a Viparspectra Par 700 light. You are viewing the growing area from above the light and the numbers are the PPFD values at certain points under the light, with the light hanging at 2 feet above the growing surface.
note that they refer to PAR values – but they are actually PPFD values in μmol/m2/s.
LED light distribution under a Viparspectra PAR700 light, at a height of 2 feet
The specifications for this light suggest a coverage area of “Core Coverage at 24″ Height is 4x3ft“. The reason why this area is longer than wide is because the shape of the light is a rectangle. It makes no sense that the above diagram shows circles and squares for a rectangular light, but lets assume the numbers are correct.
LED viparspectra par700 grow light
Directly under the light you have a PPFD value of 780, which is lots of light to grow and flower any plant. Assume you want to cover a 3 x 3 ft area, the light at the edges of this growing area have a PPFD of between 30 and 200. That is enough for growing seedlings, but not much more.
Lets look at this from a different perspective. Lets say that after doing a lot of diligent research you decide that you want to provide a minimum PPFD of 300. That reduces the growing area under this light to a 2 x2 ft area, and even then the corners will only be getting about 200 PPFD. So for your requirements (ie 300 PPFD), your coverage area is 2 x 2 ft, not the advertised 4 x 3 ft.
Without seeing this light distribution diagram and knowing the height used to measure the values, the coverage area in the specifications is of little help. At least Viparspectra provides this information; many manufacturers don’t. If they don’t, don’t buy from them.
Myth #12: PAR 20, PAR 30, etc.
This one is not really a myth, but it does confuse things. PAR 20 and PAR 30 are lamp size designations and PAR in this case stands for parabolic aluminized reflector. It describes the shape and size of the bulb and has nothing to do with the quality of the light. PAR 20 and PAR 30 are common sizes for bulbs used in the home.
The confusion arises because these sizes are now made as LED lights for the home. These are not suitable for growing more than a single plant.
Myth #13: Plants Don’t Use Green Light
Light wavelengths absorbed by plants for photosynthesis
A bit of factual information can easily lead to incorrect conclusions. Plants look green because they reflect green light and absorb red an blue. That makes sense and it follows that if they reflect green light, they don’t use it.
The absorption spectra for extracted chlorophyll shows peaks in the blue and red zones, but no absorption of green light. Again we conclude plants don’t use green light in photosynthesis.
We are wrong. Some green light (around 500 nm) is absorbed by plants, and when we look at photosynthesis in a whole leaf instead of extracted chlorophyll, it is clear that green light does contribute to photosynthesis.
We now know that plants grow best with a wide spectrum that contains all wavelengths including near IR and maybe even near UV. A good LED grow light will provide a wide spectrum which includes some green light.
See also: LED Grow Lights – Getting The Right Color Spectrum
Myth #14: LED Lights Can’t Damage Plants
LED lights tend to produce less heat than older technology, and their light intensity is relatively low. This has lead to the conclusion that you can put plants as close to the lights as you want and you won’t burn them.
The reality is that modern LED grow lights can produce a very high level of light and it can cause photo-bleaching and burn leaves. This depends very much on the plant, but a PPFD of 800 is enough to damage some plants.
Myth #15 Blue is For Veg, Red is For Flower
This was a myth even with florescent technology but it persists with LED. People using cool white (more blue light) bulbs used to add a few incandescent bulbs (very red light) when it was time for plants to flower. It was believed that red light was needed to initiate the flowering process.
Some of the early LED lights were red and blue and it naturally followed that the blue ones would be best for veg and the red for flowers. There are even lights that allow you to switch between a veg mode ( more blue bulbs on) and a flower mode (more red bulbs on).
The reality is that plants grow and flower best with both blue and red light all of the time. For production you might want to fine tune this at different stages in a growth cycle, but for home use we can ignore it.
Myth #16: The More Lumens, the Better
Lumens is a measure of light intensity so it logical to think that a grow light with more lumens is better. The problem is that lumens measure intensity based on the human eye, and we see green and yellow light much better than red and blue.
Consider this extreme case where the light is only yellow. People see a lot of light and therefore it gets a high lumen rating. But plants don’t use yellow light very well, so for a plant this light has a very low intensity.
Lumens work great for evaluating the intensity of lights for you home, but its mostly useless for evaluating LED grow lights.
Myth #17: LED Shop Lights Won’t Grow Plants
Some of the early LED shop lights did not produce much light and were not suitable for growing plants, except for some very low light level requirements. That has all changed. The newer LED shop lights provide lots of light for seedlings and low level plants like lettuce and African violets.
You can buy complete systems including the reflectors or you can buy 4 ft long LED tubes that replace traditional florescent bulbs, allowing you to continue using the existing fixtures. Even better is that the price of these has come way down.
Myth #18: Summer-to-Winter Kelvin Shift is Important
Florescent tubes and the new LED shop lights measure the color of light using a Kelvin (K) scale. A blue-white has a higher Kelvin value than a red-white. Since Kelvin is a unit of measure for temperature these lights are also called cool and warm.
Light in spring is more blue, and fall light is more red. Some people believe that it is a good idea to mimic this natural shift by using bluer light (6500 Kelvin) in spring and a redder light (3500 Kelvin) in fall.
In northern and southern hemispheres there is a real shift in color because sun light has to travel through more atmosphere in winter, but the change from spring to fall is only 300-500K. That is not significant enough to warrant changing lights with the seasons.
In the world of LED grow lights, Kelvin means very little. It is much better to compare actual spectra, but they can be hard to come by. Some manufacturer do show them on their website.
References:
- Photo Source; Tyler Nienhouse
