18 LED Grow Light Myths You Should Know About

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Robert Pavlis

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
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.

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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.

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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, provided by Fluence Bioengineering
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.

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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 lights, spaced farther apart then on commercial units.
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 Blurple

“Blurple” is the industry name for the light produced by many LED grow lights. Since most of these lights contain a lot LED grow light mythsof blue and red bulbs the result is a blurple.

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
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
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
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.

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.

Quite a few LED grow lights have switches for veg mode and flower (bloom) mode. Veg mode has more blue light relative to red light but also has less total light. This can be a problem with lower end lights. Flower mode is the veg light plus more red. This mode usually grows the best plants even if they are not flowering.

The science of the effect of color on blooming is much more complicated than gardeners are led to believe as it also depends on whether a plant is a long-day (LD) plant, a short-day (SD) plant or a day-neutral plant (ND) as explained in this diagram.

diagram showing the complexity of making plants boom
The effect of color on blooming is much more complicated than gardeners are led to believe, source: Trivellini et al

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 you should ignore it. The feature is not worth buying.

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:

  1. Photo Source; Tyler Nienhouse
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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

63 thoughts on “18 LED Grow Light Myths You Should Know About”

  1. I’m a real novice to lights and was planning on trying an led one for my house plants during winter. Can you rotate them around the led light as I ha e many plants

    Reply
    • How are you defining “rotate around”. Plants need light all the time. Moving them to a dark corner every second week will not be good for them.

      Reply
  2. Hi Robert in particular could u perhaps let me know what light set up I need to grow house plants and make them extremely happy. I have an area I want to fill with plants but my house does not fill with acceptable sun. I have a plant problem and you seem to be the best person to ask here x

    Reply
  3. Useful info for the people that want to get into growing and dont know anything about it. Although I would not trust all sites and posts. To complement this and add some more basic knowledge You should look for bruce bugbee’s channel on youtube, Lots of useful information and sciencie behind what he says. For a personal suggestion I would go with not to cheap out on lights. What you pay for is what you get so save some money, do your research to know whats best for you and decide based on experience. Ive been running with the same company of lights that I started and never had to change because their lights are the best, if something doesnt work, change it. Iluminar lighting for the win!!

    Reply
  4. Hello! Thanks for the article but I am still a bit confused by the light I am considering. Why are there 2 PPF measurements, why aren’t they providing a Kelvin rating? I know I need about 6500k for my aloe and I’ve been trying to determine if this light is sufficient. TIA for expert insight you can provide!!

    Reply
  5. Myth that white light is better than blurple is in fact true. You contradict your self saying wide range is better but you say white light is not needed. Wide range is white.

    Reply
    • Re: “white light is not needed” – this is clearly true since you can grow plants quite well with burple light.
      Re: your statement “wide range is white” – that is necessarily true. You can have a wide range of blue light and that is blue – not white.

      You are forgetting an important point. A white light can have a lot of yellow and green and look white to use. A light of similar energy output with lots of blue and read is better for the plant because it is able to use these colors better.

      That is not to say white light is bad – I never said that. But many manufacturers lead customers into thinking that just because it is white it is better. That is not necessarily true.

      Reply
  6. Thank you for the great article! I have 2 Braun 4ft. LED Shop Lights (product code: 64410) that I am considering for my grow set-up for 100% indoor plants.
    120 VAC/60 Hz/0.5 A
    Are there any concerns associated with using these lights for all the stages of plant growth (seedlings/vegetative/flowering)? I of course plan to control the distance of the lights to the plants.

    Reply
  7. Violet
    Friday February 5th, 2021
    I happened upon your article and found it interesting and informative. I have very little sun that comes into my house and I love gardenia’s. It has many buds but its been weeks now and they don’t seem to be growing. I know gardenias like a good amount of light so I invested a small amount in an LED plant light. I am a novice but I would like to know at this stage of growth should I just use the red light to promote flowering or should I use both? Also, how long should I leave it on for a day, how far away should the lights be and can the lights burn the plant? LED quality is 39 Red + 21 Blue = 60 PCS. LED light efficiency 30W and Input Voltage AC 100-240V, 50-60Hz. I’d appreciate your advice.

    Reply
    • Follow the manufacturers advice for distance. I would not bother changing light at different times of the year – it really is not needed for home use.

      Reply
  8. Hello! I can’t change the kind of light fixture overtop of my Juniper Bonsai because of it’s location, and I cannot add an extra light, but what I’ve done is change my chandelier lights to LED ones that work by wifi to turn at 4AM and stay on until 9PM. These lights are about 4 feet higher than the bonsai plant. They’re turning on as a “warm white”, would it be better to have these LED lights come on as red or blue? Thanks!

    Reply
  9. 1 am new to starting seeds indoors my question is will the led lights listed below be good for seed starting.
    Freelicht 6 Pack Linkable LED Utility Shop Light, 4000 LM, 4 FT, 48 Inch Integrated Fixture for Workshop, 40W Equivalent 260W, 5000K Daylight, Surface + Hanging Mount – ETL

    Reply
  10. I’ve been using inexpensive LED shop lights for grow lights for several years; what I’ve noticed is that the plants germinate fine but once the plants develope true leaves the leaf color is not quite as green as I would expect. I have some spinach growing now and have decided to move the trays into an unheated greenhouse on warmer days this winter to see if the color improves.

    Reply
  11. On a budget; just want to start some annual flowers. Was hoping could get away with using 6500k T5 shop lights, but concerned about the growth AFTER germination and pricking out.

    Looking to supplement my lighting with a few more bars; would it be wise to purchase something in the “warmer” spectrum of your typical shoplight–3500-4100 (less readily found but still cheap)? Or do you think it doesn’t really matter?

    Appreciate it and love your books!

    Reply
  12. Does having an LED light lower to the floor; the light is clipped lower to provide the light to the base of the cactus, have a damaging effect on our eyes or skin? I have it lite in a central location and didn’t consider the colors that reflect on the walls or floor could cause any harm. Thank you for your time.

    Reply

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