LED grow lights: What to look for

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LED grow lights: What to look for

So what colours of light should an LED grow light have?

The research into the question of what are the ideal LED grow light specs has been confusing to say the least and because of this the products on the market have varying combinations of LED colours and intensities. We think this goes a long way to explaining the differing opinions and experiences people have had with respect to the effectiveness of LED grow lights on plant growth. We think we can make more sense of what makes a good LED grow light by first analysing what wavelengths plant pigments are absorbing.

We debated on whether to include all the detail about plant pigments in this article but ended up deciding that for most people, the scientific information would be sleep-inducing, even though we find it quite fascinating knowing what's going on inside those leaves! So, instead we created a separate page to summarise the more scientific findings here. Suffice it to say, that so far, all the pigments identified in green plants absorb light wavelengths in the UV, blue, red and far-red regions of the light spectrum while ignoring (or avoiding!) the green and yellow regions. This makes sense from the standpoint of what we see: plants are green precisely because they don't absorb these wavelengths, instead reflecting them back at us (note: we only see the reflected yellow / orange colours in the fall when the green chlorophyll has degraded).

Table showing the light wavelengths which plant photosensitive pigments use to implement their physiological functions

So from the plant pigment absorption information, we can deduce that the ideal LED grow light should have the following light sources:

  1. Red LEDs within the range of 620-710nm with special attention provided for the 660nm wavelength
  2. Blue LEDs within the range of 400-495nm (preferably 430nm)
  3. UV LEDs within the range of 280-400nm
  4. Far-red LEDs within the range of 710-850nm (preferably 730nm)
  5. Orange light at 610nm??

NB: one key point to keep in mind is that plant photosensitive pigments typically absorb light over a narrow band of wavelengths around a central peak. Therefore if LED wavelengths do not exactly match the ideal wavelengths required, for instance let's say we have 650nm LEDs in our grow light instead of 660nm ones for red light, that's still OK since plants will likely absorb sufficient amounts of 650nm light to equal their 660nm light requirement, so we have some leeway.

What colours of light do we get from traditional grow lighting?

How do the electric lighting systems that we use now stand up to the lighting needs of plants? What wavelengths do they emit? Below is a table detailing the colours/wavelengths that are emitted by sources of light currently used by horticulturalists and indoor gardeners:

Table showing the peak emission wavelengths of different light sources

Clearly, most of the current lighting systems in use today don't cover all the wavelengths that are required by plants, or if they do, they also include wavelengths that are not required and possibly detrimental to plant growth. One way growers get around some of these issues is to use different combinations of lights, for example, High Pressure Sodium (HPS) in combination with Metal Halide (MH) will cover the full spectrum but this has high cost and low life-expectancy issues. The HPS / MH combo, like the full-spectrum fluorescent lighting, also emits in the yellow and green light wavelengths, which as we have said before, would be preferable to avoid. So, our current electric lighting systems appear to be both inefficient and more costly, and may even be preventing us from reaching the full potential of our gardening ambitions! So there is certainly an argument for using LED grow lamps if we could only just get the lighting characteristics right...

A word about White LEDs...

We've also seen some LED grow lights containing white LEDs which appear to be included in the light fixture to provide the 'other' important wavelengths in addition to the essential red and blue ones - how effective they are is debatable but there are only two common ways to generate white light from LEDs, neither of which seems to sufficiently cater to all the wavelengths that we've outlined and may even be producing lots of the undesired green/yellow light. We detail our findings on how white LEDs are made here and why we think they should NOT be part of your LED grow light specifications.

What else should you be looking for in an LED grow light?

OK so now that we know what LEDs should be present in our ideal LED grow light, we can go ahead and press that 'pay now' button and begin fantasising about how impressed our neighbours will be with our exceptionally green fingers, right? Not so fast.... there's more...

  1. High-power LEDs - For the serious horticulturalist, we would avoid anything in our LED grow lights that is below 3-watts (just as a point of reference, the typical electronics LED is 0.02-watts or less). The current standards in LED grow lights today are 3-watt or 5-watt LEDs. However, we did notice some chatter about the current state of 5-watt LED technology being at a stage where they are still not as electrically efficient and have a shorter lifespan than 3-watt LEDs. We are not sure how true this is, but to be on the safe side, the prudent course of action is to lean towards 3-watt LEDs for the moment since they have proven their worth so far.
  2. Brand-name LEDs - It is also best for LED grow lights to only contain brand-name LEDs. This is because big-name companies extensively test their LEDs to ensure that their life expectancy is within the range that they specify. They have a reputation to uphold after all!
  3. Metal core, cooling fans and heat sink - Even though LEDs produce far less heat than other types of lighting, they still produce some heat. While this is unlikely to be detrimental to plant growth, it is detrimental to the LED lifespan. The way high-quality manufacturers have tackled this is to place each LED on a metal base attached to a heat sink providing a path for heat to escape, and complemented by the presence of electric fans. These are an absolute must for the high-power LED in a grow light.
  4. Constant-Current  - Cheap LED grow lights are powered by constant voltage circuitry. As the LED’s temperature increases, its voltage tends to drop, causing a cheap LED driver to compensate by increasing the voltage it supplies, inadvertently increasing the current as well. This increased current ages the LED prematurely reducing its life expectancy. The better LED grow lights have circuitry that keeps the current steady instead of the voltage and probably gives you better value for money in the long run.

So what to look for in that 'Perfect' LED grow light?

For anyone who doesn't want to bother reading all of our drivel above, here's the summary of what we think an ideal LED grow lamp should have, which should help in the decision as to which LED grow light one should buy:

  1. Red LEDs within the range of 620-710nm with special attention provided for the 660nm wavelength
  2. Blue LEDs 425-495nm
  3. UV LEDs 300-400nm
  4. Far-red LEDs 710-800nm (especially around 730nm)
  5. (Avoid white LED bulbs in your grow light if you can)
  6. Bulbs should all be brand-name 3W LEDs
  7. A cooling system consisting of a metal base,  heat sink, and multiple cooling fans
  8. Constant-current circuitry to run the LEDs