Why Are Some Grow Lights Purple? 3 Key Reasons Why We Think You Should Avoid Them

Have you ever wondered why some grow lights are purple? Do purple grow lights actually work better for plants? What are the benefits of a purple vs a white grow light?

In this article we will explain both the science and history of why some grow lights continue to be purple, as well as offer 3 key reasons why we think you should avoid purple grow lights (and use a full spectrum 'white' one instead).

This blog is also available as a YouTube video, below. 


Why Are Some Grow Lights Purple?

The Science Behind Purple Grow Lights 

The most succinct answer to the question of why many grow lights emits a purple-looking light, is that they consist of red and blue LEDs (or 'Light Emitting Diodes'). When you mix red and blue together, you get purple. 

Scientists, horticulturalists, and LED manufacturers have focused on the red and blue frequencies of light as these frequencies of light are two of the most important for photosynthesis. 

As noted by Yan et al:

  • Red light plays an important role in controlling the functions of the chloroplast, stem and petiole growth, and the reproductive system. In other words, red light is important for fruiting and flowering. 
  • Blue light affects plant growth, leaf expansion, photomorphogenesis, stomatal opening, photosynthesis, and pigment accumulation. That means that blue light is important for vegetative growth. 

Essentially the idea behind purple grow lights, is that you only focus on the most important light frequencies and forget about the rest. This is a fine idea in theory, however as researchers and scientists are learning more and more about how plants respond to other light frequencies it is becoming increasingly apparent that whilst red and blue are important, they are not the only frequencies of light that are important for plant health. More on this below. Let’s first take a quick history lesson to understand how we got here. 

Why Are Grow Lights Purple? A Brief History

The world’s first LED was red in color and invented over 50 years ago in 1962 by 33 yo General Electric employee Nick Holonyak, Jr.

By the early 1970s, we had figured out how to make green LEDs, and by combining them with the original red ones meant we were able to make yellow. Now, in case you’re casting your mind back to kindergarten paint classes right now and thinking “red + green = yellow” doesn’t sound right, remember that mixing light doesn’t work the same way as mixing colors does. See below.

It wasn’t until 1994 that Shuji Nakamura used Gallium Nitride to make the world’s first blue LED. By combining red, green, AND blue we were able to generate a LED light that looks white to our human eyes. 

Philips was the first company to produce LEDs with the Edison screw fitting in 2009, which means that even if you had the oldest LED screw light in the world, it’d be barely a decade old at this point. 

Like most technologies, when they first came out LEDs were pretty lousy in terms of cost and efficiency – but they’ve come a loooong way since, as the chart below illustrates.

So how is all this relevant to today’s purple grow lights? Glad you asked. In the remainder of this article we we will cover the 3 Key Reasons Why We think You Should Avoid Purple Grow Lights

Reason 1 – The Economics Of Purple Grow Lights Don’t Make Sense For At Home Growers

Early manufacturers of LED grow lights recognized that they could essentially ‘construct’ any light recipe they wanted using specific combinations of LEDs, and given that 1) they didn’t know as much about plant health as we do today, and 2) LEDs were expensive and not that efficient at the time, they opted to leave out all of the 'non-essential' frequencies of light and focus on delivering only those frequencies that were absolutely critical to plant growth. And hey – we get it. Once upon a time, this was a very sensible approach. 

But the times have changed. LED technology has gotten waaaay cheaper and more efficient over the last decade. Both the cost of manufacturing, and the cost of operating these lights has come down significantly. What that means is that the costs savings you get from omitting these 'non-core' (i.e. non-red and non-blue) light frequencies are far less than they once were. 

A typical 10-30W grow light for indoor plants is going to cost less than $20 a year in electricity. The savings associated with leaving out the 'non-core' light frequencies is therefore only a dollar or two per year in terms of electricity cost savings. 

Now, don’t get us wrong – we’re all up for being energy efficient, and if you were going to fill an entire warehouse with grow lights and run a vertical farming operation then sure – purple grow lights might make more sense. But given the other disadvantages of purple lights (see below) we think this historical efficiency argument just doesn’t have the weight it once did. 

Reason 2 – Red and Blue Frequencies Are Not The Only Ones WhichThat Are Important For Plant Health

Source: Samsung

Much of the ‘foundational’ knowledge we have about grow lights were from the latter part of the 20th century. It includes:

  • The McCree Curve, 
  • The ‘importance’ of red and blue light, as well as
  • The idea that PAR is only 400-700nm and frequencies outside this range don’t matter to plants. 

The problem is that when McCree et al did their work on how plants respond to light, they only had access to light filters, analog equipment, and were only able to assess plants’ response to light one color at a time. They did not consider how these light frequencies work together and complement each other, nor did they have access to the sophisticated measuring equipment that we have today. As a result, a lot of this ‘foundational’ grow light knowledge is now in the process of being proven wrong.  

The invention of single-color LEDs has allowed us to learn much more about how plants respond to light. Professor Bruce Budgee of Utah University is one of the leading researchers in this field. Thanks to the work of Professor Budgee and others, we now understand that:

  • UVA / UVB (less than 400nm) – impacts essential oils, taste, and flavor
  • Green – is incredibly good at canopy and leaf penetration, much more so than red and blue
  • Far Red (700-760nm i.e. not even counted as ‘PAR’ light) – enhances cell expansion, and is a particularly good complement to blue

To purchase a blue/red (purple) grow light means you are ignoring the last 20 years of research and science on how plants respond to light. We just don’t think that ignoring science is a smart thing to do.

Reason 3 – Purple Grow Lights Are Just Not That Pleasant To Live With

The first ever ‘beta’ test we performed at Urban Leaf involved setting up a group of about 12 people with a very basic Kratky hydroponic setup and a grow light. We split the group in two – half had white lights and the others purple. Although the purple light users thought they were ‘cool’ at first, the novelty quickly wore off. By the end of our trial, most of the purple grow light users had either found a way to cover their units with a cloth to shade the light, or they’d relocated them to a cupboard or wardrobe. 

Given the hours we’d spent assembling these units by hand, we were pretty disheartened to see people going to these extents to ‘hide’ our products. It taught us however that purple light is just not a pleasant color to have in a living space.

Want another reason? Purple light is also shown to make you feel sleepy and reduce sexual desires. Doesn’t sound very fun, if you ask us :)

Purple vs White LED Grow Lights – A Better Option?

Hopefully by now, it is clear why we do not recommend purple grow lights for small-scale at-home growers who are looking to set up a garden in their living space. 

A better choice, in our opinion, is a full spectrum light that will ensure optimal plant health and a more pleasant environment for you, too. The Kelvin scale is a useful reference for evaluating ‘white’ grow lights. If you live in a very hot environment, and you want the inside of your home to feel cool then look for a light rated 5000-5500K. The spectral distribution of these lights skews towards blue making them feel a little cooler. If you live in a colder climate or have a small space, we recommend a warmer light temperature, in the 2700 – 3500K range. These skew red, and have a warmer glow, making them arguably a little better for fruiting and flowering compared to a higher K globe which would be better for vegetative growth. 

If you’d like to learn more about finding the perfect grow light for your at-home setup, we recommend Growing Plants Under Lights: A Guide To Buying Your First Grow Light.

aspect grow light

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