Guys. As an average joe who just wanted to help my indoor tropical plants out a little bit during Winter, I have never been more bombarded with more confusing information in my life.

Grow lights are a science. This blogpost is meant to give a very BASIC understanding and summary of what general information is out there. If you’re looking for a professional setup, please contact a professional.


I’m going to throw the complicated stuff at you first, then we can move on you can always come back and refer to it.

Kelvin (K): Colour Temperature of light. The higher the Kelvin the cooler and bluer the light (think of a bright blue sky). The lower the Kelvin the yellower and warmer the light (think of candle light). In general 2000 – 5000 Kelvin will be good enough to grow plants.

Lumens: The total output of visible light at the light source. Typically, the more lumens a light fixture provides, the brighter it is. LUX is the measurement of luminance on a particular surface and is calculated using the Lumens, distance from the light source, and other factors like the angle of the light source. One lux is equal to one lumen per square meter (lux = lumens/m2). Essentially, as light travels from the emitter, it will disperse throughout an area. The further the light has to travel the more it will be dispersed. Therefore, the amount of lux in an area or on a surface can vary depending upon the distance the light travels and the angle at which it is dispersed. Many lighting manufacturers quantify the output of their sources in this unit of measure, but it is not necessarily a good comparison value in horticultural applications.

Watt: A watt is a measure of energy consumption and really has nothing to do with how much light your plants will actually receive. Also wattage is based on a traditional incandescent light bulb energy consumption. So when you see wattage on newer bulbs they’ll often have two wattage measurements for example 15W, 100W. What this means is this new bulb uses 15 W to produce the same quantity of light that would take an incandescent bulb 100W to produce. So don’t get nervous that you’re going to blow your electric system with a 100W fixture. You’re really using a 15W bulb that has the equivalent of 100W ‘power’.

What do grow lights do?

We all know that sunlight is the most important factor in a plant’s growth. The light’s rays are a series of coloured wavelengths. Plants convert these wavelengths through photosynthesis into foods like starch.

A grow light is meant to try and replicate these wavelengths. Although nothing is as perfect as the sun, this technology can help a lot for plants that are grown in less than ideal situations.

The main spec a growlight needs to fulfill in order to stimulate photosynthesis and provide something meaningful for your plant is: A certain colour range. There is still some debate what this colour range should exactly be.

Red and Blue colours are reportedly the most important for a plant’s growth. Which is why you’ll see purple lights everywhere when you google ‘growlights’. However you’ll also see something called ‘broad spectrum’ growlights which essentially are white in colour. Remember the perceived white light is still made up of all the other coloured wavelengths including the blue and red. At a certain colour temperature (measured in Kelvin) broad spectrum light can provide the right mixture of colours. Recently there has been some speculation that these are better than the purple growlights because the other colours present also contribute in a meaningful way to a plant’s growth.

Lighting tech

Incandescent light bulbs: When you think of a typical light bulb, one thinks of Edison’s bulb with filaments. Society is moving away from these bulbs because they are highly energy inefficient. These would not work well as grow lights because they amount of energy they would have to use. The bulbs also give off a lot of heat (like 90% of their energy is dedicated to creating heat) and would likely damage your plant.

CFL (Compact Fluorescent Lamps): You’ll recognise these bulbs easily due to their coils. These guys are a lot more efficient than incandescent lighting options and for a long time we’re labelled them as the energy saving alternative to the traditional incandescent light bulb. So these bulbs produce a high amount of energy that will help grow your plants. They’re great for small spaces and are cheap compared to other technologies out there.

HIDs (Hight Density Diode Lighting): This technology has a very high output of light and are often used by commercial growers in the cultivation of indoor plants. There are two types, HPS (High Pressure Sodium) and MH (Metal Halide). I won’t be focusing much on this technology because its more suited to large scale growers.

LED (Light Emitting Diode): We’ve all heard of this technology and associate with the huge neon lights on buildings. LEDs are great for using as grow lights because the individual chips or diodes are capable of producing a specific narrow ranges of colours where needed. The purple grow lights previously mentioned are LEDs. You also get Broad Spectrum LEDs that emit white light.

How to make your choice

LED vs CFL | Broad Spectrum vs Purple

  • Budget: LEDs are more expensive and CFLs are cheaper.
  • Space: A smaller space with fewer plants will be fine with a CFL bulb. The larger the space, the more likely you’ll need a more powerful LED setup.
  • Environment: If your plants are in a green house or if they have access to a reasonable amount of natural light, then a purple light set up which promotes important colours in good. However if your plants have limited access to natural light and all the other colours, then the broad spectrum white light is probably best.
  • Usage: If you’re in the plant game for the aesthetics of it, the last thing you want is a great blaring purple light in your room. So then a Broad Spectrum LED lightbulb or a CFL would suit your purpose. These technologies also come in different forms. You get LED flood lights, little dotted LED light strips, very normal looking LED bulbs, CFL bulbs that could easily fit into any fitting and lampshade, CFL bulbs that need additional fittings. So if aesthetics is your end game, you might want to start with the approach on what you want your set up to look like and go from there.

Positioning of the light:

There is no one set standardised distance that a light will need to be positioned to produce a required effect. Apart from the bulb itself, you’ll need to consider the total light in the room, the amount of space you want to illuminate and any present reflective surfaces. When looking at a bulb, some sources point to looking at the Wattage to determine the distance it needs to be positioned, other methods of determining the quantity of light or brightness a plant needs is through measurements such as Lumen (or lux ), Foot Candle or PAR.

The science behind these measurements can get quite convoluted and make your head hurt. Also for every argument to use one measurement, there’s another argument against it. Also, remember a lot of the internet’s growlight resources are either dedicated to vegetables or cannabis- both which will need more light than your average indirect light seeking tropical plant.

However, a good rule of thumb, is to hold your hand up between the plants and light and is if it’s too hot for your hand, then its definitely too hot for you plants.

So what now?

Remember you want something between 3000-5000 Kelvin. And since manufactures just love using Lumen as a measurement, you might want to look for a bulb with around 300 and 800 lumens per square foot- according to the interwebs your plants should be happy with this. You can find a lux/ lumen calculator here to help you make the calculations should you wish.

Now let’s spend some dough! You want a CFL? Try looking here. No? A simple LED bulb? Then look here. Oh I see you, you want a funky purple lounge, try this.