So you’ve got your grow bulb, something to hold it in place and a timer. Now it’s time to set everything up.
Getting the placement of your grow light correct is a crucial, but often over looked step. The difference between a correctly and incorrectly placed grow light can be the difference between a plant that thrives, and one that dies. In this blog we’ll be discussing how to get the placement of your grow light correct.
Point Sources of Light: The Key To The Right Distance from Your Plants
One of the most crucial concepts to get your head around here is how point sources of light behave with distance. The sun is located about 93 million miles away from earth, so moving something a few inches further away or closer to it is inconsequential. The artificial lights that we use to replicate the sun are know as a point source, and the impact of moving a plant a few inches further / closer to a point source are significant. There’s actually a mathematical relationship that describes the relationship between distance and light intensity from a point source – it follows what’s know as an inverse squared relationship. Essentially what that means is that every time you double the distance from a point source (moving from say 4 inches to 8) the intensity of the light goes down by 1/4, or a factor of 0.25 – in other words a useful light intensity of say 1,000 PAR becomes 250.
We recently purchased and tested 4 of the top selling grow lights on Amazon (details below) to see just what this light intensity / distance relationship looks like, and we’ve summarized it below.
You’ll notice that these curves don’t follow the ‘inverse square’ ( x 0.25) relationship exactly, and that’s because they not all ‘pure’ point sources. Many of these lights have what’s know as optics – or small lenses right near their light emitting components – which help channel the light in a specific direction (i.e. towards the plant). The manufacturers of these globes are evidently aware of this light degradation issue, and they’re doing what they can to offset it via these optics.
What is PAR Value?
Plants use different types of light to fuel their growth than what we use to see with, so one of the tricky things about assessing light intensity on behalf of a plant is that our human eyes are incredibly bad at judging it. That’s why in the plant world PAR meters are used. A PAR meter measures light intensity which is available to plants.
If you’re willing and able to fork out for a PAR meter (they’re a few hundred bucks) then they are your best option, and if you do then we recommend placing your grow light or lamp such that you’re getting a reading of between 500 – 1,500 PAR on the plant’s leaves. For some grow lights this will mean 8″ away or more, for others it would mean less than an inch. No two lights are the same, and if you’re interested in learning more about what to look for in a grow light we recommend this YouTube video.
how to Figure out the Best Distance from your light to your plants without a PAR meter
We realize however that not everyone is going to want to shell out for a PAR meter, which is why we’ve come up with another option 🙂
- Make sure you have a plant-specific grow light. If you don’t get this right, then the subsequent steps won’t work.
- Download a light metering app. There’s a bunch of free options. We use one called Light Meter.
- Now open the app and take a reading from either outside on a sunny day, or when you’re standing in a sunny window with the sun shining directly in. Take not of this reading.
- Now go back to your indoor garden / grow light setup, and take a measurement of the light intensity at your plant’s leaves when it’s under your grow light.
- Divide one number by the other, and you will have a sense (in the form of a ratio) of your bright or intense your grow light is relative to sunshine.
If you’re like most people, you will be surprised at just how ‘weak’ your grow light is compared to the sun. Chances are your grow light, at even a few inches away, puts out a fraction of the energy that the sun does. Kinda surprising when you remember that the sun is 93 MILLION MILES AWAY, huh??
Unfortunately there’s no magic formula to convert lux (which you measured with the app) to PAR (which is what matters to plants), but hopefully the above experiment helps you appreciate just how critical distance is for plants. Perhaps you could try the light meter app at different distances as well to get a sense for the inverse square relationship we were talking about.
Some Rules Of Thumb for estimating distance
Unfortunately there’s no ‘one size fits all’ answer to the grow light placement issue, however if we had to generalize then this is what we recommend for fast growing, lighting loving, edible plants such as the herb kits that we sell.
- For grow lights without optics (i.e. no lenses on the surface to direct the light) – make sure you use a lamp shade and place the globe within 1-2″ of the leaves. You’ll need to check on your plant regularly to make sure the leaves don’t get burnt as they grow towards the light!
- For grow lights with optics (such as the one we sell) – you can afford to be a bit further away. We recommend something in the 4-6″ type range.
Start Growing Today with LED Grow Lights
Thanks to huge advancements in LED technology indoor plant lighting is more affordable and accessible today than it’s ever been. If you’d like to learn more about indoor plant lighting then we recommend our separate YouTube playlist.
If you’re ready to get setup, then here’s a shopping list of parts and accessories we recommend:
LED Grow Lights
We like these ones because they have built in optics and good PAR per Watt energy efficiency. In other words they create a lot of plant-available light for each watt of energy they consume.
Lamps and Stands (for your LED Gor Light)
This is highly subjective. Just pick one you like the look. These ones all work well.
Timers (to automatically schedule light)
Other Stuff (For the true plant geeks)