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How Far Should Your Grow Light Be From Plants?

Getting the placement of your grow light right is critical to making sure it is delivering the right amount of light in your plant. In this blog, we will 1) start with a quick recap of the ‘theory’ behind this concept while offering some links for additional reading (in case you’re interested), and 2) provide some more practical recommendations that are quickly and easily implementable. We won’t be offended if you want to skip ahead 😉

Grow Light Distance From Plants – The Theory (aka Long Answer)

Grow Lights and Daily Light Integral (DLI)

In order to know how far your grow light should be from your plants, there are a few things we need to know. Firstly, we need to understand the DLI or Daily Light Integral for your plants. This is the plant worlds’ equivalent to us humans and our calories. If you don’t give them enough light energy each day, they will wither and likely die. In our accompanying article titled How Long Should You Leave Grow Lights On For? we include a comprehensive list for edible plants, but if you’re looking for an approximate range then most edible plants will require 10-30 mol/m2/day . 

Grow Light Distance is Impacted By Grow Light Timing

The next thing we need to know is the optimal light duration for this plant. As we discussed in How Long Should You Leave Grow Lights On For? all plants have an optimal duration of light – which will either encourage or discourage flowering – depending on what your objectives are. 

Once we know a) the total volume of light that a plant needs to be happy (i.e. DLI), and b) the ideal time/duration in which we should deliver it, then we can simply divide one number by the other in order to calculate the ideal delivery rate of that light. This is exactly the same math you would use if I told you we had a 10-gallon bucket and it needed to be filled over a 5 hr period. You’d simply divide 10 by 5 to deduce that you needed to be filling the bucket at 2 gallons per hour. 

This equivalent of a ‘2 gallon per hour’ flow rate for light is called PPFD, or photosynthetic photon flux density, and it is measured in umol/m2/sec. Growing a healthy plant indoors will require some knowledge of both the target PPDF and the duration (i.e. “5 hours”) such that you can deliver the right volume of light, or DLI (i.e. “10 gallons” in our bucket example).

Grow Light Distance is Determined by Grow Light Power (PPF)

Once we know the ideal DLI and duration of light we have an ideal PPFD. The PPFD that a grow light delivers is a function of two things: 1) distance from the grow light, and 2) power of the grow light.

Grow light distribution is similar to that of a sprinkler

Going back to our bucket analogy, think of your grow light as a sprinkler head. Just as sprinklers distribute water in multiple directions, grow lights do the same with light. If you stand right next to a sprinkler, you will get drenched pretty quickly. But if you were further away, you might only feel a very subtle sray. The same concept applies to grow lights. 

Let’s say you found your ‘sweet spot’ under the sprinkler with your 10-gallon bucket, and you find a spot where you can get exactly 2 gallons per hour for a total 5-hour fill. What would happen if we suddenly doubled the water output of the sprinkler? Well, we’d probably expect your fill rate to increase from 2 to say 4 gallons per hour. In order to reduce our fill rate back to 2 gallons per hour, what would you do? You’d move away from the sprinkler of course! Again, the same concepts apply to grow light. 

The best distance for placing a plant from a grow light is a function of the target PPFD you are trying to achieve (flow rate of light) as well as the power of the light. In the lighting world, we measure this ‘power’ as PPF or photosynthetic flux. All grow lights perform a similar function; they convert Watts from your wall socket into PPF for the plants. All grow lights are not created equal, however, and some are much more efficient at converting Watts into PPF than others. In fact if we divide these numbers together we get PPF/W, PPE, or ‘efficacy’.

If you are interested in learning more about grow lights and how all of these terms and measurements are related we recommend Grow Lights For Indoor Plants – How To Measure It, and Understanding Watts, PPF, PPFD and DLI

Grow Light Distance From Plants – In Practice (aka The Short Answer)

Thank you, fellow plant nerds, for sticking with us through the geeky bits there. We think you will agree that once you understand some of the underlying principles around how light is emitted, spread and accumulated, this whole grow light thing becomes a lot easier to wrap your head around. 

What follows is some practical guidance for grow light usage based on various types of indoor edible plants. These recommendations are based both on the above ‘theory’ as well as our own experience in testing, measuring and evaluating dozens of commercially available grow lights. 

Best Grow Light Distance For Seedlings?

When plants are young, at the seedling stage, their energy requirements are typically pretty low. Some varieties of edible plants actually prefer complete darkness to germinate. More on that here. 

Generally speaking though, any window that has a decent amount of light coming through it is going to provide ample light for seed starting. If natural light isn’t an option for you, then don’t get too hung up on grow light placement – just use whatever you have and make sure it is no closer than the ‘lettuce’ recommendations below for grow light distance from seedlings.

Best Grow Light Distance For Lettuce and Leafy Greens?

Once you start getting into leafy greens like lettuce and herbs, the light requirements do start to increase, and it is, therefore, important to be delivering the right quantity of light. If you wanted to take the ‘scientific’ approach to this, the theory above as well as Grow Lights For Indoor Plants – How To Measure It, and Understanding Watts, PPF, PPFD and DLI will guide you through it. Alternatively, you can run with the guidance we have compiled below. We’ve included 6 columns, representing 10, 20 and 30W lights as well as a ‘with optics’ and ‘without optics’ scenario for each. We’ve also included some approximate average PPF/W (or PPE) assumptions as well as our CU assumptions for each scenario. The values we include are typical for E26 screw-in globes such as the GreenLite and Aspect ones we offer. If you know the actual values for your grow light, and they differ from our assumptions, then you’re welcome to input your own assumptions using our Grow Light Distance Calculator (below).

Depending on the wattage of E26 you are using, and its inclusion of optics, lettuce may require anywhere from 3 to 13” distance from the grow light, as shown above. This table assumes that we are delivering a DLI of 10-15 which is typical for lettuce. 

Best Grow Light Distance For Tomatoes and Other Flowering Plants?

When it comes to tomatoes the DLI requirements increase significantly, and hence your grow light will need to be a lot closer. 

Again, depending on the wattage of your E26 grow light and its inclusion of optics, we recommend a grow light placement of 2 – 10” from the plant. Lower powered (eg 10W) lights that don’t have optics are not even worth bothering with for flowering plants, in our opinion. You ideally need a more powerful light and/or optics to help channel the light towards the plant. This table assumes that we are delivering a DLI of 20-30 which is typical for flowering plants such as tomatoes and peppers.

Grow Light Distance Calculator

We hope you’ve found this guide to grow light placement useful for setting up your indoor garden. If you’d like to play around with some other scenarios or run these numbers yourself, then you’d be welcome to check out our free online LED Grow Light Distance Calculator. We’re putting the final touches on it right now, but leave a comment below if you’d like to receive a copy so that we can email you when it’s done. 

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