How Far Should Your Grow Light Be From Plants?

Getting the placement of your grow light correct is super important for delivering the right amount of light to your indoor garden, and having happy indoor plants.

Depending on the type of light, and the plant you're growing, appropriate distances from plant to light can range from as little as 2 inches, up to 2 feet or more.

The goal of this blog is to help you understand the main factors that determine optimal grow light placement, so that your indoor garden can thrive. 

How Plant Type Impacts The Correct Distance For Your Grow Light 

Decorative indoor plants (eg Monstera, Snake Plant) have very low requirements and can survive on less than DLI of less than 4 umol/m2/day. Lettuce, herbs and leafy greens need at least 4-5x more light, and flowering plants like peppers and tomatoes require more still! (DLI's of 20-30 umol/m2/day are typical). All else equal, this means decorative plants are ok with grow lights being placed further away. 

How Wattage Impacts Grow Light Placement

Generally speaking, more watts (energy consumed) translates to more plant available light energy. All else equal, higher wattage globes will be placed further from the plant and low wattage globes need to be placed closer. Note that some manufacturers attempt to mislead consumers by quoting an "effective" wattage. This is irrelevant for plants, and we recommend ignoring it. 

How Beam Width Impacts Grow Light Placement

You'll notice that some grow lights just have 'naked' or exposed LED chips, which are basically throwing light in all directions, while others emit a much more concentrated array of light - this is usually achieved with some sort of light-concentrated lens, or optics. All else equal, a grow light with optics can be placed further away. The diagrams below will help make this point a bit clearer.

How Plant Maturity Stage Impacts Grow Light Placement

Seedlings (like babies) need very little light (energy) therefore greater distance are generally ok. Mature plants (adults) need more energy, therefore distances will generally need to be shorter such that the light is more intense.

Grow Light Placement For Lettuce and Leafy Greens

The chart below summarizes typical grow light placement for various types of E26 screw in grow lights. 

Let's check out a couple of examples:

• If you have a ~10W grow light with a lens (like our compact grow light) then you'd want it 7-9" from the foliage. 
• Or if you were using something more powerful like the Vita Grow Light (which also has optics) then 8.5 - 11" is going to be more appropriate.

Grow Light Placement For Fruiting & Flowering Plants

And here's the same chart for Fruiting & Flowering plants like peppers & tomatoes

You'll notice that the distances in this second chart are overall much closer. This is because of the higher light intensity these plants need. Grow lights without lenses/optics really don't make much sense here. At the 10W end of the chart you'll notice that the light would need to be within an inch or two in order to deliver enough light intensity. Since this is so close that you're likely to burn the leaves by touching them, we'd suggest you don't even bother trying fruiting plants if you're using this sort of light.  

Grow Light Placement For Decorative House Plants

Our focus here at Urban Leaf is on Edible Plants, so we are less familiar with the exact requirements for decorative plants (eg Monstera, Snake Plant), but if you're looking for a rough guide then we recommend taking the numbers from the 'Lettuce & Leafy Greens' chart and multiplying these distance by ~3x. For example, if you have a 30W globe with optics then you should try your grow light at 30 - 39". 

WARNING: The Remainder Of This Blog Is Intended For Plant Nerds Only

Grow Lights and Daily Light Integral (DLI)

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 

1. the total volume of light that a plant needs to be happy (i.e. DLI), and 
2. 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 (flow rate of light) 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.

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 spray. 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. 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 indoor plants 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’. One of the reasons that we offer the grow lights we do here at Urban Leaf, is that we have tested these units ourselves and found them to be amongst the best in their class in terms of their efficacy at converting watts into plat-available light.

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.  Looking for more gardening tips and tricks? Join our newsletter below!