LED Grow Light Distance Chart From Plants. How Far Should Grow Lights Be From Plants?

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. provide some more practical recommendations that are quickly and easily implementable (aka Short Answer).
  2. offer an explanation of the science behind light distribution and placement in case you’re more of a details person (aka Long Answer).

Since Urban Leaf’s focus is on edible plants (i.e. food) our recommendations are specific to these types of plants. Light requirements for ‘decorative’ houseplants are very different. We should also note that the recommendations and guidance we are offering here is most-applicable to E26 style screw-in fittings, like the Vita

This blog is also available as a YouTube video if you’d rather watch than read!

How Far Should Grow Lights Be From Plants (SHORT Answer)

The correct distance between a grow light and plant is determined by the following variables:

  1. The type of plant. Within the (edible) plant world we can categorize most varieties into one of two categories. 
    1. Lettuces, herbs and leafy greens are generally going to require a DLI of 10 - 15 umol/m2/day. 
    2. Fruiting plants like peppers and tomatoes will require more like 20 - 30 mol/m2/day.
  2. The wattage of your globe. More watts generally means more light energy emitted. More light energy means greater distances. In our store, you will find lights that range from 7W up to 40W.
  3. Whether or not the globe has optics. Having a powerful wattage is one thing, but if that light is uncontrolled and its ‘leaking’ all over your living room wall it’s not doing your plants much good. Grow lights with optics help channel the available light energy in the direction of the plants. Since the light is more focused, distances can generally be great when using lights with optics (also known as a ‘narrow’ beam angle). Because we care about grow light efficiency and plant health, the only type of lights we offer are ones with optics. 
  4. The plants stage of maturity. Seedlings (like babies) need very little light (energy) therefore greater distance and acceptable. Mature plants (adults) need more energy, therefore distances will generally need to be shorter such that the light is more intense.

Our recommended grow light distance for lettuce, herbs and leafy greens is 3 - 13 inches as shown above, with the exact value being based on the wattage and whether or not your light has optics (narrow beam) or not (wide beam). 

Our recommended grow light distance for fruiting and flowering plants (eg tomatoes, peppers) is 2 - 10 inches, again based on the same wattage/optics variables. 

Grow Light Distance from Plants – The Theory (LONG Answer)

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 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 bulbs 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, herbs & leafy greens’ recommendations provided below. 

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, Vita 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 Chart or Calculator (access that here).

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 fruiting and flowering plants like tomatoes and peppers 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

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