How High Should Your Grow Lights Be Above Your Plants?

Photo: Gardeners

You've got grow lights, your plants, all of your liquid nutrients, and everything you need to grow.

You set up your grow tent, set your plants in place, strap your rope hangers to your lights and hang 'em. But before you switch those lights on, are you sure you know how high you need to hang them above your plants?

Hanging your lights too low can burn your plants, but hanging them too high can leave them stretched and weak. Some grow lights need to be hung far away from plants so they don't get bleached, but some lights are too weak to hang more than 10 inches away- so what do you do?

The trick is to hang your lights right in that sweet spot of effectiveness and safety. The distance of your lights to your plants is all about getting your plants the power they need to turn nutrients into food for growth. So before we start talking numbers, let’s talk about the idea behind hanging your grow lights:

Why do different grow lights need to be hung at different heights?

If your indoor garden is supposed to mimic a plant’s natural (outdoor) environment, then the grow light is the sun from which everything grows- only you have control over what type of light and how much of it your plant receives. That's partially why growers say more light=better yields. But does that mean you should get more lights and hang them as close to your plants as possible without touching them?

Of course not.

It means making sure they receive enough energy to grow quickly without overpowering them. But to assure that your plants are getting exactly what they need- no more, no less- you’ll want to check the type of light you’re giving your plants: L.E.D., H.I.D., or T5 Fluorescent lighting. Some lights offer more heat energy, and others offer better spectrums, and each needs to be hung in a different distance from your canopy:

LED grow lights, for example, need to be hung farther away from your plants than HPS/MH grow lights to avoid bleaching the canopy of your grow room from strong spectrums.

On the other hand, T5 Fluorescent lights need to be hung closer to plants than HID’s in order to have an effect on them because the energy coming from T5's isn’t strong enough to get plants growing when hung from a distance of more than 12".

HID Grow Lights

When it comes to HID grow lights, it's best to keep them at least 12" (30cm) away from your plants at all times. The stronger the light, the further you'll need to hang it, so if you have a 1000w you may want to hang it at least 15" (38cm) away. Depending on the strength of your light, you won't want to go higher than 30 inches (76cm) above your plants. Smaller sized lights like 400w's can only be around 20" (~51cm) away from the plants, and medium-sized 600w's can be around 25" (64cm) above the plants without losing vitality.

HID's need to be hung above your plants at a good height because of the heat they generate, so if you're using HID's it's important to have a tall growing space:

(400w HID Grow Lights) 12"-19" away from plants
(600w HID Grow Lights) 14"-25" away from plants
(1000w HID Grow Lights) 16-31" away from plants

LED Grow Lights

Now, despite LED grow lights not generating lots of heat like HID's, their light spectrum can be overwhelming to lots of plant species if they're hung too close. When this happens your plants will suffer from light bleaching, which will impact how your plants take in sun and convert it into energy for growth.

Simply put, what makes a light a "cool" blue light, or a "warm" red light, is the temperature of the light's spectrum(s). LED's are much more focused on light spectrum output than energy generation, so you actually need to hang them higher above your plants than HID's to avoid light bleaching. Light bleaching can be just as bad for your plants as overheating them and will result in an underwhelming crop.

LED's don't generate lots of heat, but you'd be surprised by how high you have to hang them above your plants. Supplemental LED lights used to grow little patches of wheatgrass, herbs, and spices (15w-90w) need to be at least 15" away from your plants. For higher wattage lights it's even further:

(240-400w LED Grow Lights) 16-30” away from plants
(450-550w LED Grow Lights) 20-30” away from plants
(600-850w LED Grow Lights) 24-26 away from plants
(900w+ LED Grow Lights) 26-42” away from plants

Note: while HID's need to be hung at a consistent distance throughout the plant's growth cycle, LED's will need to be moved closer to the plant during the flowering stage.

Fluorescent Grow Lights

Fluorescent grow lights give you a nice, easy blend of energy and spectrum output. They effective enough to grow nearly any plant, but to do so, you'll need to hang them pretty close.

What makes hanging fluorescent lights so tricky is that not all of them are the same. For instance, CFL bulbs can generate some decent heat over an extended period of time because of their compact size, whereas a longer 54w 2ft T5 grow light won't generate lots of heat because it can dissipate it along its tube. Not only that, but the more bulbs you use the more heat you'll generate, which is dangerous if it's hung too close to the plants.

Fluorescent lighting isn't known for kicking off lots of heat, but they will generate some heat while they run so you'll need to watch your plants carefully for signs of burning or bleaching.

In general, you'll want to hang your light no more than 12" away from your plants.

This one's not an exact science, unfortunately, because fluorescents come in different shapes and sizes. That said, we recommend the following:
- Start out at 5 inches above the canopy
- Compact CFL bulbs usually generate more heat than a long T5 grow lights, so measure the canopy temperature to assure you're at a safe level to grow
- If canopy temps are too high, raise the light another inch and check the temp again
- Raise if the temps are too high, lower if the temp is too low
- Remember: after 12" fluorescent lighting loses its power, so make sure you don't raise it any more than 12" above your canopy otherwise your plants aren't getting light energy.

Editor's Note: This article was originally published July 2017 and has been updated on 3/23/2018 for quality, accuracy, and comprehensiveness