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How High To Hang Grow Lights Above Plants – Based On Science!

grow light height

The most frequent question I’m asked by indoor growers is how high above plants they should hang their grow light. The short answer is “it depends,” but I hope to explain the deciding factors below.

Whether you are growing cannabis or any other plant that provides fruit or vegetables, your plants require a certain amount of light which means the grow light’s distance from plants will make a difference.

Recommended HPS / CMH / HID Grow Light Distance From Plants at 100% Power:

Watts:Grow Light Height Above Plants:
150w:3ft (seedling); 2ft (veg/flower)
250w:3-3.5ft (seedling); 2.75ft – 3ft (veg/flower)
400w:4ft (seedling); 3ft – 3.5ft (veg/flower)
600w:4.5ft (seedling); 3 – 3.5ft (veg/flower)
1000w:5ft (seedling); 4-4.5ft (veg/flower)

Recommended LED Grow Light Distance Above Plants:

Generally speaking, LED grow lights can be kept at 24-26 inches during the seedling phase of growth (first 3 weeks), and 18-22 inches during the vegetative and flowering phases. Hang the grow light a few inches higher if the light is powerful, or a few inches lower if it’s not too powerful (cost under $100).

Unfortunately, almost every LED grow light is different because they use different brands of LEDs, different magnifying lenses (or no lenses at all), and run a different amount of watts through the LEDs.

Most LED grow light manufacturers provide a recommended hanging height, but these are only estimates and are not always applicable to every situation.

These recommended LED grow light hanging heights are available on the manufacturer’s website and usually on the website you purchased the grow light from.

Below is a table of the hanging heights I recommend for the most popular LED grow lights during each phase of cannabis growth at 100% power. This is based on manufacturer provided light intensity readings, and may or may not be perfect.

Cannabis Growth Phase:Seedling HeightVeg & Autoflowers HeightFlowering Height
Mars Hydro SP-15028 in24 in18 in
Mars Hydro SP-25032 in26 in18 in
Mars Hydro TS-60024 in14 in12 in
Mars Hydro TS-100024 in18 in15 in
Mars Hydro TSL-200030 in24 in18 in
Mars Hydro TSW-200030 in24 in18 in
Mars Hydro TS-300030 in24 in18 in
Spider Farmer SF 60020 in15 in12 in
Spider Farmer SF-100030 in18 in14 in
Spider Farmer SF-200026 in22 in16 in
Spider Farmer SF-400030 in24 in20 in
HLG 300L28 in24 in18 in
HLG 55032 in26 in20 in
HLG 650R48 in36 in30 in
Bestva 600w24 in15 in12 in
Optic 6 Gen 448 in40 in32 in
Optic 4 Gen 4 40 in34 in28 in
Optic 2 Gen 434 in28 in22 in
Bestva 1000w26 in18 in12 in
Bestva 2000w30 in24 in18 in
Bestva 4000w42 in36 in30 in
Viparspectra P100022 in14 in10 in
Viparspectra P150022 in14 in10 in
Viparspectra P250024 in18 in14 in
King Plus 1000w24 in18 in14 in
King Plus 2000w28 in22 in16 in
King Plus 3000w34 in30 in24 in
Phlizon PH-200024 in20 in14 in
Phlizon PH-300028 in24 in18 in
Phlizon 1200w28 in18 in12 in
Phlizon 1000w COB28 in24 in18 in
Phlizon 2000w COB36 in30 in24 in
Phlizon 3000w COB46 in38 in30 in
Scynce Raging Kush40 in30 in24 in
Optic LED Slim 600H40 in30 in22 in
Green Sunshine ES 18030 in24 in18 in
Green Sunshine ES 30042 in32 in24 in

Keep reading below for a more exact & scientific approach to placing your grow light the proper distance from your plants.

Recommended T5/T8 Grow Light Distance Above Plants:

T5 and T8 bulbs can vary widely in the power output, and therefore the light strength and the hanging distance above the plants can vary. For example, a fixture with two bulbs can output 60 to 100 watts. But a fixture with four or six bulbs can output two to three times at much light.

Low power T5 and T8 grow light setups can be hanged 6 to 10 inches above plants. Higher power T5/T8 grow light setups can be hanged 12 to 14 inches above plants.

When Should You Adjust Your Grow Light’s Distance From Plants?

Growing too close to your grow light:

When your plants mature and grow closer to your grow light, you’ll need to either dim the grow light or adjust the distance from the plants. If you don’t adjust the distance or dim the grow light, you’ll risk burning your plants.

Heat Stress:

You need to adjust your grow light’s height above the plants if the plants are heat stressed. Heat stressed leaves appear to be curling upwards towards the light. Immediately increase the distance between your grow light and your plants if you see this happening.

heat stress cannabis
Stretched Plants:

Plant stems stretch when the distance from the grow light is too far. The plant intentionally stretches its stalk and stems in order to get closer to the light source.

Genetics can play a role in plant stretching as well, but if you notice this happening, you should hang the grow light closer to the plants. Remember every 12 inches closer to the grow light increases intensity 75%.

stretching cannabis plant

While this picture is an extreme example, ideally cannabis plant node spacing should be as tight as possible. If you experience significant stretching, you’ll likely need to top the plant to prevent it from growing too large.

Use The Inverse Square Law To Determine Grow Light Hang Distance Above Your Plants:

Grow Light Hang Height - EASIEST METHOD!

The inverse square law is a physics law that can help you to determine how intense your grow light is at a specified distance from your plants. All you need to know is the number of Lux emitted by your grow light at a certain distance.

The inverse square law also helps you understand how grow light intensity changes from distance to distance. Once you figure out how strong your grow light is at “x” height above the plants, you will know whether you need to raise, lower, or maintain the grow light distance from the plants.

The mathematical formula is Intensity = Light Output/Distance².

You only need to know one intensity reading at one distance to determine another intensity reading at another distance.

For example, if the manufacturer states that their 1000w HPS grow light emits 140,000 lux at a 1ft distance from the light, but doesn’t give you any other measurement, that may not help if you need to grow seedlings that’ll be burned by 140,000 Lux. But you can determine the light intensity at other grow light heights by using the law of inverse squares:

If 140,000/1 foot ² = 140,000 Lux,

Then 140,000/2 feet ² = 35,000 Lux.

And 140,000/3 feet ² = 15,555.55 Lux.

The law of inverse squares helps you understand the rate at which light intensity changes for each distance from the light source. The examples show that light intensity is much more sensitive to distance than you might think.

Likewise, if your 140,000 Lux HPS grow light has the capability to dim at 25% increments, then the formula would look like this:

140,000 Lux At 75% power at 1 foot distance: (140,000 *0.75)/1 foot ² = 105,000 Lux

140,000 Lux at 75% power at 2 foot distance: (140,000 *0.75)/2 foot ² = 26,250 Lux

140,000 Lux at 50% power at 1 foot distance: (140,000 *0.50)/1 foot ²  = 70,000 Lux

140,000 Lux at 50% power at 2 foot distance: (140,000 *0.50)/2 foot ²  = 17,500 Lux

Answer These Questions To Determine A Proper Grow Light Hang Height:

If you don’t want to use the inverse squares law or manufacturer recommended height, there are a few main questions to answer in order to figure out the proper grow light height for your grow space. You need to Measure, Convert and Compare.

What amount of light intensity do your plants receive at each height?
For LED Grow Lights:

You need to figure out (roughly) how much light your plants are receiving, and compare that number to what they should be receiving.

The easiest way to do this is by using a Lux meter. Unlike PAR meters they’re very inexpensive. You don’t need a PAR meter because you already know the light is within the PAR spectrum range.

Purchase a Lux meter and measure how much Lux your plants are receiving at the current distance from your grow light.

Then measure the number of Lux and convert it using the Waveform Lighting calculator. Insert the number of Lux, then select Red + Blue + White LED, and run the calculation. You’ll end up with a light intensity number measured as umol/second.

How many micromoles of light do cannabis plants need during each growth phase?”

  • Plants in the seedling phase require between 200-300 uMol/second.
  • Plants in the vegetative phase receiving 18 hours of light should receive between 306 and 617 uMol/second.
  • Plants in the flowering phase receiving 12 hours of light should receive between 460 and 925 uMol/second.

If your plants are currently receiving light intensity within the above range, you do not need to adjust the height of your grow light. Below this range would require moving the grow light closer to the plants. Above this range would require moving the grow light further away from the plants.

Remember, higher light intensity = higher yields. To read more about PPFD and light intensity levels required to grow cannabis, read my full article on the topic here.

For CMH and HPS Grow Lights:

People using CMH or HPS grow lights ask me about Lux and what Lux intensity is good for plants.

Here’s my advice for people using CMH or HPS grow lights: Purchase a Lux meter and measure how much lux the plants are receiving at each height.

Next, you need to convert your Lux reading to micromoles. Luckily, Apogee Instruments, the manufacturer of the light intensity meter created a conversion table for converting Lux to micromoles.

At 50,000 lux, a double ended HPS grow light emits 650 uMols of light per second and about 28 Moles of light during a 12 hour flowering light cycle, or 42 moles of light during an 18 hour vegetative phase light cycle.

That’s more or less on target with what cannabis plants need to grow. So if you’re getting this reading from your HPS light at 30 inch hang height, you’re in a good position. Close to or below 20 moles and you’ll want to lower the lamp until you reach a number up to 40 moles.

Here’s the math: (Your Lux reading * .013 Lux to PPFD conversion rate * 60 seconds in one minute * 60 minutes per hour * hours in your light cycle = Moles of light per day).

Is your CMH or HPS grow light intensity within this range?

  • Plants in the seedling phase require between 200-300 uMol/second.
  • Plants in the vegetative phase receiving 18 hours of light should receive between 306 and 617 uMol/second.
  • Plants in the flowering phase receiving 12 hours of light should receive between 460 and 925 uMol/second.

If your plants are currently receiving light intensity within the above range, you do not need to adjust the height of your grow light. Below this range would require moving the grow light closer to the plants. Above this range would require moving the grow light further away from the plants.

Remember, higher light intensity = higher yields. To read more about PPFD and light intensity levels required to grow cannabis, read my full article on the topic here.

Every grow light’s intensity at a given height is different, that’s why you need to determine these numbers by looking at the manufacturer provided chart or by using a Lux meter. If every grow light was the same, I’d be able to tell everybody to hang their light “xx inches from the plants.”

If your grow light is within the above mentioned range, there’s a good chance it’s hanging at the right level. But there are two more questions to answer to determine what height to hang your grow light.

Is your grow light dimmable?

If your grow light is dimmable, you may be able to dim the light instead of adjusting the light’s distance from your plants. Again, the most critical aspect of this whole process is having a Lux meter handy so you can measure, convert and compare the light intensity levels to the actual light intensity levels your plants need to grow.

If your grow light is dimmable, you can usually adjust the light intensity via a screw or knob on the ballasts.

Another Video About Grow Light Hang Height:
Grow Light Hang Height For Cannabis - LED & HPS Hang Height Advice

Good Luck! Want to learn more home growing tips? Learn how to calculate grow light coverage. Learn how to lower temperature in your grow tent.

FAQ:

How high do I hang my grow light for growing cannabis?

The proper grow light height depends on the power of your grow light. A low power grow light needs to be closer to plants than a high power light. The plant’s phase of growth also plays a factor. Generally speaking LED grow lights can hang between 16 and 24 inches. HPS lights need to hang at 36 or even 48 inches depending on the power output.

Do I need to adjust grow light height as plant grows?

Yes, as the plant gets closer to the light it receives more intense light and you risk burning the plant unless you adjust your grow light height.

References:

Chandra, Suman, Hemant Lata, Ikhlas Khan, and Mahmoud Elsohly. “Photosynthetic response of Cannabis sativa L. to variations in photosynthetic photon flux densities, temperature and CO2 conditions.” Physiology and Molecular Biology of Plants 14.4 (2009): 299-306.

Backer Rachel, Schwinghamer Timothy, Rosenbaum Phillip, McCarty Vincent, Eichhorn Bilodeau Samuel, Lyu Dongmei, Ahmed Md Bulbul, Robinson George, Lefsrud Mark, Wilkins Olivia, Smith Donald L. (2019). “Plant Growth-Promoting Rhizobacteria for Cannabis Production: Yield, Cannabinoid Profile and Disease Resistance.” Frontiers in Plant Science 10. 10.3389/fpls.2019.00495

Nick
 

My name is Nick, and I'm a home grower who spends time writing about indoor growing tools and setups. My goal is for 420ExpertGuide to be a one stop shop for indoor growers to find the information they need.