black friday cannabis seed sale

What Height Do I Hang My Grow Light For Cannabis Growing?

grow light height

The most frequent question I get from cannabis growers – by far – is the question of what height they should hang their grow light at. The short answer is “it depends,” but I hope to explain the deciding factors below.

I get this question not only from new cannabis growers sprouting seedlings, but also from concerned growers who are afraid their plants are stretching too much mid-grow, and are starting to wonder if they hung their grow light correctly.

Most grow light manufacturers provide a recommended hanging height, but these are only estimates, and are not always applicable to every situation. These recommended heights are available on the manufacturer’s website and usually on the website you purchased the grow light from.

The Lazy Method For Determining Grow Light Height:

I’m about to lay out a slightly complicated step by step process to figure out exactly how much intensity your grow light is providing at its current height, and then compare that to how much light your plants actually need. If you don’t want to read through all that, I advise you use the lazy method:

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

HPS grow lights are a bit more tricky because the height requirement can vary widely depending on the power level. During the vegetative and flowering phases HPS lights can hang at the following heights:

 

Watts: HPS Grow Light Height (at full power):
250w: 2.75ft – 3ft
400w: 3ft
600w: 3.5ft
1000w: 3.5ft – 4ft

 

That’s it. If the leaves start to curl upwards they’re probably heat stressed (too close). If the plants are stretching, you can probably move the light closer.

Keep reading for the more precise answer.

Answer These Questions To Determine A Proper Grow Light Height:

If you don’t want to use the 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.

When You Need To Adjust Your Grow Light Height:

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 light or adjust the height. If you don’t adjust the height or dim it, you’ll risk burning your plants.

Heat Stress:

You need to adjust your grow light height if your plants are heat stressed, or if your plants appear to be stretching.

Heat stressed leaves appear to be curling upwards towards the light:

heat stress cannabis

Immediately increase the distance between your grow light and your plants if you see this happening.

Stretched Plants:

Stretched plants happen when they don’t receive enough light, so the plant attempts to grow larger to get closer to the light source. Genetics can play a role as well, but if you notice this happening, you should move your grow lights closer to your plants immediately.

stretching cannabis plant

While this 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.

Good Luck!

 

 

 

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

 

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.

Nick
 

").after(c.lastChild)}d&&(f.id=d+"-script"),f.src=e,c.appendChild(f)})},TVE_Dash.send_ajax=function(b,c){a.ajax({url:tve_dash_front.ajaxurl,xhrFields:{withCredentials:!0},data:{action:"tve_dash_front_ajax",tve_dash_data:b},dataType:"json",type:"post"}).done(function(b){b&&a.isPlainObject(b)&&(b.__resources&&(b.__resources.css&&TVE_Dash.ajax_load_css(b.__resources.css),b.__resources.js&&TVE_Dash.ajax_load_js(b.__resources.js),delete b.__resources),a.each(b,function(a,b){if("function"!=typeof c[a])return!0;c[a].call(null,b)}))})},a(function(){setTimeout(function(){var d=new a.Event("tve-dash.load");return a(document).trigger(d),!a.isEmptyObject(b)&&(!(!tve_dash_front.force_ajax_send&&tve_dash_front.is_crawler)&&(TVE_Dash.send_ajax(b,c),void(TVE_Dash.ajax_sent=!0)))})})}(ThriveGlobal.$j)}catch(e){console.log(e)}try{!function(d,l){"use strict";var e=!1,o=!1;if(l.querySelector)if(d.addEventListener)e=!0;if(d.wp=d.wp||{},!d.wp.receiveEmbedMessage)if(d.wp.receiveEmbedMessage=function(e){var t=e.data;if(t)if(t.secret||t.message||t.value)if(!/[^a-zA-Z0-9]/.test(t.secret)){var r,a,i,s,n,o=l.querySelectorAll('iframe[data-secret="'+t.secret+'"]'),c=l.querySelectorAll('blockquote[data-secret="'+t.secret+'"]');for(r=0;r