How to Set the Height of Your Cannabis Grow Lights?

Here’s a fun fact: moving your grow lights from 18 to 36 inches from the canopy can make a difference of 75% in the light’s intensity. That’s a difference between night and day for your cannabis plant.

This is one of the reasons why you must place your grow lights at the right height so your cannabis plant only gets the light it needs — neither less nor more. Just right. 

But getting to that sweet spot of light can be difficult, and sometimes, downright confusing. Especially if your grow lights are unconventional. If you are facing the same problem, this article is here to help you. Keep reading to know all about setting the right height for your cannabis grow lights.

Why is Grow Light Height Important When Growing Cannabis?

Right from the moment a cannabis seed germinates to the pre-flowering stage where the plant experiences explosive growth, your cannabis plant continually keeps growing. It goes from an inch to a few feet in a matter of months. This means that your grow light height needs to constantly change as the plant grows.

Leaving your lights at a set height is not recommended as it can have tremendous consequences on your plant’s health. Some of the common problems that your plant can experience if the grow lights are not properly placed include the following:

• Stunted growth or reduced vigor
• Bleached leaves due to light or heat burns
• Lower yield than usual 
• Airy, fluffy buds with low trichome content 
• Long internodes 

Apart from these problems, even your grow room can face a couple of problems. First, because the lights are not correctly set, you may have to use a lot more light which can lead to wasted wattage/lumens and high energy bills. At the same time, you don’t utilize your grow room space efficiently.

On the other hand, setting the right height for your grow lights can have several benefits, such as the following:

• Better yield with bushy buds that are rich in trichome and terpene content
• Faster bloom process 
• Healthy and quick plant growth throughout the growth stages
• Short internodes that grow healthy flowers 
• Thick stems and branches that can handle the weight of the buds during bloom

And again, placing the lights at the right height can lower your energy bills as the light gets distributed more evenly while helping you maximize your grow room space. 

Keep reading to know the three ways you can find the right height for your cannabis grow lights.

How to Find the Right Height for Cannabis Grow Lights Without a Lux Meter?

Finding the right height for your cannabis grow lights is a little tricky, but if you know the right steps, you can find the sweet spot easily. All you need is a little patience, some trial and error, and a keen eye (and maybe a lux meter, but we’ll get to that later).

Follow these steps to find the right height for your cannabis grow lights manually, without any confusing calculations or a lux meter.

Step 1. Start with a Baseline Height

The first step is to set a baseline — for this, you need to place your light at a generic distance from your cannabis plant’s canopy. In the beginning, you'll do good to maintain the light at a distance of 12-16 inches, which is your baseline. However, you need to adjust the distance if the plant starts stretching. 

Try to keep the grow lights between 12 inches to 50 inches from the canopy throughout the cycle. But you'll need to adjust it, depending on how your plants react. 

Step 2. Adjust Your Grow Light 

Now comes the hard part. Start by observing your grow light’s footprint on and around the plant. Typically, the light should cover the entire plant evenly, without a hotspot in the center or dark spots on the edges. 

If there is a noticeable hot spot in the center of the canopy, you need to raise the light’s height. A hot spot may not always be visible, but it can lead to the leaves bleaching in the center of the canopy. 

On the other hand, if the edge of the plant is not receiving enough light, you need to lower the light. Again, do not lower the light more than 12 inches to the canopy — if you need to, it’s a sign that your grow light is inadequate for your plant. Consider upgrading your grow lights or getting an auxiliary panel to fill in the gaps.

We could give you a one-size-fits-all guideline on the height, but that’s not possible. While many growers are switching to LED lights, many more still use HID lights like HPS and MH, and some novice growers also use CFL lights. 

And different types of light panels produce different kinds of light. For instance, HID, which stands for high-intensity discharge, produces a powerful beam with high lumens that can cover a lot of distance. In comparison, a CFL bulb uses fewer watts and only produces light that is effective for a couple of feet.

So, here are some rough guidelines on the ideal height for HID, LED, and CFL grow lights.

Ideal Grow Light Distance for HID Lights

• 400W HIDs: 12 to 20 inches from the canopy
• 600W HIDs: 14 to 26 inches from the canopy
• 1000W HIDs: 16 to 32 inches from the canopy

Ideal Grow Light Distance for LED Lights

• 240W to 400W LEDs: 14 to 28 inches from the canopy
• 450W to 550W LEDs: 18 to 32 inches from the canopy
• 600W to 850W LEDs: 22 to 36 inches from the canopy
• Over 900W LEDs: 26 to 46 inches from the canopy

Ideal Grow Light Distance for CFL and T5 Lights

• 20W to 50W CFLs: 1 to 2 inches from the canopy
• 55W to 100W CFLs: 2 to 4 inches from the canopy
• 150W to 250W T5 lights: 4 to 10 inches from the canopy
• Over 300W T5 lights: 10 to 16 inches from the canopy

NOTE: This is just a general guideline to help you get started, but you may still need to finetune the height of your grow lights. Various factors come into play here, like your plant’s genetics, growth environment, temperature, humidity, and a lot more. 

This is especially true for LED lights because they come in all kinds of varieties. Some LED lights that use better lights or lenses are much more powerful than other LEDs, even if they share the same wattage ratings. 

Most grow light manufacturers, including LEDs, offer a recommended hanging height. If your grow light comes with that, always follow that. 

Step 3: Observe Your Plant

Once you have placed your grow lights as per the recommendations, you should then observe the plant for some time. If you have misplaced the lights’ height, your plant will tell you.

Here are some positive signs you should be relieved to see on your cannabis plant:

• Vigorous growth 
• Leaves growing upwards (not curling) as if reaching for the light
• Tight spacing between the internodes
• Proliferated calyxes during the flowering stage
• High resin content on the buds

On the other hand, these signs should worry you:

• Bleaching on the leaves or buds
• Heat burns on the leaves 
• Slowed growth of the plant
• Uneven canopy
• Excessive, atypical stretching of the plant due to lack of light

If you have found only positive signs, you have figured out the right height for the lights. But if you find any negative signs as mentioned above, you need to act quickly and retrace the steps mentioned above until you reach the sweet spot.

How to Find the Right Cannabis Grow Light Height Using Inverse Square Law?

If you are a little nerdy, you can go further than rough trial and error with the inverse square law. This law can help you figure out how intense your grow light is at a specific distance from the cannabis plant.

The inverse square law applies to grow lights, where if you increase the distance between the grow light and the canopy, the light’s intensity decreases at the canopy. This difference in the light’s intensity is drastic yet precise — when you double the light’s distance, the intensity decreases by 75%. 

Confused? Here’s an example.

Let’s say your grow light produces 1000 micromoles of light on the canopy when it is 18 inches from the plant. If you double the height of the grow light, from 18 inches to 36 inches, the light’s intensity at the canopy will decrease by 75% to 250 micromoles (μmol).

The mathematical formula for this is:

• Light intensity = light output/distance square

For this, you only need to know the light’s intensity at one distance and use this formula to figure out the best height for your cannabis plant. Typically, you should aim for the following intensity of light for growing cannabis:

• For a seedling: 200 to 300 micromoles
• For a vegetative plant: 300 to 600 micromoles
• For a flowering plant: 600 to 900 micromoles 

How to Use a Lux Meter to Find the Right Cannabis Grow Light Height?

The third method to find the right height for your cannabis grow light is perhaps the most reliable and straightforward. Here, you have to measure the light, convert it, and compare it until to find the sweet spot. 

First, you need to measure the amount of light your plant receives at varying heights of the grow light and compare the readings to what your plant should be receiving. For this, you should first know how much light your plant needs at each of its growth stages, which is as follows:

• Seedling stage: 200 to 300 μmol/second
• Vegetative stage: 306 to 617 μmol/second in 18/6 light cycle
• Flowering stage: 460 to 925 μmol/second in 12/12 light cycle

If you have purchased grow lights from a high-quality manufacturer, the packaging will likely contain light intensity measurements at varying heights from the canopy. But if you can’t find it or if your plant has unique light requirements, you should get yourself a lux meter.

Ideally, we would recommend you get a PAR meter as it measures photosynthetic active radiation — the specific wavelengths your plant needs — but these meters can cost a bomb. A good one would set you back by a few hundred dollars.

So, the next best thing is to get a lux meter that measures the intensity of light on a surface, which is the canopy of your plant in this case. Lux meters are relatively cheap and a worthy investment for a serious cannabis cultivator. 

Measuring the Light Intensity for CMH or HPS Lights

If you are using CMH or HPS (or any kind of light except LED lights), you need to measure the light intensity using your lux meter and then convert the reading to micromoles. You can easily convert the reading using one of the online conversion charts or calculators.

If your plant is receiving the right intensity of light at the current distance, you are good. If not, you need to adjust the light until you get the right reading in your lux meter. 

Measuring the Light Intensity for LED Lights

Measuring the light intensity for LED lights is the same as that of CMH or HPS lights, but things get a little tricky with some LED lights. 

A lux meter cannot really estimate the light intensity of LEDs because they have been calibrated to measure incandescent lights. LED lights have a unique spectrum of color, which the meter cannot read well. 

So, after you measure the intensity of light from your LEDs, you need to convert it using a waveform lighting calculator. Here, input the lux reading, select red + blue + white LED, and run the calculator. The answer would be the intensity of light in μmol//second.

When Should You Readjust the Light’s Height from the Plant?

You can’t set the light’s height once and forget about it until harvest — that would be disastrous for your cannabis plants. Instead, you need to continually readjust the light’s height as your plant grows and gets closer to the light. (Or you can dim the lights, too.)

Doing so will prevent your plants from burning or experiencing light or heat stress.

Usually, you should check and readjust the light every few days, especially if the plant is growing fast. However, if you notice any signs of heat or light stress on your cannabis plant, you need to act immediately and move the lights up. 

Also, look out for atypical stretching, where the plant starts growing too tall — this is the plant trying to get closer to the light source. If you notice this, you need to lower the lights immediately. 

Protips for Adjusting Light’s Height for Cannabis Plants

There is always room for improvement, even if you use the best methods and equipment. So, to help you get there, check out the following tips.

1. Don’t Rely on Wattage Alone

In the past, growers would solely rely on the light’s wattage and place them at the recommended height based on the wattage. But things have changed now. Wattage is no longer a reliable measurement of light intensity, thanks to the advent of LED lights. 

LED lights consume a lot less energy than conventional lights while producing the same levels of light intensity, which renders wattage pointless for intensity measurement. Even conventional lights have gotten more efficient.

So, it is wise to not rely on wattage alone but use lux or PPFD to measure the light’s intensity. PPFD is by far the most reliable way, but measuring that can be hard, so for most growers, lux works just fine.

2. Learn to Read Your Plant

Your plant is way more communicative than you would think — if it is experiencing any stress due to light, it will tell you. Look for signs of light stress on the plant. If you notice any of the leaves curling, yellowing, drying, bleaching, etc., especially those closer to the light, you need to check your light’s height. 

If the lights are too close or far, readjust them to the right height immediately before the problem worsens.

3. Use Your Hand to Test the Light

An old gardener’s trick is to use your hand to check if your lights are getting enough light. Just place your hand under the light, right atop the canopy. If you can keep your hand there for 30 seconds without feeling uncomfortably hot, your plant is safe. 

This isn’t the most accurate method, but it is quick and easy to ensure you have not placed your grow lights too close to the plant. 

4. Consider Training Your Cannabis Plant

Apart from simply moving your grow lights up and down, you can take the entire process a step further by training your cannabis plants. Use training methods like the screen of green (ScrOG), bending and tying, LST, etc. to take advantage of the light’s sweet spot.

Essentially, you have to train your plants in a way where most of the branches get an equal amount of light. Doing so can significantly increase your yields and allow you to grow bigger, bushier buds. 

5. Be Very Careful with Seedlings

Cannabis seedlings are delicate and it is easy to give them too much or too little light. And since the seedling stage sets the foundation for your plant’s following growth stages, you must be extra careful to not stress out the seedling. 

The most important tip here is to never use HID lights on seedlings as they are way too powerful. Instead, use CFL lights or dimmable LED lights. If you are using a CFL light of low wattage, place it 1 to 6 inches from the seedling.

On the other hand, for a medium-intensity LED light, place it 10 to 14 inches from the seedling. Again, use the hand test to ensure the seedling is not getting too much light.

6. Prevent Light Wastage

If finding the sweet spot is getting tricky, where a lot of the light is being lost, you can use reflective materials to prevent light wastage. Use reflective materials like mylar, panda film, or foylon film around the plant to redirect unused light back to the foliage.

Doing so can increase light penetration in the lower parts of the foliage, allowing the lower bud sites to grow bigger buds. 

Summary: How to Set the Height of Your Cannabis Grow Lights?

Setting the right height for grow lights is often overlooked. Many growers make the mistake of setting the lights at a recommended distance without testing the light’s intensity or learning how to read their plant for signs of light or heat stress.

You mustn’t be one of them. 

Your cannabis plant can grow wonderful buds if the light conditions are ideal — the right intensity of light can help your plant grow big buds with a rich trichome profile. Such buds have a high shelf appeal, rich flavor and aroma, and are quite potent. 

So, use the methods listed above to measure the light falling on your cannabis plant’s canopy and adjust the light if it's too much or too little. The trial and error method is great as it is free and works well enough for beginners — although it is a little time-consuming and prone to errors.

On the other hand, inverse square law is an accurate way of reading light but you have to rely on the manufacturer’s measurements, which can sometimes not be so accurate. Plus, this is only ideal for folks who don’t mind dabbling in mathematics.

For other growers, especially experienced growers, lux meters work best. They offer an easy and accurate measurement of the light’s intensity. And if you don’t mind splurging a bit or are growing a commercial quantity of cannabis, you should even consider getting yourself a PAR meter. Yes, they are expensive but so worth it. 

And always remember to keep checking the light intensity every few days. You need to raise the lights as your plant grows over the weeks. We also recommend training your plant — doing so with constant light height adjustment can reward you with a yield like never before.