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PSA: Too Much Light?!

by Sirius Fourside

Cannabis plants love bright, direct sunlight

We all know that light is important to cannabis plants!

In the wild, growing cannabis plants generally like bright, direct sunlight. Outdoor cannabis plants given lots of light can grow to the size of trees!

That’s why indoor growers have been trying for years to make a system that mimics or even improves on the power of the sun.

The suns awesome penetration power remains unmatched; however, we can mimic its power at close range. In fact, grow lights can outdo the sun in terms of light levels received by the plant because we can position grow lights about 93 million miles closer than the sun!

When setting up grow lights, we position our lights a good distance away from the top of our plants (depending on the type of grow light) because we don’t want our plants to get too much heat. But is that the only reason why? If we could reduce the heat in the area, could we give our plants more light by moving the grow lights even closer? If more light equates to more bud, I could be harvesting more using the same amount of electricity...right?

Read More About Light Burn!

In one of my past grows, I decided to try keeping my lights as close as possible without heat/light burning the plants to see if they would capitalize on the abundance of light. My Liberty Haze and Critical Kush had become the unfortunate guinea pigs in a test to see if a plant can get too much light!

A Natural Amount of Light

Natures grow light in all its glory!First, it’s good to know how much light a cannabis plant could possibly get if it was being grown outdoors.

A plant being grown outdoors in a location with relatively low levels of sunlight could get as low as 32,000 lux (lux is a measurement of light) on a bright sunny day in direct sunlight. Light levels can fall down to 10,000 lux (or even less) on an overcast day. On the flip side, a desert in the height of summer can see light levels as high as 100,000 lux on a sunny day. Cannabis plants can usually survive at either end of these ranges as long as the temperature, soil, etc. are acceptable.

That being said, there is definitely a desirable range when it comes to the amount of light a cannabis plant receives. Although that range varies depending on the type of plant (Indica vs. Sativa) and strain, most plant's ideal light levels fall into the range below:

Vegetative: 35,000 - 70,000 lux
Flowering: 55,000 - 85,000 lux

Note: Lux is the measure of how much light is being received at a specific point in space. You can measure the amount of lux at different places in your grow tent using a lux meter. Using a lux meter gives you the power to accurately position your lights so you get the biggest yields without accidentally light burning plants.

When a cannabis plant is having its other needs fulfilled, being in the desirable light range means that it grows at a pace that isn’t slowed down by any factor besides its genes. A plant in light levels below this range will produce spindly stems and buds and will take significantly longer to develop. Conversely, a cannabis plant getting higher levels of light than this range will usually experience nutrient problems, heat burn, light burn, or a combination of all three! Let's take a look at what too-high levels of light can do to your cannabis plant.

A Too-High Amount of Light

In the aforementioned grow while the plants were flowering, I lowered the temperature in my grow tent with an AC so I could get my lights closer. I also used a fan to blow air right under the light to further reduce the amount of heat on the plants. Generally, a 600W HPS grow light should be kept ~16" away from the tops of the plants in order to give the same brightness level as direct sunlight. What distance should my MH/HPS light be from my plants? 

Light bleached buds under an LEDI ended up keeping my 600W light only 8 inches above my plants, but the heat was reduced enough that I could stick my hand right over the plants without my hand getting uncomfortably warm. I thought that the plants would be able to use all that extra light because they didn't need to worry about heat.

When we measured the amount of light the plants were getting with a lux meter, we got readings between 105,000 and 120,000 lux! This means that these cannabis plants were getting more light than they would on a sunny desert day, but with temperatures in the 70s. We thought this would make the plant grow even faster and better than in the wild. However, even with the heat being controlled, we didn't get faster growth or bigger yields and problems started showing up…

Tip Burn, Nutrient Deficiencies & Dying Leaves

As a plant gets higher levels of light, it increases the amount of nutrients being taken in by the roots. This is most true for growers using powder or liquid nutrients.

This is because cannabis plants drink more water when it's sunny out. And taking in more water at the roots also causes the plant to take in more nutrients.  In other words...

Cannabis plants take in more water and nutrients in bright light

Why? Photosynthesis (the process of making energy from light) causes the plant to evaporate more water from its leaves. As a plant evaporates water from the leaves, it creates negative pressure which sucks up water from the roots like a straw. This capillary action - known as transpiration - is how plants get water from the roots to the leaves. Though plants can also absorb nutrients through their leaves, cannabis plants generally get the majority of their nutrients during the process of taking water in through the roots. 

Did you know? In addition to light levels, several other environmental factors can cause plants to drink more, including high temperature, low humidity, and a higher amount of air blowing over the leaves. 

So if a plant is drinking more, it's also taking in higher levels of nutrients from the roots, even if it doesn't need them.

Fish, fries, chocolate AND a chicken sandwich?

Unfortunately, cannabis plants don’t seem to uptake nutrients a la carte. Rather, they uptake all the available nutrients in the water and get whatever is on the menu. Cannabis plants aren't really able to sense if there's too much or the wrong type of nutrients, they just take whatever nutrients show up. This is most true when growers are using powder or liquid nutrients since these provide minerals in the most chemically available forms, making them extremely easy to uptake.

So as plants are using more light, they take in more nutrients than they otherwise would, which can lead to a buildup of nutrients in the plant, causing...

Nutrient Burn

If a plant gets too-high levels of nutrients at once, it doesn't know what to do with it all and this causes problems in the plant. Generally, the most common symptom is nutrient burn where the tips and edges of leaves get a brown, burnt appearance.

Though a plant can handle a small excess of nutrients, they won’t be able to handle a huge extra load of minerals being piped in through the roots, especially if it happens all at once. An excess of a certain nutrient can show itself in many ways (like with Nitrogen Toxicity where leaves turn dark green), but when mineral levels get too high for the plant to process anymore, you get this:

These nutrient burn symptoms are actually caused by the grow light being too close

No one likes nutrient burn...

So you realize your plants are getting nutrient burn, and your thought may be to simply lower the nutrient levels. Although that's a logical guess, it'll probably just lead to...

Nutrient Deficiencies!

That's correct: a cannabis plant can simultaneously have nutrient burn and have nutrient deficiencies?

When the whole plant is being overworked, it starts showing signs of what appear to be nutrient deficiencies. At this point, it might seem like a good idea to increase the nutrients you're giving your plants.

Unfortunately adding more nutrients at this point can do more harm than good, often leading to even worse nutrient burn. Because the true problem is the light being too close, upping the nutrients isn't going to help, even though it may look like the plant is suffering from nutrient deficiencies.

These overworked leaves began dying early! They showed signs of both nutrient burn and nutrient deficiencies at the same time, though the true problem was the distance of the grow light. Giving the plant more nutrients at this point would only make things worse.

This light burned leaf is not happy...

The leaves below show different stages of a mix of what appears to be nutrient deficiencies from an "over-lighted" plant. Changing the nutrients won't help in this case, but if you fix the conditions by moving the light further away, these symptoms will stop spreading to new leaves. Unfortunately, the damage done to the remaining leaves is permanent.

More unhappy leaves...


Top Leaves (or Buds) Getting Bleached

This is closely related to the problem of the plant showing nutrient deficiencies when lights are too bright. Leaf bleaching often looks a lot like a nutrient deficiency.

The fact is, leaves start breaking down when they've been getting too much light for too long. They can show a few different symptoms that may look like different deficiencies, but noticing where the deficiencies/bleaching are happening will help alert you to the true situation.

So when you see the top leaves closest to the light are getting lighter than the rest of the plant, especially if they seem to be yellowing fast while the rest of the plant stays green, that's a big warning sign that the light is too close. 

In cases where too-high light levels are being used, leaves can't keep up with removing the waste products from photosynthesis. As the leaves get more and more damaged, they begin to look bleached and stop being effective. 

This bleaching reaction seems to be most prominent when buds are fattening fast in addition to high light levels - the leaves just get tired out. They need to "retire" early after a hard life.

Here's a pic of my Liberty Haze (left) and Critical Kush (right). It might look like one big plant, but you can see a change in the size of colas near the middle of the pic.

Notice how the leaves closest to the light have turned completely yellow, yet the Critical Kush plant that is lower down and further away has leaves that are still green and healthy

Leaves are getting light-bleached from too-close grow lights

Without green leaves to support growth, the buds on the left pretty much stopped growing. Non-green leaves can't do photosynthesis anymore, so unfortunately, buds stop fattening when all the leaves turn yellow.

In order to get the best yields, it's important to try to keep a healthy amount of green (but not too green) leaves for most of the flowering stage.

When you see bleaching directly under the grow light (while the rest of the plant is green and healthy), it is a clear sign that this particular part of the plant is having a hard time keeping up with processing the light energy it’s receiving and the light should be moved further away. If the light can't be moved away, that part of the plant should be gently bent and tied down further away from the light, as best you can. Learn how to use low-stress training to control too-tall plants.

And it's not just leaves, buds can also get bleached or light-burned, lowering their potency and reducing their smell!

This picture shows a bud too close to the light where the top has been bleached white (notice that all the surrounding leaves have also been bleached and look unhealthy, too)

Bleached white cannabis bud

Some plants/strains are more sensitive to light, while others can take incredibly bright light levels like a champ. For example, Indica strains tend to be less resistant, while Sativa strains (which hail from the equator) seem to be better equipped to deal with a lot of light. You can have two plants in the same tent that are reacting differently to the same amount of light. It's important to watch your plants closely during the flowering stage to see how they're reacting to the grow lights!

Now that you understand why keeping your grow lights the right distance away is important, let's look at some methods to help keep your plants happy!

Solution - Find the "Sweet Spot" for Your Grow Lights

The main idea is you want to keep your grow lights as close as possible (to maximize the amount of light to the plant) while keeping them far enough away they won't cause damage.

Here are some tools and techniques you have available to you so you can find the "sweet spot" for your own grow lights.

The Hand Test

A totally non-edited picture pf someone performing the 'hand test'

Ok, you know that you don't want to give your plants too much light. So how can you check if your lights are the correct distance away?

One way is to use the "hand test" to check if a grow light is too close to your plants.

Although this is not the most accurate method, the hand test is still a good technique to get a measure of the heat and energy a plant is receiving from your grow light. 

With the hand test, you place your hand (palm down) just over the plant for about 30 seconds - if it’s uncomfortably warm for you it means you need to move your lights further away. 


  • Easy & Free!
  • Great Estimate for CFLs and T5 grow lights.
  • Relatively Good Estimate for MH/HPS Lights.


  • Not effective for LED grow lights (which often burn if kept too close, even at cool temps).
  • Not effective if grow space is being cooled since the low temperature can mask the high levels of energy from the grow lights.

As we've learned in this article, this technique should only be used as a general guide, because a cool grow area can prevent your hand from being able to tell if there's too much light. 

Manufacturer's Specifications

When it comes to choosing the correct distance for your grow lights, a great way to find out is to ask the person who made them.

For LED grow lights, this is pretty much the only good option you have besides trial and error or talking to other growers who have the same LED grow light model. The hand test does not work for LEDs because they run very cool, yet still put off a lot of light, often focused downward with lenses to make the beams of light even more powerful.

Each model of LED is different and needs to be kept different distances from your plants. Unfortunately, there's no "standard" yet for LED grow lights since they use various ways to focus light downward, and these have a huge effect on how much light is experienced by the plant. That being said, it's generally safe for growers to start at 18" and move the light closer as needed.


  • Free!
  • This is the only real way to know where to start when it comes to how far to place LED grow lights from your plants since LED models can be quite different from each other.
  • Some grow lights have been standardized to the point you don't even need to talk to the manufacturer. For example, MH/HPS grow lights follow a pretty standard formula when it comes to placing the light.


  • Definitely not a perfect solution but this method is still a great starting point.

Lux Meter

For growers who aren't using LED grow lights, you may appreciate a tool known as a "lux meter". These inexpensive devices measure the level of light in a specific spot and can be used to help you know whether your plants are getting too much or too little light.

Unfortunately, lux meters aren't an effective way to estimate light levels for LED grow lights, however, from our testing, it appears they can give you a really rough estimate.

When used right, a lux meter can help you maximize your yields from both big and small grow lights, and they’re excellent for growers who are worried about giving their plants too much light. A lux meter will help make sure you’re giving your plants exactly the amount of light they want. 


  • Inexpensive
  • One of the more accurate ways to measure light levels without paying a lot of money.
  • Lets you easily measure light levels at different points in the tent.
  • Can be used to measure if there's too much light or not enough.


  • Not as accurate for LED grow lights

Learn how to use a lux meter for your cannabis grow!

Meet your cannabis plant's new best friend!

Watch Plant's Reaction

Now you know about light levels and you have a few tools to help make sure you're avoiding light problems. The hand test is a good start, the manufacturer's specs are great for LED grow lights, and a lux meter is great for everything else.

But the most important thing to look out for is your plant’s reaction. A plant will tell you it's getting too much light by displaying nutrient problems and bleaching. If a plant is not getting enough light, it will grow tall and spindly.


  • Free
  • Hands-down most effective way to place your grow lights at the right distance


  • Takes time to watch your plants closely, at least until you get experience with your grow light.
  • Plants can get damaged with this method since it's reactive while the other methods are proactive. Ideally, you want to fix problems before you notice damage to your plant.



Used together, these tools ensure you are giving your plants just the right amount of light. Just like humans, plants need a happy medium when it comes to sustenance. Now, you have the information to make sure your babies are getting what they want!