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When is Too Much Light? Is there such a thing? (Viewed 1077 times)
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When is Too Much Light? Is there such a thing?
Reply #5 Posted on February 16, 2015

We all know how important light is to cannabis plants!
That’s why indoor growers have been trying for years to make a system that mimics the sun or even improves on it if possible.
The sun’s awesome penetration is still unmatched; however we can mimic its power at close range. In fact, grow lights can outdo the sun in terms of light received by the plant because we can position them only inches away!
We tend to leave our lights farther away from our plants because we don’t want them to get too much heat, but is that the only reason why? If we could reduce the heat in the area, couldn’t we give our plants more light? If more light equates to more bud, I could be harvesting more while wasting less electricity...right?
For my last grow, 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 became the unfortunate guinea pigs in our test to see what happens if a plant gets too much light!
A Natural Amount of Light

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 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 plants fall into the range below:
Vegetative: 40,000 - 70,000 lux
Flowering: 65,000 - 85,000 lux
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 just plain take too long to develop. Conversely, a cannabis plant getting more light than this range will usually experience heat burn, light burn, or a combination of both!
Our Slightly High Amount of Light
In this last grow while the plants were flowering, I lowered the temperature in my grow tent and got 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. I ended up with 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.
When we measured the amount of light the plants were getting with a lux meter, we got readings between 105,000 and 110,000 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. However, even without heat/light burn, problems still showed up…
Drastically Increased Nutrient Consumption

When your plants take in and convert sunlight via photosynthesis, they also use up nutrients in the process.
It’s also important to understand that plants don’t seem to uptake nutrients a la carte. Rather, they uptake nutrients and they get whatever is on the menu. Meaning, when it’s time to uptake nutrients, cannabis plants don’t seem to distinguish much between nutrients as they just take what’s available.
With these last two things in mind, it makes sense that the more light a plant gets, the more nutrients it needs. And the more nutrients the plant needs, the more it uptakes.
This is where having ‘too much light’ becomes a problem: In my case, my plants were already getting enough light and as such took up enough nutrients to process that light. But when they got more than 25% more light than what they needed, they had to uptake more nutrients to process all that extra light.
At this point, the plants are now in excess of light and nutrients, which always leads to...
Nutrient Burn
Even if a plant can handle an excess of one or two minerals, they certainly won’t be able to handle an excess of all available minerals. An excess of a certain mineral can show itself in many ways (like with Nitrogen Toxicity), but when mineral levels of differing types get too high you get this:

Nutrient burn is also normally associated with an excess of heat. Heat causes your plant to drink more and inadvertently uptake more nutrients. Although heat was managed in this case, the symptoms look very similar to if I hadn’t managed the heat at all.
Nutrient Deficiencies Galore!
Wait a minute...how can the plant be taking in too many nutrients and have nutrient deficiencies?
 
My plants started uptaking more nutrients to process the extra light, and that overworked the leaves until they eventually began dying early, as they had ran out of all the nutrients I provided for them! I could give them more nutrients, but they’re already taking in way too much, and the leaves were working so hard they couldn’t keep up. This puts you in a bit of a bad place as there aren’t many good choices to save your leaves besides moving the light away.

 
Return of the Lux Meter
This is another time where a Lux Meter is a great tool to have. Although we’ve recommended getting one to make sure your plants are getting enough light, they’re also excellent for growers with larger HID grow lights. A lux meter will help make sure you’re not giving your plants more light than they can use (or less than they need).

Another thing to look out for if the top leaves closest to the light are getting lighter than the rest of the plant, especially if they seem to be yellowing fast. In cases where a high intensity light is being used, the plant will use up the chlorophyll and nutrients in the leaves being exposed to the most light and they will appear bleached. This is a sign that this particular part of the plant is having a hard time keeping up with processing the light energy it’s receiving.
 
The ‘hand test’ is still a very good option for those with HID lights. The hand test is when you place your hand where the plant is for 30 seconds, if it’s uncomfortable warm it means you need to move your lights further away. Just remember that this technique should only be used as a general guide, while a combination of a lux meter and your plant’s reaction are the best tools you can get to give your plants just the right amount of light. Just like with humans, plants need a happy medium with everything, even if it’s good for them. Now I know that for sure!
 

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