How Much Light Is Too Much Light?

We all know how important light is 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 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 levels received by the plant because we can position them only inches away!

When we set up grow lights, we position our lights several inches or even farther away from the top 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, couldn’t 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?

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 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 plant’s ideal light levels fall into the range below:

  • Vegetative: 35,000 – 70,000 lux
  • Flowering: 55,000 – 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 higher levels of light than this range will usually experience nutrient problems, heat burn, light burn, or a combination of all three!

As your plant gets higher levels of light, it increases how much nutrients are 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…

Photosynthesis (the process of making energy from light) causes the plant to lose extra water out of the 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 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. 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 much 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.

Find The Best Spot 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.

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

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 your grow lights. 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.

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

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.

Watch Plant’s Reaction

So now that you know about light levels, 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.

Used together, these tools ensure you are giving your plants just the right amount of light. Just like humans, plants need a happy medium with everything, even if it’s good for them.

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