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Sufficient light is critical for the growth and production of marijuana plants. When growing marijuana, the rule of thumb is: 1% more light = 1% higher yield. The plant uses the energy contained in light for its growth and for the production of buds. The color of the light and the length of the day (light period) also make a big difference to the growth. A good understanding of the effect of light on marijuana plant is a prerequisite for anyone who deals with marijuana plants.

How light is used

The plant can do three things with solar energy:

  • Photosynthesis
  • Fluorescence
  • Warming
How light is used Photosynthesis
Photosynthesis

Photosynthesis is the utilization of solar energy to create sugars so the plant can grow. The solar energy puts an electron in the chlorophyll into a higher energy state. In a big chain of reactions, it falls back into its original energy state. The captured energy is transferred to all kinds of chemical substances, and is eventually used for the conversion of water and carbon dioxide into sugars and oxygen. The chapter Photosynthesis provides more details on this phenomenon.

How light is used chloroplast chlorophyll
Fluorescence

The solar energy puts an electron in the chlorophyll into a higher, excited energy state. The electron can however also fall right back into a lower energy level. If this is the case, it will emit light. This is called fluorescence, which can be measured by an instrument. The plant always fluoresces a little, but an excited state means there’s something wrong with the photosynthesis. In this case, the plant is unable to utilize the vast majority of the solar energy to produce sugars. This occurs when there’s too much light, but also if the sugars can’t be transported sufficiently. The latter is the case if there aren’t enough locations the sugars can go to, such as to growing leaves or buds.

Marijuana leaves heat stress
Warming

The third effect of light is the warming of the plant. This can be useful; the rate of many processes increases at higher temperatures. But too much warming leads to heat stress. The plant can maintain an acceptable temperature through evaporation. If there’s not enough water in the soil, the plant can’t absorb enough water to evaporate. The warming of the plant could cause much more damage this way.

Amount of light

The plant is very efficient in the conversion of solar energy into sugars in order to grow. If us humans could develop such a system, we wouldn’t be able to do it as efficiently as plants.

More light often is better for the functioning of the plant, because it will produce more sugars. This larger amount of sugars creates a more powerful plant with a faster growth and more buds. Also, the plants stay more compact when they catch a lot of light, so you can fit more plants on the same surface area.

There is however a limit the plant can handle regarding the amount of light. At a certain point, the plant can no longer convert more light into more sugars, because the photosynthesis systems can’t use the light anymore. More light will therefore have no use and could become harmful. The plant will warm up (also at low environment temperatures) and could cause damage to the edge of the leaves. So there’s a limit to the amount of light you should expose your plants to. More light is usually better than less light, but don’t overdo it.

Marijuana plants have a maximum of 600 watt for each 12 square feet. There is no minimum, but the more the better, up to a maximum of 600 watt. You could also get a 1000 watt lamp, but with that much light the plant will need more water, nutrients and CO2. And CO2 is particularly difficult to add. You would need a major investment and a lot of knowledge on growing marijuana. The spectrum of the light is just as important as the amount of light.

Different grow lights

Sunlight consists of a range of colors from blue to red, all the colors of the rainbow. The colors of light the plants receive determine certain developments of a plant. White-blue light for instance promotes growth and orange-red encourages flowering. People have best vision at yellow-green and blue-green light, so our lights our homes are adjusted to this. You therefore need different lights for growing marijuana. Lately, a lot of developing and experimenting has been going on with different types of lamps, such as high-pressure sodium lamps, colored LEDs and plasma lamps. Here is a short overview of the different types of lamps:

Different grow lights regular
Light bulbs

During transplanting, the soil has to be a bit moist, so the clod will easily come out of the pot. A very dry soil won’t come off the pot and can harm the roots. It’s also not good for a plant to be so dry. If the soil is soaking wet, it could fall apart when putting the plant in the pot, causing the root system to break off.

Different marijuana grow lights CFL
Different grow lights- CFL

These lamps were often used for cultivation in the 70s and 80s. The spectrum is very wide, but the cool white version is very suitable for cultivation, especially for during the vegetative stage. Cuttings and seedlings are often cultivated under fluorescent lighting. These lamps are not as suitable for flowering, but it’s possible. Especially the compact fluorescent lighting with a high wattage can provide a nice yield.
LED

Different grow lights Led
Different grow lights- CFL

LED lighting is increasingly on the rise, and increasingly better lamps are being developed. They virtually give off no heat, so you won’t have to extract hot air. A drawback is that they’re very expensive and that the yield doesn’t outweigh the investment, in my opinion. Once professional vegetable cultivators move on to LED, the technique is sufficiently developed. I haven’t had a successful harvest under LED lights yet.

Different grow lights hpi
Metal halide

These lamps are much more powerful than fluorescent lighting and are perfect for the vegetative stage, because of the blue spectrum. I always use an MH lamp during the vegetative stage and an HPS during the flowering stage. You can also let your plants flower with MH lamps, but you’ll end up with plants with some more leaves.

Different grow lights hps
HPS

These are the best lamps for the flowering stage, because of the red spectrum and the high light output. All professional vegetable cultivators use them in their greenhouses to provide their crops with additional light during cloudy days. I always use them during the flowering stage, and also during the growing phase if I don’t have an MH lamp. HPS lamps come in various wattages, and this is the ideal surface area they can illuminate;

  • 250 watt – 30×30 inches or 20×40 inches
  • 400 watt – 40×40 inches
  • 600 watt – 4×4 feet
  • 1000 watt – 5×5 feet

Light cycle

When using lamps, the question of how long they can stay on automatically arises. marijuana plants must have a dark period, otherwise they get impaired growth. If the lights are always on, the plant can’t get rid of the products from photosynthesis as well so the produced sugars won’t be sufficiently transported. In this case, starch granules will accumulate in the chlorophyll, which causes damage to the chlorophyll and therefore affects photosynthesis.

The light duration also depends on the phase the plant is in. Plants need more hours of light when they’re in the vegetative stage than when they’re flowering. Just like in nature; when plants germinate in the spring, the days get longer and they’ll grow. Once the days get shorter and the plants receive fewer light hours, the plants will automatically start flowering in order to reproduce before winter arrives.

Indoors, we can determine how much light a plant receives. We provide the plant with 18 hours of light and 6 hours of darkness during the vegetative stage. As long as they receive 18 hours of light, your plants will continue to grow. So we can determine how big they will grow. If we want our plants to start flowering, we give them 12 hours of light and 12 hours of darkness. From that moment on, it takes another 8 weeks (for indicas) before they’re ready for harvest.

It’s important to make sure that the plants receive no light at all during the dark period. Any light causes stress to the plants and could mess up their rhythm. Flowering plants could even revert back into the growing stage, so make sure your room is completely lightproof. Since plants can’t absorb green light, you could light your room with a green lamp if you want to go inside and take a look.

I used a 400 watt MH lamp during the growing stage and a 600 watt HPS lamp during the flowering stage for the grow journal in this course. In my opinion this is the best and most cost-effective setup. If you’re not willing to invest as much, or if you want to start out a bit smaller, fluorescent lighting is also a good option. Feel free to post on the forum if you have any questions about lighting.

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