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Maintaining Optimal Lighting

Mar 4, 2020
Optimizing Yields

Maintaining Optimal Lighting for Marijuana Growing

Sunlight provides a complex array of wavelengths including the visual light spectrum required by plants. Indoor marijuana lighting synthesizes the spectrum of visual light. Regular lightbulbs intended for regular light are not functional grow lamps.  Indoor marijuana growing lights emit visual wavelengths that extends between 440 and 700 nanometers (nm) on the Photosynthetically Active Radiation (PAR) light measurement scale.

Light Measurement

Understanding light measurement is key to achieving optimal lighting for indoor marijuana growing. Voltage and amperage are the units used to express measurements of electrical current. Similarly, lumens and lux are used to express light intensity measurements. Lumens measure the intensity of light from the source, while lux is the measurement of light in a given square meter of space. Understanding both is important for indoor marijuana growers.

The Life-Light Cycle of a Plant

Maintaining optimal lighting is a constant challenge throughout the growth cycle, because the amount of light a plant needs varies based on its current stage of growth. Experts recommend that marijuana plants receive 5,000 – 7,000 of lux during the seedling stage; between 15,000 – 50,000 lux during the vegetative stage; and 45,000 – 65,000 during the flowering stage.

The Color of Light

Essentially, the wavelength of light depicts the color of light. Many growers do not realize that the color of light effects how the plants grow at different stages of life. During the vegetative stages, blue light is best. The most common way to get blue light is to use MH lights. Blue light mimics sunlight in the middle of summer, when the sun is high in the sky. HPS lights mimic the red light of the sun at the end of the summer. Use red glowing HPS lights for the plant during its flowering stages to effectively mimic late-summer’s red spectrum light.

Distance of Light

Lights should be five inches from the plant’s canopy when using a 150-watt light. With 250-watt lights the distance increases to six inches. With 400-watt lights the distance is 8 inches, 9 inches for a 600-watt light, and eleven inches for a 1000-watt light. Please note these recommendations are for high intensity discharge lights, not LEDs.

New growers should also note that the distance of light can vary based on the strain of marijuana and requires a certain amount of trial and error to get the ideal distance. Sometimes the most difficult part of maintaining optimal light is discovering which plant-to-light distance is optimal. Unfortunately, this can often require several crop cycles.

During the earliest stages of the plant’s life, growers must ensure that plants receive enough light to develop strong branches and stems. A light source that is too far away from the plant’s canopy will cause the plant to reach for the light and stretch toward it. The plant knows when it’s not getting enough light to survive and will therefore attempt to save itself. The stretching toward the light often causes the stem to develop inadequately, resulting in a weak stem prone to bending or snapping under pressure.

It is critical that indoor marijuana growers learn the recognize the signs of a plant that is too far away from the light source. Too little light causes plants to form weak stems that cannot support the weight of the final crop.

The Daily Schedule of Lights

Perhaps the thing that makes maintaining optimal lighting for marijuana grows most difficult is the changing schedules of light that photoperiod marijuana plants need to grow. The lighting schedule is essentially how many hours of light a plant receives in a day compared to how many it spends in darkness.

Like humans, plants need time to “sleep”, and they can’t sleep in broad daylight. Unlike humans, the amount of “sleep” a plant gets is directly proportional to how much time they spend in the darkness.

During the vegetative stage of life, plants require more light than darkness. Marijuana cultivation experts recommend that plants receive 18 hours of light and 6 hours of darkness during this stage. During the flowering stage, they recommend that plants receive and even 50/50 balance of day and night, or twelve hours of light followed by twelve hours of darkness.

As with all other aspects of growing marijuana, this too depends upon the strain. Cannabis strains that are primarily Sativa are not as sensitive to the light cycle. The reason is that these varieties of marijuana originated in tropical environments where they naturally receive long days of sunshine. Marijuana strains that are largely Indica, however, are far more temperamental, and require absolute darkness to flourish.

Light Positioning

It is important that grow lights shine directly on the plants. If the light hits the wall or the floor more directly than the plant itself, then the plant will attempt to grow to reach the light. Growers know by looking at their plants when they’ve correctly positioned the lights. All young plants will grow directly toward the light.

As the plant canopy rises in the light’s direction, the grower must periodically reposition the light to ensure that the plant does not get too close to the source and become burned. A marijuana plant is like a moth: it naturally wants to get as close to the source of light as it can.

The Reflectivity of Light

Something that professional photographers understand well is the importance of reflected light, and how it illuminates the subject of a photograph. For indoor marijuana growers, that subject is the plant’s leaves, and the goal is to get as much light to hit the surface of them as possible.

The best way to achieve this is to position plants in a room that has good reflective surfaces. Reflective surface light bounces and increases the amount of light distributed over the growing plants. Maintaining optimal lighting is largely about preventing wasted light. For young marijuana plants, the reflected light would ideally reach the underside of their leaves.

Optimize the Grow Space

When seeking to maintain optimal lighting for indoor marijuana plants, remember that the lights themselves are only part of the equation. Every surface in an indoor growing space that the light touches should be as reflective as possible. For the plant’s surrounding area, materials such as Mylar and dimpled aluminum are the most frequently recommended by field experts. Paint the walls of a grow room with white primer. Primer paint offers the purest white and is a better reflector than most other paints and colors.

Remember, light that doesn’t reach the plant’s leaves is nothing more than a waste of electricity. Light waste significantly increases the grower’s expenses and cuts into their profit margin. Nobody wants to spend extra money for nothing. That is the situation marijuana growers face if they don’t make every surface in their grow space reflective.

Measure, Adjust and Measure Again

The best general advice that anyone can receive about marijuana lighting (other than listen to what the plant is telling you) is to measure, adjust, and then measure again. Refer to the “Distance of Light” paragraph above to see the exact distances from light to plant. One of the surest signs that a grower is maintaining optimal lighting is that they keep having to move the lights according to the size and shape of their weed plants.

Prudent marijuana growers should keep a ruler next to their plants. Measure the distance between the marijuana plants’ leaves and their light source. Avoid letting the plant’s canopy get too close to the light. Remember that the grow light fulfills the function of the sun. Like anything else, too much of a good thing is harmful. It is important to make sure plants are not so close to their source of light that the leaves wilt or burn. If your hand gets too hot under the lights, the plants will be too hot as well.

Metallic Thermal Tape

Metallic thermal tape is a product used primarily to fasten heating ducts and dryer vents. Its strength and highly reflective surface make it the preferred tape used by indoor marijuana growers. It is no easy task to cover every surface of a growing space with sheet aluminum, but it is possible to cover almost anything, no matter the shape, with highly reflective metallic thermal tape. An additional benefit of metallic thermal tape is how it tolerates heat without the adhesive being compromised.

The process of photosynthesis wouldn’t be possible without the visual spectrum of light that provides plant leaves with photons. The chlorophyll in marijuana plant leaves converts the spectrum of light energy into glucose with the help of CO2.

While deciding on what light source to use, it is important to know which strain(s) you’ll be working with. To purchase seeds from an online marijuana seed bank, visit

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