Maintaining Optimal Lighting for Cannabis Plants

When growing cannabis, lighting is essential.  The best possible lighting is the natural light produced by the sun.  It has the perfect mix of UV rays that plants crave, and the closer you are the equator, the better those rays become.

Marijuana needs plenty of light to produce quality plants and buds.  On average a marijuana plant needs twelve hours of darkness each day to begin flowering. Generally speaking, the more light your plants receive, the better and stronger they grow, leading to higher yields.

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    When growing outdoors, nature provides what’s needed for the plant’s photoperiod. The color spectrum is naturally regulated by the seasons, and the frequency is managed by day and night. Plants grown near the equator have the shortest distance and highest intensity of light.

    However, when growing indoors, you get to control the photoperiod so you can regulate when your plants will flower.  Although the sun is free and provides the perfect light for marijuana plants, some places on earth just do not see enough sun.  If you live in such an area, then growing your marijuana plants in a greenhouse with controlled lighting is probably best.

    Using grow lights

    Using grow lights
    Using grow lights – Image powered by

    While outdoor growing has its benefits, some growers prefer using grow lights because they provide greater control over their plants and growing season. However, just shining random light bulbs at your plants isn’t enough to help them grow properly. For lighting to act as a replacement for the sun, it needs to be produced by bulbs and grow lights specially made for growing marijuana.

    Here’s how Stefanie Chan of Grobo, a grow box company, explains how the four lighting basics apply to grow lights.

     How many weeks this stage lasts forColour of lightLighting scheduleDistance of lightIntensity of light (brightness)
    Seedling1-2 weeksBlue24 hours of lightDepends on lightDepends on light
    Veg3-5 weeksBlue18 hours on 6 hours offDepends on lightDepends on light
    Flower7-10 weeksRed-far red12 hour on 12 hours offDepends on lightDepends on light

    Distance of Light

    Distance of Light
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    Light distance is critical to the growth of your plants. Too far away, and your cannabis won’t get all the light that it needs, however, if too close, you risk giving your precious plants light burn.

    Keeping your lights at the correct distance is crucial during your plant’s seedling stage. One of the biggest mistakes that growers make is keeping their lights too far away. This results in “leggy” or stretched stems that flop over and can’t support the weight of the plant. These stretched stems occur as the seedlings reach for more light.

    The actual optimal distance of your light will vary depending on the type of light you use, and your growing space. Here are some basic guidelines:

     Light distance from the plant canopy
    Grow LightClosest~ SunlightFurthest
    150W5″ (13cm)7″ (18cm)11″ (28cm)
    250W6″ (15cm)9″ (23cm)13″ (33cm)
    400W8″ (20cm)12″ (30cm)19″ (48cm)
    600W9″ (23cm)16″ (41cm)25″ (64cm)
    1000W11″ (28cm)21″ (53cm)31″ (79cm)

    *Chart is based on HID lights (MH, HPS, etc.), and not LED lights.

    Intensity of Light

    Plants that get more light tend to grow better and output higher yields – it’s a fact. However, it is easy to over saturate your plants with bright light and cause a light burn. It is also easy to set your lights too dim, causing your plants to receive inadequate amounts of light and “stretch” or have stunted growth.

    Light intensity, or brightness, can be measured in both lumen (lm) and lux:

    • Lumen – measures the flow of light which is emitted from a source. The higher the lumen, the brighter the source of light.
    • Lux – measures light intensity that falls on a surface. Because plants only pick up the light that falls on its surface, growing guides will usually measure light levels using lux.

    Similar to light distance, the optimal light intensity will also depend on the type of light that you choose to use in your setup. Below are some guidelines to help you get started:

    Life StageMaximumGoodMinimum
    Vegetative~70,000 lux~40,000 lux~15,000 lux
    Flowering~85,000 lux~60,000 lux~35,000 lux

    Color of Light

    Color of Light
    Color of Light – Image powered by

    Many people don’t realize that the color of light can affect the growth of your plants. Visible light actually behaves as a wave, and it displays varying properties depending on the length of its particular waves.

    For example, a light with a wavelength of 400nm will be detected by the human eye as being purple in color.
    Certain types of lighting solutions display a specific color of light.

    For example, MH produces a predominantly blue color light, whereas LED lights can isolate and display a variety of colors.

    In terms of plant growth, blue light is best used during the seedling and veg stage, as it influences the formation of chlorophyll, a chemical that makes plants grow faster and stronger.

    MH lights are often used for this, as its blue light is designed to mimic the summer months when the sun is high in the sky.

    Conversely, HPS lights mimic the end of the summer, with the sun’s rays passing through more of the earth’s atmosphere, causing it to display a red spectrum, which is great for flowering cannabis plants.

    Want to grow seeds that will thrive in an indoor environment? Check out our selection of indoor marijuana seeds in our shop.

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    Lighting Schedule

    The final important component to proper lighting is the actual amount of time your plants receive light. Just like humans, plants need their sleep and cannot normally be hit by sunshine for 24 hours a day. Plus, cannabis lighting can become expensive when it comes to the amount of electricity used. This is another reason why it is a good idea to have a schedule for your cannabis lighting.

    You cannot keep marijuana under continuous light because it will not flower or produce any buds without darkness.  The only time that plants can use 24 hours straight of light is when they are in their seedling stage, as they are still babies and need lots of light to grow.

    While in the dark, plants produce the hormones needed to produce buds that eventually flower.  If the darkness is not ever disturbed by light, they will continue to flower and stop growing.  You need a good combination of both to yield a good crop of marijuana.

    For the duration of their vegetative stage, your plants need 18 hours of light and 6 hours of darkness.

    Once you are ready for your plants to flower, you can trigger its flower stage by switching the lighting schedule to 12 hours of light, and 12 hours of absolute darkness.

    Preventing Waste When Using Grow Lights

    As explained above, light is essential for growing high quality, high yielding cannabis plants. Exposure to light is what triggers plant photosynthesis, and not getting the proper type or amount of light will result in stunted growth for your plant.

    Unfortunately, many growers miss out on high yields because their indoor setting is wasting light.

    If quality wasn’t reason enough to want to control your lighting, finances might be.

    Marijuana is dependent on the hours of light it receives to grow effectively, yet the cost of the grow lights, combined with the cost of electricity to use them, can become quite expensive.

    Luckily there are some ways to lower this, by being more efficient with your light.

    One of the easiest ways to do this is by making simple adjustments to your grow room. Use reflective materials and keep cannabis plants as close to the wall as possible. The will allow less light to be wasted.

    Preventing Waste When Using Grow Lights
    Preventing Waste When Using Grow Lights – Image powered by

    By carefully choosing the wall’s surface, you can increase the amount of light received by the plants. The reflective materials help direct the light to the plants and keep it on them. They can also help shine light on places that would otherwise be dark, providing light, heat, and energy for the lower parts of the garden.

    Ensuring Reflective Walls

    The material that you use on your walls can provide a reflective surface for light. To measure how effective a particular material is, use a light meter.

    Measure by placing an opaque board a few inches away from the wall with the meter below it.

    The golden rule is to make sure that both measurements are at the same distance from the light. When the light reflects onto the light meter, it should provide different numbers for the two surfaces.

    There should be a difference between those numbers – that difference reflects how well your wall (or reflective material) reflects the light.

    Another thing to have in mind is that the wavelength of radiant energy, which is also referred to as electromagnetic radiation, is 400-700 nm and EM radiation is correlated to radiant heat energy which has a wavelength 800-2000 nm.

    Reflective walls in a grow room
    Reflective walls in a grow room

    Creating Reflective Grow Spaces

    If your current walls are not reflective enough, you can fix that! Here is a list of the most frequently used materials for the walls of a marijuana grow room:


    This polyester film has a thickness of 1-2 mm and a high level of reflection. There’s also a more expensive, but resistant and cleanable version called foylon.

    Mylar may be less durable, but it is far more reflective than foylon; however, since foylon is easy to clean, it can provide better reflection over the long term.

    C3 anti-detection film is yet, another, type of mylar with the same qualities as the 2mm thick one. In addition to providing high levels of reflection, it is also infrared proof.

    Good ventilation is required if you used either of these materials because of the high percentage of reflection of radiant heat energy. Avoid creating hotspots in the room while attaching Mylar, foylon or C3 film to the walls.

    You can use Velcro when attaching the materials to your walls to prevent them from being damaged. Be sure that no pockets of air forms between the two surfaces.

    White Paint

    Flat white paint on a marijuana grow room
    Flat white paint on a marijuana grow room

    Flat white paint is also perfect for grow rooms. It reflects well, is low maintenance, and there is no need to worry about hot spots. You should add fungicide, however, while painting.

    The paint must be pure white, and flat. Glossy paint and eggshell colors do not reflect as well. Also, make sure that you do not stain the walls because it will damage the reflection. Titanium white is highly reflective, but it is rarely used due to its high price.

    Specialty Paint

    Elastomeric paint is another surface treatment that provides good reflection. It is also rubberized, so it is quite resistant. The Kool Seal Roof Coating is one cost-effective version of this paint. It forms an expanding and contracting rubber-like blanket that is perfect for almost any surface.


    The white or black plastic known as panda plastic or ‘’poly’’ is an easily cleanable surface for a temporary room. It will not damage the walls and helps prevent hotspots, but it can melt if it’s too close a light source.

    There are many options when it comes to light for your marijuana plants.  Everything from the type of bulb you use to the arrangement of the lights will affect the growth of your marijuana plants.

    Setting up your lighting system perfectly is essential for a great yield, but it can take many tries to get it right. If you don’t want to experiment, you can also use an automated grow box, but it’s not required. With some practice, you can learn how to create the best lighting for cannabis plants.

    FAQs About Cannabis Lighting

    Practice and learning are all you need to become an expert in growing marijuana. Let my blog help you get started.

    What other tips can you share about cannabis lighting? Please share your comments or leave questions below.

    Happy growing!


    Avatar for Robert Bergman

    Robert Bergman

    Robert Bergman is an Amsterdam-based marijuana grow expert who has years of experience from small grows to massive operations. His passion for growing lead him to develop his own Gold Leaf strain. Now, Robert is dedicated to sharing his knowledge with the world.... [Read full bio]


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      18 comments on “Maintaining Optimal Lighting for Cannabis Plants”

      1. seedlings are leggy (4-5 inch stem topped with 3-4 leaves) Plants have now dPlants have toppled over. Following directions carefully for indoor growing conditions. Is there a rescue plan for more leaves and stronger ste?

      2. I am trying to find info about autoflower plants and 12×12 light cycle. I have some autoflowers that are about 14 days from harvest. Also some femenized Blue Dream that have been in veg for 10 weeks or so. I would like to begin flowering the BD but do not wish to negatively affect the Autoflower. All stock from ILGM btw.

      3. Yes, similar to what I use. My kit contained everything you’ll need (bar seeds, nutes etc)

      4. Yeah you do. That 600W globe will make that tent into an oven. You need an exhaust fan to pump that hot air out.

      5. I have a small space in the house 7′ x 7′ walls are brown paneling and I don’t want to paint them white. Should I get a growing tent? Do I really need to buy a fan kit? I was looking at the vivosun 4′ x 4′ tent with the 600w led light. It’s 230 for the tent then 110 for fan kit. Is spending 340 worth it in your opinion?

      6. Start with a small pot then bigger for the next faze and then another for the final faze flowering and just miracle grow potting soil, bloom and feed

      7. Sun reflectors for your car or truck & there cheap. Or hot water tank blankets which cost around 50$ Canadian

      8. I use 4 4 ft T8 fixtures in my grow tent. I also have a 2000 watt grow light hanging from the top of the tent that I adjust as needed. My last crop was excellent. Using Super Skunk seeds. I didn’t use self flowering or scrip, so they grew all around the tent!

      9. Keep them in dark until you see them raise their heads, then 18 hours of light, 6 dark.

        I just planted 8 year old cannabis seeds and they came up in 2 days.

        5 for 5.

        They are under a 400 watt LED that is dimmed to about 1/3.

      10. I just planted my seeds yesterday. Someone I read put lights 18-24 in the dark. I also read leave in in dark with no lights. Which is correct?

      11. Allan. We are not set up in the blog for Q and A. I Suggest You Go Here: Support.Ilovegrowingmarijuana.Com Our Members And Staff Can Help Give You Informed Advice. We Have A Lot Of Topics That You Can Research.

      12. I’ve bought an auto-flowering Sativa strain. What size pot should I get ie. 1 gallon 2 gallon. What type of potting soil and what nutrients (and when). I would prefer to buy from you but I live in Canada. Is there any way I could get your nutrients to Canada???

      13. Muy buenas las enseñanza y le pienso sacar el mejor provecho gracias amigos.

      14. Your “light distance from plant canopy” chart is a little out in imperial to metric eg ‘ 8″ (15cm)’ in reality 8″ is 20cm. 5″, 6″, 9″ and 11″ all seem to be 1-5cm out to. I know the measurements are for approximation of heights but 8″ is not 15cm.