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.

Main lighting considerations for growing cannabis

Having enough light isn’t the only issue. There are actually four main things to consider when it comes to light if you want to create the optimal environment. They are:

  • The distance of the light
  • The intensity of the light
  • The color spectrum of the light
  • The frequency of the light (lighting schedule)

When growing outdoors, nature provides what’s needed for the plant’s photoperiod.

Growing marijuana indoors with grow lights
Using grow lights for growing marijuana

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

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.

Using grow lights
Using grow lights

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.

Download my free marijuana grow guide for more lighting tips and growing your own marijuana.

Grow Bible

marijuana grow bible
  • Grow with my Quick Start Guide
  • Discover secrets to Big Yields
  • Avoid common grow mistakes


    How a marijuana plant grows

    How a marijuana plant grows is determined by the way the grow lights are used.

    The best types of bulbs to use are metal halides and high-pressure sodium lamps.

    Fluorescent bulbs are good too since they emit the blue light that is best for foliage growth.

    The MH bulbs work their best during the vegetative stages, while HPS bulbs work their best during the flowering phases.

    The best way to enjoy the advantages of both bulbs is to use them together.

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

    Stage
    period
    ColourLighting
    schedule
    DistanceIntensity
    (brightness)
    Seedling1 – 2
    weeks
    Blue24 h of 
    light
    Depends 
    on light
    Depends 
    on light
    Veg3 – 5 
    weeks
    Blue18 h on
    6 h off
    Depends 
    on light
    Depends 
    on light
    Flower7 – 10 
    weeks
    Red –
    far red
    12 h on
    12 h off
    Depends 
    on light
    Depends 
    on light

    Distance of light

    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.

    Growing marijuana indoors with grow lights
    Using grow lights for growing marijuana

    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 LightClosestSunlightFurthest
    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.

    Growing cannabis indoors with high quality grow lights
    Intensity of grow light

    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
    Stage
    Max.
    (Lux)
    Good
    (Lux)
    Min.
    (Lux)
    Veg~70,000~40,000~15,000
    Flower~85,000~60,000~35,000

    Color of light

    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.

    Color of light
    Color of light

    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.

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

    Indoor marijuana seeds

    Buy indoor marijuana seeds

    • Wide selection of indoor seeds
    • High THC levels
    • Consistent heavy yields
    • Easy going high for everyday use

    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.

    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. 

    Installing grow lights for growing marijuana indoors
    Installing grow lights

    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, it 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.

    best lighting system for growing marijuana
    Cost effective lighting system

    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. This will allow less light to be wasted.

    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.

    Know how to perfectly time your harvest so you can maximize your yield. Download our free mini harvesting guide.

    Harvest Guide

    marijuana harvest guide
    • Time your harvest for perfect taste
    • Get THC levels for a perfect high
    • Don’t waste any precious bud


      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:

      Mylar

      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 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.

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

      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.

      Plastic

      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

      What types of bulbs are best during vegetative and flowering stages?

      The MH bulbs work their best during the vegetative stages, while HPS bulbs work their best during the flowering phases.

      How many hours of light do marijuana plants need?

      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.

      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!

      Robert

      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 led him to develop his own Gold Leaf strain. Now, Robert is dedicated to sharing his knowledge with the world.... [Read full bio]

      GROW BIBLE

      marijuana grow bible
      • Grow With My Quick Start Guide
      • Discover Secrets To Big Yields
      • Avoid Common Grow Mistakes


        21 comments on “Maintaining optimal lighting for cannabis plants”

        1. Reflective paint they use for train signs, and such, is a killer, for reflecting light, ask ? And you shall recieve, god bless!

          Reply
        2. I started my Second grow and i only had 4 plants this time i got 6 grown and cured Off One viper Spectre P1000 had a good first grow so i Bought the p1500 and just recieved it and Wasn’t going to use it in this grow but i havE a white widow that is growing Faster and taller then the other 3,can i add my new light and use the more powerful one on the 3 and use the P1000 on just the white widow so i can Keep more light focused on all plants i guess what im saying Is it ok to add light to indoor already Vegging plants,

          Reply
          • Hi Troy,

            Thanks for your question. Adding additional lights to your setup should not be an issue. Make sure the plants aren’t too close to the lights though as this could cause light stress.
            And if you really want to be careful about this, you can acclimatize the grow room but putting it on for a couple of hours at a time at first. Though this really should not give you any issues.
            I hope this helps, but let us know if you have further questions, of course!
            Kind regards,
            ILGM

            Reply
        3. 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?

          Reply
        4. 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.

          Reply
        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?

          Reply
        6. 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!

          Reply
        7. 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.

          Reply
          • 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?

            Reply
            • 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.

        8. 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???

          Reply
          • 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

            Reply
        9. 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.

          Reply