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.

Having enough light isn’t the only issue, however. 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. 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 Visiondemujerperonista.com

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.

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.

How many weeks this stage lasts for Colour of light Lighting schedule Distance of light Intensity of light (brightness)
Seedling 1-2 weeks Blue 24 hours of light Depends on light Depends on light
Veg 3-5 weeks Blue 18 hours on 6 hours off Depends on light Depends on light
Flower 7-10 weeks Red-far red 12 hour on 12 hours off Depends on light Depends on light

Distance of Light

Distance of Light
Distance of Light – Image powered by Ledgrowlightsexpert.com

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 Light Closest ~ Sunlight Furthest
150W 5″ (13cm) 7″ (18cm) 11″ (28cm)
250W 6″ (15cm) 9″ (23cm) 13″ (33cm)
400W 8″ (20cm) 12″ (30cm) 19″ (48cm)
600W 9″ (23cm) 16″ (41cm) 25″ (64cm)
1000W 11″ (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 Stage Maximum Good Minimum
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 Growlightsinfo.com

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.

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.

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.

Preventing Waste When Using Grow Lights
Preventing Waste When Using Grow Lights – Image powered by Growweedeasy.com

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.

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 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.

The founder of I Love Growing Marijuana, Robert Bergman, is a marijuana growing expert that enjoys sharing his knowledge with the world. He combines years of experience, ranging from small-scale grows to massive operations, with a passion for growing. His articles include tutorials on growing... [read more]


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  1. By Roger dixon ,13 Oct 2018
    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 […]Read More
    1. By latewood_ILGM ,16 Oct 2018
      Roger, Thanks. We will look into getting this corrected. :)
  2. By osvaldo reyes ,07 Mar 2019
    Muy buenas las enseñanza y le pienso sacar el mejor provecho gracias amigos.
  3. By Allan ,04 Aug 2019
    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 […]Read More
  4. By latewood ILGM ,05 Aug 2019
    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.
    1. By Merle ,12 May 2020
      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?
      1. By Nod ,09 Sep 2020
        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 […]Read More

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