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The Importance of Light for Growing Cannabis

What weed looks like in different light - UVA, UVB, and UVC light spectrum in a grow house.

Why Does Light Matter?

Cultivating cannabis at its core is not much different from cultivating other plants in the grand scheme of things. It, like most other plants, needs several things in order to survive, grow, and bloom, whether it is in the wild or in a growhouse. These essentials can be distilled down to water, nutrients, and light!

The subject of choosing light for growing weed is a complicated one and something that a cannabis dispensary like Seed and Smith takes pretty seriously. We have to understand the science behind light. Call us the Einstein’s and Edison’s of marijuana. We could write a book on the subject and how light affects cannabis growth something we discuss on our Denver dispensary tour  extensively. But for now, we want to talk about the basics and why light is so important for growing weed.

In this blog, we will be focusing on light and the role it plays in the life cycle of the cannabis plant. At the very least, you probably remember the term photosynthesis from your middle school science class. This is the process by which plants harness the energy from light and turn it into chemical energy from which they can stimulate new growth. [1] The process is a little more complex than that, but for our purposes, this definition will do just fine.

Light Spectrum & Life Cycle

So, what exactly is good light for growing weed and what is the life cycle of a cannabis plant? For that, we have to discuss the relationship between Cannabis and UV light. However important light may be, it is a contextually vague term; there are many different types of light! For starters, light as we know it is made up of electromagnetic radiation, which can range from gamma rays or x rays, to radio waves and visible light, and a whole host of other names. [2] These different types of radiation are determined by the wavelength of the radiation. Some wavelengths are harmful and can cause serious problems such as skin cancer with prolonged and repeated exposure. Other wavelengths are harmless and manifest themselves simply as visible light, such as the radiation emitted from your desk lamp. What does this have to do with the best light for growing weed… cannabis craves UV light in a specific wave length. But we’ll get to that. 

Visible light ranges from about 380-760 nanometers(nm) in length, ranging from violet on the lower end, to red on the higher end. Radiation on either side of this spectrum is not visible to the human eye, but just because we can’t see it doesn’t mean it doesn’t affect us. Radiation shorter than ~380nm is referred to as ultraviolet radiation, or UV light. Radiation longer than ~760nm in length is referred to infrared radiation. Each of these types of light has their own uses such as disinfecting medical tools or controlling your TV! 

Within ultraviolet light, there are three further classifications; UVA, UVB, and UVC light. UVC is the shortest wavelength and the most harmful to living organisms, UVA has the longest wavelength and is the least harmful. Organic UVC light is almost completely absorbed by the ozone layer of our atmosphere, shielding us from the most harmful radiation the sun sends our way. 

UVB light is the second shortest wavelength and is the main culprit of sunburn. If you’ve ever had a nasty case of sunburn from laying outside too long, you know just how powerful this type of UV light can be! UVB light penetrates the top layer of our skin in as little as 15 minutes and can cause lasting damage, including skin cancer.

UVA light is the longest of the three types, and makes up 95% of the UV light that actually enters our atmosphere. This type of UV light can cause skin aging. You may also be familiar with it as the type of radiation people use to tan. While not as harmful as UVB or UVC, prolonged and repeated exposure to even UVA light is linked to skin cancer as well.

The sweet spot, the light for growing weed that is the most beneficial for the plant, is said to lie in that UV range. Adding in a UV spectrum can significantly increase yield… but we’re getting to that.

UV Light Spectrum


So how do these different lengths of radiation or light affect the growth process of cannabis? Besides providing the basis for photosynthesis as mentioned, light also helps plants know what stage of its life cycle it is in. In an outdoor setting, light levels are changed naturally with the seasons, and the amount and intensity of sunlight received. This is why trees will shed their leaves in fall when there is not as much sunlight, and bloom in the spring when the days start becoming longer again. These light levels can also be manipulated artificially in an indoor setting by mimicking the typical light levels found during the day and at night.

The cannabis plant is no different! There are two distinct stages to the life cycle of the cannabis plant; the vegetative stage and the flowering stage. [3]

The vegetative stage is when the cannabis plant prepares for its flowering stage. This means it is fortifying its roots, growing leaves, and strengthening its limbs. In order to stimulate the vegetative stage of the cannabis plant, it must be exposed to shorter wavelengths, in the 400-500nm range. [4] This means blue light! Cannabis can be kept in the vegetative state by providing it with 18+ hours of light per day, which mimics spring/summertime. This can range anywhere from 18 hours of light and 6 hours of darkness, to constant 24 hours of light. Different botanists will have their own preferences, and other factors can affect the split including temperature, electricity costs, and the specific strain being grown. 

Once a cannabis plant has been kept in the vegetative stage for the desired time and has built up a sturdy base, it’s time for the flowering stage. In order to force a cannabis plant into the flowering stage, it is recommended to shorten the available light to ~12 hours a day, as well as change the wavelength to ~620-780nm. This mimics the shortening days and the lower angle of the sun in the fall and winter. This type of light is more red or orange in color. [6]

Cannabis under the blue light spectrum on the left, versus under the red light spectrum on the right


When it comes to how much a certain cannabis plant produces in terms of its final yield of plant matter, there isn’t too much research that supports any one method as being superior. The important thing is to let your cannabis plant spend adequate time in the vegetative stage so that it can grow big and strong, and able to support more nugs when it reaches maturity, then blast it with high intensity light in order to stimulate the flowering stage. And this creates much more potent cannabis flower.

This is where the true creativity of the individual grower and the holistic methods they employ have a chance to shine! One grower may prefer to leave their cannabis plants in the vegetative state a little while longer than usual. Another grower may prefer to enter the flowering stage as soon as possible in order to keep a tight schedule.

Another interesting factor to consider is the yield of a cannabis plant in terms of its THC, CBD, terpene, cannabinoid, and flavonoid content. The cannabis plant has its own ‘natural sunscreen’–defense mechanisms it uses to guard itself from harmful UV light which can damage the plant’s DNA. This defense mechanism involves increased production of trichomes in order to deflect and absorb some of this harmful radiation. As we know, increased trichome production equates to a greater abundance in cannabinoids!

Obviously, growers would like this increased trichome production, but without exposing their crops to harmful UV light. The trade off is exposure to UVA light, which is relatively non harmful to cannabis plants, yet triggers the same defense mechanisms described above and thus increases trichome production. [7] UVA light is in the ~315-400nm wavelength, and it is becoming more and more common for cannabis cultivators to incorporate this type of radiation into their growing methods in order to stimulate increased THC, CBD, and terpene yields.

Types of Lighting for Growing Cannabis

Now that we’ve gone over the importance of light, let’s go over some of the different types of lights used in cannabis cultivation, and the pros and cons of each. [5]


Fluorescent lights are the basic, low wattage lights that growers use when they are on a tight budget or working in a small grow operation. These lights work fine for the seedling and vegetative stages, but lack the intensity needed for the flowering stage. Fluorescent lights are prevalent in many different areas, and you may recognize them from their trademark, twisted appearance in everyday life.

High Intensity Discharge

Metal halide lights are a type of HID (high intensity discharge) light. This type of light is powered by filling a tube with gas and then sparking it in order to create radiation. These types of lights are very efficient, however they mostly emit blue light. This means, similarly to fluorescent lights, they are effective for the vegetative stage but not so much for the flowering stage. [8] That is why growers using HID lights will usually incorporate a combination of HID lighting; a softer light such as metal halide with a stronger light such as HPS (high pressure sodium) lights to stimulate the flowering stage. In fact, this combination has been nicknamed the ‘golden standard’ within the cannabis realm, and for good reason. [9]

HID lights are also infamous for getting extremely hot – that is why if you use this type of lighting you must have a spacious grow to let the heat dissipate or have some other exhaust method in order to get rid of excess heat. Be careful, as they can also rack up your electricity bill! As a ballpark, HID lights can usually last for about 20,000 hours – however their efficacy drops off quite a bit before that threshold.


Light emitting diodes (LED’s) are a relatively new addition to the cannabis cultivation industry, but are gaining in popularity quite rapidly. LED’s have certain advantages over other types of lighting that make them very popular with modern day grows. They are more efficient than other types of cannabis lighting for one. They are also more versatile, as one LED setup can cover both spectrums needed for the vegetative and flowering stages, reducing the need for a combination of different lights. They also run much cooler than HID lights which allows you to save space in your grow as well as worry less about dissipating excess heat. They can also last for an astounding amount of time, in some cases nearing 100,000 hours!

As you can tell, lighting in your cannabis grow operation is very important, and it can be quite complex to choose and build a lighting system once you get granular with grow space, time limitations, budget constraints, and available technology. While there are baseline good practices for cultivating cannabis, there is a lot of creativity involved as well depending on the preference, skill level, and end goal of the cultivator.

At Seed & Smith, we are always looking out for new advancements in technology at any stage in our process, and lighting is no exception. The better we make our product, the more we can experiment with powerful strains and products like the dart pod. We look forward to seeing how this part of the industry progresses as more and more research becomes available. Thanks for tuning in!


  1. [1] LiveScience – What is Photosynthesis
  2. [2] Nasa – Tour of the Electromagnetic Spectrum
  3. [3] CannaConnection – The Importance of Light for Growing Cannabis
  4. [4] Royal Queen Seeds – Cannabis Cultivation: The Light Spectrum and Ways to Raise THC Levels
  5. [5] Grow Weed Easy – Grow Light Breakdown: Heat, Cost & Yields
  6. [6] Grow Weed Easy – Learn About The Cannabis Vegetative Stage
  7. [7] Greenhouse Grower – Lighting Strategies for Higher Terpene and THC Content in Cannabis
  8. [8] Grow Light Info – Metal Halide Grow Lights
  9. [9] Grow Weed Easy – MH and HPS Lights for Cannabis
January 19, 2021
Cannabis Education
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