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Cannabis Cultivation: The Light Spectrum and Ways to Raise THC Levels

Creating an ideal environment for cannabis plants is only achievable by understanding the principles of nature – the light spectrum is a factor that cannot be ignored.

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Most cannabis growers have multiple objectives in mind when planning an indoor grow. Drafting scenarios to achieve higher yields, increase THC levels, or simply to improve the overall health of a plant is an integral part of their hobby. This element of strategic planning involves the challenge to link knowledge of different scientific fields and to match those findings to a technical solution that helps to achieve predefined goals. Besides dedication and passion, it is the willingness to learn that differentiates good growers from future experts – so let us try to grow the royal way and learn what it takes to cultivate cannabis of exceptional quality. Today, we are looking at fundamentals of physics, and learn how the light spectrum affects the growth of a cannabis plant.

WHAT IS THE LIGHT SPECTRUM?

The sun emits energy in the form of solar radiation including gamma rays, x-rays, ultraviolet light, visible light, and even radio waves. Life on Earth is only possible because the ozone layer blocks this radiation, and reflects most of it back into space. This filtering process only allows wave lengths between 300nm and 1100nm to reach our plants and an even smaller portion of this light is visible to us. It is often referred to as the light spectrum, color spectrum or visible spectrum, and ranges from 380nm to about 750nm.

  • 180-280nm – UVC: Extremely harmful and luckily almost completely absorbed by the ozone layer
  • 280-315nm – UVB: Cause of sunburn and suspected to increase THC levels (!)
  • 315-400nm – UVA: Not absorbed by the atmosphere, commonly known as black light
  • 380-750nm – The visible light spectrum: Bands of wave lengths represent visible colours
  • 700nm-1mm – Infrared light: Not visible above 750nm but noticeable as heat on our skin

COLOUR TEMPERATURE (KELVIN) AND HOW IT AFFECTS YOUR PLANTS

When shopping around for a grow light, you will likely come across the term “colour temperature”. This is essentially a way to describe the light appearance provided by a bulb, and is measured in Kelvin (K).

Colour temperature doesn’t mean the physical temperature of your light, but the degree of warmth or coolness of a light source—the “visual temperature”. When a light has a higher degree of Kelvin, it has a more blueish appearance. Thus, we call it a “cool” light. On the other hand, a bulb with a lower degree of Kelvin emits a “warmer”, reddish light.

IS COLOUR TEMPERATURE THE SAME AS LIGHT SPECTRUM?

In a strictly scientific sense, no. Colour temperature is normally used as a way to describe how the light produced by a lamp looks to the human eye. For some types of lights, such as LEDs or fluorescent lamps, it doesn’t describe a light’s spectral distribution or wavelength.

Without going too deep into physics here, the light from an incandescent bulb radiates light spanning the entire visible light spectrum. The white light from the bulb is the result of a mix of wavelengths (colours in the spectrum) “contained” in the light.

Other lights, such as LEDs or fluorescents, may emit light from a number of narrow wavelengths, with gaps or peaks within the spectrum. In other words, even if the light appears the same to the eye, it may be missing certain wavelengths (colours) that plants require for healthy growth.

Because LEDs tend to emit light in a very narrow colour spectrum, LED grow lights are usually outfitted as “full-spectrum” setups. They consist of a number of different-coloured LEDs that together cover most of the necessary spectrum for cannabis plants. These full-spectrum LEDs are comprised of different reds and blues, often mixed with additional white LEDs. Other, newer LEDs, such as COB lights, emit a light spectrum that more or less approximates natural sunlight; there’s no “gap” in the colour spectrum.

WHERE DOES KELVIN COME IN WHEN CHOOSING A GROW LIGHT?

For vegging your cannabis plants, go with a cool light, one that emits a “daylight” colour with a high Kelvin of 6,000–6,500K. For flowering, a warm light with a reddish tone, about 2,800K, is optimal. You can also find grow lights with a “best of both worlds” colour temperature of about 3,500K, which you can use for both vegging and flowering.

HOW THE LIGHT SPECTRUM AFFECTS GROWTH

Every organism living on Earth needs information what is going on around them to react to environmental changes, and ideally, get a slight advantage over other members of their species regarding natural selection and evolution. Interestingly, cannabis plants receive a lot of their information from the light they’re exposed to, and almost instantly react to different bands of wave lengths – a complex topic to fill books with, but let us focus on the basics first.

1. Vegetative Stage – “Blue” light for healthy leaves (range: 400-500nm; ideal: 460nm)

During the vegetative stage it is recommended to aim for as many leaves as possible, and to make sure plants stay rather compact, don’t stretch too much, and develop strong stems. Indoor growers tend to use metal halide bulbs, compact fluorescent lamps (CFL’s), or T5/T8 lighting fixtures with a blue band of light for the first few weeks to achieve these goals. When cannabis grows in nature, the high angle of the sun in spring and summer allows more “blue” wave lengths to penetrate through the atmosphere, a signal for cannabis plants to grow strong, large and healthy leaves.

2. Flowering Period – “Red” light for giant buds (range: 620-780nm; ideal: 660nm)

When cannabis plants enter the flowering period, highest yields can be achieved by exposing them to a light spectrum that contains lots of “red” wave lengths to promote budding. The rate of photosynthesis peaks when plants are subjected to “red” wave lengths of 660nm although latest NASA findings suggest that even “green” wave lengths, which are not associated as a major factor in photosynthesis, can also have an impact on how plants grow. Seeing a cannabis plant as simple photosynthesis factory is consequently a little hasty. But for now, choosing a lighting solution with a high degree of “red” in its spectrum remains the best way for growers to imitate the shallow angle of the sun in late summer and autumn.

INCREASING THC LEVELS WITH UVB LIGHT – MYTH OR REALITY?

Have you ever wondered why potent cannabis strains often originate from landraces that naturally grow in high altitude regions? There are experts who suspect ultraviolet light, especially a high exposure to UVB wave lengths (280-315nm), to be responsible for an increased THC production. The theory is based on the fact that a high elevation means lesser atmosphere between cannabis plants and the sun, leading to a higher exposure to UV rays. These ultraviolet wave lenghts knowingly damage our skin, and the human body reacts by producing melanin as protection – a cannabis plant assumingly does something similar – it produces more resin and THC as a form of natural sunscreen. It is too early to say if we are dealing with a theory or a cost-effective method to grow better cannabis but the concept seems plausible enough for hands-on experiments. UVB bulbs for reptiles only cost a few bucks; perhaps we should give it a try.

Seeking methods to increase THC production feels natural for growers – learn how the light spectrum can affect growth and potency of weed.

How To Select the Best Grow Lights for Your Marijuana Grow

Throughout our articles we have discussed several success factors of an indoor production. Here is a short summary: Growing indoor will provide you with the opportunity to influence all environmental factors. This way you can optimize soil, air, water and grow light to create the perfect conditions for your plants. The more you take care of them, the better your yield. And with light it is easy. Always remember: More light = bigger yields!

Lights are essential because they largely determine your plants’ grow cycles, their photosynthesis and therefore their health and their buds. Lighting is the food of your marijuana plants. When first creating your grow room make a blue print to decide what system for water management, light and air flow you want to use. In this piece we will discuss the details about lighting.

Which Lighting is Best for Growing Weed?

There are 4 main types of lamps that you can use for growing cannabis:

Fluorescent grow lights (T5 and CFL)

Metal Halide grow lights

High Pressure Sodium grow lights

While each of these options has pros and cons, as well as different stages of growing cycle and different sizes of operation they work best for, in this article we’ll focus more on the types that will be your bread-and-butter for a commercial-scale grow operation.

Pro Tip 1

Normal light bulbs won’t work for your plants 🙂

LED Grow Lights

LEDs are super easy to set up. Small ones are literally plug and play. Once you have plugged them into a wall you may just hang them above your plants. They are the way to go for an easy setup that still grows great yield.

They don’t run very hot but for a professional production you still will have to set up exhaust fans and organize airflow and temperature throughout your growing facility.

Make sure to keep enough distance to your plants with LEDs to avoid light burn. Also, be sure to get a light that includes green and white light for full spectrum. Only red or blue light will not be enough for your plants.

LEDs are the way to go for small production up to 1 ounce of cannabis per month, but produce slightly lower yield per watt than HPS and Metal Halide lamps, and are generally less optimal for professional grows.

Fluorescent Grow Lights for Cloning Young Plants

Fluorescent lights will be your go-to for the very first stages of your plants’ lives. They don’t use much electricity, they’re cheap, and they’re also very popular with many hobby gardeners and so are very easy to find.

Compact bulbs (CFL) that you can find in any hardware store can great for small or narrow areas, or grow tent set ups for beginner grows. For your professional production, however, you are going to want to look for T5 Grow Lights (the long tube-like ones) which can be found in a home and garden center.

Those lamps are best used for cloning, seeding and young plants. Without burning the plants you are able to place them close to the plants and save electricity. Also, they do not produce a lot of heat, and the purple-white spectrum is ideal for seedling plants.

However in the vegetative and flowering stages, keep in mind that fluorescents will lead to smaller yields. While T5 lights are great for small plants, in the later stages you will want to use a higher powered light like HPS or Metal Halide.

Pro Tip 2

Use T5 flourescent grow lights for cloning, propagation, and seedling stages

Metal Halide Grow Lights for Vegetative Stage

These lights are one of two types of High Intensity Discharge (HID) grow lights, and are incredibly efficient. Metal Halide (MH) lights typically come in an integrated fixture with an external ballast and reflector hood, and will need proper ventilation due to high heat output.

These bulbs, along with High Pressure Sodium (HPS), the other type of HID grow light, produce the highest yields per watt of electricity out of any grow lights available. For this reason, they are the go-to for professional growers in the later stages of plant development.

Metal Halide produces a bluish spectrum which is ideal for the vegetative stage.

Pro Tip 3

In the vegetive stage switch from T5 fluorescents to HIDs. Use a ratio of 2 Metal Halide lights to 1 HPS light for maximum yield.

HPS Grow Lights for Flowering Stage

Simlar to Metal Halide, HPS grow lights must be used with reflector hoods and ventilation. The fixtures used for Metal Halide bulbs and HPS are typically the same, so you can use the same fixture for both vegetative and flowering stages, and simply swap out the bulbs.

HPS produces a more yellowish/full spectrum that promotes budding and is ideal for the flowing stage. These bulbs are an absolute must-have for a professional grow.

Pro Tip 4

In the flowering stage, switch to using only HPS bulbs to promote budding and maximize yield.

Pro Tip 5

HPS and Metal Halide lights will get very hot and you want to use them with an exhaust fan to not burn your plants.

To understand the climate and temperature that your plants need, check our article on air flow and ventilation which will teach you how to keep the right temperature in your sealed room.

The lamps are well suited for growing marijuana and if you are looking for the highest yield HPS/Metal Halide is the way to go. Just consider the extra amount of work and cost for setting them up including cables, exhaust fan and fixture. Make sure to get a digital ballast. They use less electricity and bulbs will live longer.

Pro Tip 6

High quality HPS and Metal Halide grow lights will typically create 2 to 4 times more yield than fluorescents.

Why Your Plants Love Light

Your plants ability to perform photosynthesis is the key to your commercial success. Make sure to put some thought into your lighting before getting started. Shining just about any light on your plants is not going to do the trick.

Light waves come in a colour spectrum. The sun is a full spectrum light. A low light intensity might lead so long plants because they stretch to receive enough light. When having high intensity your plants may turn our shorter. You can adjust the light intensity by choosing the distance from your plant.

In the vegetative stage your plants will want up to 18 hours of light. In the flower stage they grow best in 12 hours of light and 12 hours of darkness. Your ability to choose the rhythm of lighting lets you create the perfect light timing for your plants.

Make sure you provide a minimum of 6 hours of darkness for your plants. The 12/12 hours flowering cycle will make your plant believe it is late summer which is their time to develop buds. You want to start this after 2 to 4 weeks of growth in the vegetative phase. This is the time where you can switch from fluorescents to HPS grow lights.

Pro Tip 7

Gently switch from one cycle of light to the other. Take a couple of days to switch from 18/6 to 16/8 to 14/10 to 12/12 hours.

Pro Tip 8

Use a timer to do the switching. This way your lights are always on schedule and you do not have to worry about it. Timers can be easily shopped online or in any store that has electronics for home improvement.

Grow lights are the heart and soul of your plants’ health. If you are small or on a budget you might start out with fluorescent lights but keep in mind that this will affect your yield. Use a mix of fluorescents and LED grow lights for vegetative phase and HPS for flowering stage for professional purposes.

Updated: December 17, 2019

Lights determine your marijuana plants' grow cycles, photosynthesis, health, and the quality of their buds. Which Lights are Best for Growing Weed? There are 4 main types of lamps that you can use for growing cannabis: LED grow lights, Fluorescent grow lights (T5 and CFL), Metal Halide grow lights, and High Pressure Sodium grow lights.