Cannabis Cultivation Lighting Explained

Cannabis sativa is a short day plant that possesses great medicinal qualities. Short day plants change from the vegetative growth stage to the floral growth stage when they are exposed to critical short day lengths. For cannabis this is 12 hours of light to trigger flowering. Most growers will keep cannabis is 16 to 18 hours of light during the vegetative growth stage.

When growers do not have the option of growing cannabis outside, they will have to decide on the best cannabis cultivation lighting for indoors. This is a challenging task. Aside from picking the right type of light, there is an abundance of grow light terminology that can be confusing. Understanding grow light terminology will help guide decision making and allow growers to optimize their indoor cannabis grow setup. Though, it does not make sense to learn about this before you've finalized your choice of indoor strain. If you're still deciding on a strain, we urge you to check out our linked resources and to finalize your decision before continuing to read this article. Check out the following: What is the Highest Yielding Indoor Strain for Cannabis?, Cannabis Strain Selection, and Master Grower Tips!

Light is essentially waves of electromagnetic radiation which are measured on the electromagnetic spectrum. There are three ways to measure radiation on the electromagnetic spectrum: frequency, wavelength, and photon energy.

PAR, PPFD, and DLI are common lighting terms that cannabis growers will encounter. Since light is not tangible, it can be hard to understand how it is measured. Measurements of light describe its different qualities like color, intensity, and the amount of energy it can deliver to the plant.

Photosynthetic photon flux density (PPFD) is expressed as μmol m-2s-1, which is micromoles per square meter per second. PPFD describes the number of energy particles (photons) in the photosynthetic active radiation (PAR) range that fall on a one square meter area in one second. Low PPFD irradiance may limit photosynthesis at the leaf surface. High PPFD irradiance in excess of photosynthetic needs at the leaf surface may be a threat to plant metabolism.

Photosynthetic active radiation (PAR) are the wavelengths of light that can be used in photosynthesis. The measure used for PAR is watts per square meter (W/m2). PAR tells growers the wavelength or color of photosynthetic light, but to describe the photon energy, or intensity of the light, another measure is needed.

Daily light integrals (DLI) are another important measure of photosynthetic light that describes the number of photosynthetically active photons that reach a particular area in a 24 hour period. DLI is expressed as mol m-2s-1, which is moles per square meter per second. DLI is the sum of PPFD in a day. Where PPFD expresses light intensity in a second, DLI expresses the sum of those measurements in a 24 hour period (86,400 seconds per 24 hours).

Lumens measure of the total quantity of visible light emitted by a source. Lumens help to describe light intensity but do not include all types of light that are usable to plants. Lumens are are based on light that is visible to the human eye, so growers should pay more attention to a bulb’s PPFD, which will be expressed in micromoles.

Watts (W) are a unit to measure the electric currency a bulb draws, a watt is equivalent to one joule per second.

Plants use pigments to harvest light to fuel photosynthesis. The best cannabis grow lights put off colours and intensities of light that will pass the most amount of energy to pigment molecules like chlorophyll. Chlorophyll mostly captures blue and red light wavelengths. Plants also have accessory pigments to capture other colours of light.

Red light falls between 600 nm and 700 nm on the electromagnetic spectrum and is absorbed by the chlorophyll b pigment. The brightest red light occurs around 660 nm on the spectrum and will be the most efficient red wavelength for the plant to absorb. Growing plants under only red light produces undesirable elongation, so some amount of blue light is needed.

Red light plays a major role in plant flowering by activating phytochrome pigments. When low intensities of red light (1 μmol∙m-2∙s-1) interrupt darkness, they can inhibit flowering in short day plants. As cannabis is a short day plant, lighting for the vegetative growth phase may emit red and far-red light.

Blue light falls between 400 nm and 500 nm on the electromagnetic spectrum and is absorbed by the chlorophyll a pigment. In photosynthesis, blue light is less productive than green and red light, but a minimal amount of blue light is needed to prevent irregular growth. At intensities greater than 20 µmol m2s1 blue light can inhibit flowering in short day plants like cannabis, and promote flowering in long day plants.

There are several different types of grow lights for cannabis growers to select from. LED lights have become very popular and are becoming more affordable alternatives to high intensity discharge lamps. In order to select proper lighting, growers need to consider PAR, PPFD, DLI, and other factors that describe a light’s electrical use and lifespan.

LED grow lights have gained popularity because they can be customized to emit only the most productive wavelengths for plant growth. They can be placed closer to the canopy of a cannabis crop because they do not emit as much heat as high intensity discharge lamps. Full spectrum LED lights can come with 2 settings, a red setting and a blue setting. The blue setting will support vegetative growth in cannabis, and the red setting will support flowering. Growers should select lights with 32 watts per square foot of space, or about 32 watts per plant.

High pressure sodium (HPS) lamps and metal-halide (MH) lamps are two popular types of high intensity discharge (HID) lamps used in growing cannabis. HPS lamps have a lot of red light and are commonly used in the flowering growth stage. MH lamps have more blue light, cannabis growers will use them during the vegetative growth stage. HID lamps produce a lot of heat, so it is important to have very good ventilation when they are used. HID lamps should have about two feet of space above the crop canopy to avoid burning the plants.

Lighting is one crucial variable the grower can control to ensure a healthy and successful plant. Certain types of lighting can overheat and even burn your plants, which is why it's so important to thoroughly plan your operations. For more information, and actionable tips from our Master Grower on how to improve your yields, please get in touch with us today by filling out the form, or starting a chat with us to the right.

Speak To A Cannabis Consultant Now