what should the ppfd be for marijuana plants

The Ultimate Lighting Guide for Cannabis Cultivation

A Science-Based Guide for Understanding Grow Light Terminology and Lighting Options

Sabine Downer
Nov 30, 2018 · 20 min read

Grow Lights for Cannabis

When growers do not have the option of growing cannabis outside, they will have to decide on the bes t cannabis grow lighting for indoor growing. Growers have a challenging task when deciding what grow light to use for cannabis indoors. Grow lights can also be needed for growing cannabis in greenhouses. Greenhouse grow lights for cannabis will have very different functions than grow lights for indoor cannabis cultivation. Aside from picking the right type of light, there is an abundance of grow light terminology that can be confusing. Understanding grow light terminology is essential for creating the best indoor cannabis grow.

Cannabis is Photoperiod Sensitive

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. Another way to reduce energy use during the vegetative growth stage of cannabis is to use a gas lantern routine.

The Gas Lantern Routine

The gas lantern routine runs lights on a schedule where they are on for 12 hours, off for 5.5 hours, on for one hour, then off for 5.5 hours again. This cycle repeats and maintains vegetative growth by tricking the plant with the one hour of lighting between off periods. Instead of using 16 or more hours of light, only a total of 13 hours is used with the gas lantern routine.

Photoperiod Lighting for Cannabis

Cannabis grown outdoors will move through growth stage changes as conditions naturally change. When grown in a greenhouse or grow room, growers can take advantage of this trait, called photoperiodism, to shorten or extend cropping time. Lighting specifically for inducing growth stage responses in day length sensitive plants is referred to as photoperiod lighting.

Photoperiod neutral plants are not sensitive to day length and will flower based on other cues. Autoflowering cannabis plants flower based on the plant’s age. These unique cannabis plants have this adaptation because they are native to areas near the equator that do not experience short days soon enough in the natural growing season, and to areas like Siberia where day lengths can be shorter than 12 hours before the plant has reached sufficient vegetative growth to move on to flowering.

Daylight Extension Lighting vs Supplemental Lighting

Long day plants like spinach and lettuce also exist. These plants need a critical long day length to initiate flowering. Greenhouse growers may need daylight extension lighting to flower long day plants or keep short day plants from flowering. Greenhouse growers of short day plants will also need to use blackout systems to ensure successful flowering of short day plants like cannabis. Supplemental lighting is a different type of lighting that can help support greenhouse plants when days are very overcast and there is not sufficient natural light.

Photosynthesis in Cannabis

Photosynthesis is an important metabolic process in plant growth. More than 90% of the dry matter of a plant is created from photosynthetic conversion of atmospheric carbon dioxide (CO2). Elevations in CO2 can accelerate plant growth and improve photosynthetic productivity. Doubling CO2 concentration has been shown to increase total crop yield up to 30%.

In order to convert maximum amounts of CO2 plants need to have good lighting and proper temperature to fuel the process. Photosynthetic uptake of CO2 and production of plant material in cannabis are optimized when temperatures are kept around 25°C.

How Plants See Light

Plants can differentiate light based on its electromagnetic wavelengths. Light that is not visible to humans, like ultraviolet (UV), are visible to plants. Plants can also tell the time of the day from light, and based on the length of daylight, the can tell the season. In order to do this, they must be able to sense brightness and intensity of light, light exposure length, and location of light sources. Phototropism is a common phenomenon in plants where they bend towards light sources. There is evidence that blue light wavelengths help the plant locate the light source.

Plants do not have a nervous system to translate light into pictures like humans, but they are very much able to “see” in a way that is as complex as human sight. Cryptochromes are blue light receptors that plants and humans both have. Blue light is not photosynthetically active, but it helps cue the plant’s internal clock.

Phytochrome is another special plant molecule. This molecule becomes primed when it is exposed to red light, then phytochrome can absorb the next wavelength which is far red light (nearly infrared). Phytochrome regulates many stages of plant development like germinations, stem and leaf growth, and flowering.There is also evidence that phytochrome helps the plant sense temperature.

Measuring Light on the Electromagnetic Spectrum

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. Frequency is measured in hertz (Hz) and corresponds to wavelength which is measures in nanometers (nm) or micometers (μm). Light can have low or high frequency and wavelengths can be long or short. Based on the range of light’s frequency, it can be divided into different bands in a spectrum.

Photon energy is the amount of energy carried by a single photon. Photons are packets of energy that have no mass, they are measured as an electronvolt (eV) or as joules or microjoules (1 joule = 6.24 x 1018 eV). Their energy is proportional to the electromagnetic frequency, and inversely proportional to the wavelength of light. If a particular frequency is high, the photon will have high energy. If a wavelength is long, the photon will have lower energy. Photons with the same color of light will have the same frequency, and the same energy. Intensity of the radiation is not captured by photons, so light from a campfire and light from the sun will have the same photon energy.

The electromagnetic spectrum contains high frequency and high energy radiation like gamma rays, and radiation with low energy photons like radio waves. Photosynthetic light falls mostly in the visible light spectrum between 400 and 700 nm. Ultraviolet (UV) radiation falls between 100 nm and 400 nm. UV-C light at 100 nm to 280 nm is very damaging for plants, but UV-A (315 nm to 400 nm) will not damage plants. For cannabis, UV-B light between 280 nm and 315 nm may stimulate increases in THC production.

Photosynthetic Light for Cannabis Growth

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 Active Radiation

PAR stands for photosynthetic active radiation. PAR are the wavelengths of light that can be used in photosynthesis. PAR sensors can be used at different levels of the cannabis plant canopy if the grower wants to measure how much useful light is penetrating through the leaves. The measure used for PAR is watts per square meter (W/m2).

Photosynthetic Photon Flux Density

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. Photosynthetic photon flux density (PPFD) is expressed as μmol∙m-2∙s-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. A 2010 study found that the most productive PPFD for cannabis is 1500–2000 μmol∙m-2∙s-1 with environmental conditions at 25–30°C with CO2 concentration elevated to 750 ppm. Under these indoor conditions photosynthesis in cannabis was optimized.

Lighting with intensities of 1500–2000 μmol∙m-2∙s-1 can draw a lot of electricity and be very expensive to run. Most growers will use light that is less intense and produces a good yield. Seedlings, clones, and mother plants can be successful with just 200 to 400 μmol∙m-2∙s-1 PPFD. For plants in the vegetative growth phase of cannabis 400 to 600 μmol∙m-2∙s-1 PPFD can be sufficient. Flowering plants will benefit from more intense light with 600 to 900 μmol∙m-2∙s-1 PPFD.

Daily Light Integrals

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-2∙s-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).

Natural DLI changes based on latitude. In the northern latitudes of North America days will be shorter and nights will be longer, so the DLI will be lower than in southern latitudes where days are longer. This has big implications for greenhouse growers working with short day plants like cannabis. Cannabis begins to flower when light exposure periods are less than 12 hours. Day lengths can naturally drop under 12 hours earlier in the year in the north than in the south. Many greenhouse growers in the north will have to use daylight extension lighting to make sure their cannabis plants have sufficient vegetative growth before they move on to flowering.

Plant Pigment Molecules

Plants use pigments to harvest light to fuel photosynthesis. Chlorophyll is the most common and abundant plant pigment. Chlorophyll mostly captures blue and red light wavelengths. Plants also have accessory pigments like carotenes and xanthophylls to help harvest additional light from the full spectrum of wavelengths. If light wavelengths are too short (like UV-C) they can damage pigment molecules in a plant. If a light wavelength is too long it won’t carry enough energy particles to provide the energy needed for the photosynthetic process to activate.

Red 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. Red light overall is as good as stimulating photosynthesis as blue light.

Growing plants under only red light produces undesirable elongation, so some amount of blue light is needed. The ratio of red light to far-red light also influences leaf growth and stem elongation. When the amount of far-red light increases relative to the amount of red light, stem growth is elongated. When using LED lights with little to no far-red light, plants can have very compact overall growth.

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 or stimulate flowering in long day plants. As cannabis is a short day plant, lighting for the vegetative growth phase may emit red and far-red light.

Blue 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. A minimal amount of blue light is needed since it prevents irregular stem elongation and leaf shrinking. Plants grown with blue light tend to be shorter, thicker, and have darker green leaves. Plants that are deprived of blue and UV light can have purple leaves because short wavelength radiation stimulates production of leaf coloration compounds.

Blue light is also important for regulating opening of the stomatal pores on the underside of the leaves. Stomata need to open to take in carbon dioxide. Some water is lost when they open so it is important for them to be well regulated.

Typical photoperiodic lighting intensity for blue light is 1–2 µmol∙m-2∙s-1. At this intensity blue light will not influence flowering in long day or short day plants. At intensities greater than 20 µmol∙m-2∙s-1 blue light can inhibit flowering in short day plants like cannabis, and promote flowering in long day plants.

The Best Grow Lights for Cannabis

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, lifespan, and color temperature.


Watts are a unit to measure the electric currency a bulb draws, a watt is equivalent to one joule per second. Amperes measure the strength of the electric current. Some electronics draw large currents that require outlets that can put out more amperes. It is very important to be sure that lighting equipment is compatible with the outlet capacity. High intensity discharge lamps and fluorescent bulbs must also be compatible with the ballast where they are screwed in.

Radiant Flux and Lumens

Radiant flux is the amount of radiation a light source emits, including infrared (IR), ultraviolet (UV), and visible light. The amount of radiant flux that is visible to the human eye is expressed in lumens. One lumen per square foot is equal to one foot-candle, which is a term that describes the density of light that reaches a surface.

Lumens are not a good measure of lighting for plant growth because they measure light visible to the human eye, but many bulbs will talk about their efficiency as lumens per watt of energy consumed.

Lumens can also help to describe the life expectancy of a bulb, they are expressed as lumens per watt (LPW). As light bulbs are used, the initial lumens will decrease. The life expectancy of a bulb is reached when lumens have dropped by 50%. High intensity discharge (HID) lamps have a life expectancy of 10,000 to 20,000 hours, and LEDs can last up to 50,000 hours.

Light Bulb Color Temperature

The “whiteness” of a light is measured by its color temperature in units of Kelvin (K). Light with a lower Kelvin rating has a yellowish tint, and light with a higher Kelvin rating has a bluish tint. In photomorphogenesis plants respond to colors in the light spectrum by altering their growth patterns. Seed germination and the switch from vegetative growth to floral growth are two important photomorphogenic reactions.

Natural daylight has a color temperature of 5000 K to 5800 K. Natural visible light color perception can vary based on weather and season, but this is not to be confused with the electromagnetic spectrum light colors which do not change. A light bulb that is identical in color to natural daylight would be rated as a 100 on the color rendering index.

Fluorescent Grow Lights

Fluorescent lights come in a variety of spectrums. “Natural sunshine” fluorescent lights have a broad spectrum of peaks, where yellow and conventional fluorescent lights have just a few wavelength peaks around 540 nm and 630 nm. Full spectrum lights will emit all colors of the visual light spectrum from 400 nm to 700 nm.

T5 Fluorescent Grow Lights

Tubular fluorescents are labeled T5, T8, and T12. The “T” indicated that the bulb has a tubular shape, and the number indicates the diameter of the bulb. T5 is the thinnest type of fluorescent bulb. T5 bulbs make good grow lights because they are very efficient and come in a wide range of colors. T5 bulbs cover a grow space around 2×4 ft, additional coverage can be gained by use of multiple bulbs wired together in a “daisy chain”. PLL T5 bulbs are “U” shaped and can provide the efficiency of a tube bulb in a more compact space.

Fluorescent Grow Light Ballasts

Fluorescent lights plug into a ballast that regulates the electrical current flowing into the bulb to ensure it is supplied at the correct voltage. There are three main types of ballasts: preheat operation, rapid start operation, and instant start operation. Preheat operation is used mainly for bulbs under 30 watts, and is more energy efficient than rapid start operation. Rapid start operation is the most popular fluorescent ballast operation mode. This is the mode where the stereotypical fluorescent light flicker occurs. Instant start operation is more efficient than rapid start, but instant and preheat start operation will reduce bulb life.

Compact Fluorescent Grow Lights

Compact fluorescent (CFL) bulbs are more efficient than traditional fluorescent bulbs and they can be screwed in to a regular (E27) light socket. No additional ballast is required for CFLs. CFL bulbs come in a range of brightnesses, from 13 watts to 300 watts. They can also be purchased in different colors such as warm spectrum bulbs (2700K) or cool spectrum bulbs (5000K to 6500K).

LED Grow Lights

Light Emitting Diodes (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 pressure sodium lamps or metal halide lamps.

Heat sinks in the LED light housing help them dissipate heat well. LED lamps consist of several small lights in a grid, called an array. The LED driver functions like a ballast and regulates the input power, the driver also protects the light emitting diodes from voltage fluctuations as they heat up and age.

Viewing Angle

The diodes of an LED light are covered and sealed in by a lens. The qualities of the lens can influence the viewing angle of the light. Viewing angle refers to the pattern the light disperses in. High intensity discharge bulbs and fluorescent bulbs have a viewing angle of 360°, but LED grow lights typically have a 120° viewing angle.

Color Settings

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. Full spectrum LED provides 730 nm (IR), 660 nm, 630 nm, 610 nm, 580 nm, 460 nm, 430 nm, and 410 nm (UV) light wavelengths. Growers usually need LED grow lights for cannabis that have 32 watts per square foot of space, or about 32 watts per plant.

Halogen Lamps

Incandescent bulbs are very familiar light bulbs that are used in homes. Halogen lamps are a type of incandescent bulb that has a small amount of a halogen gas such as iodine or bromine inside the bulb. Halogen lamps are more efficient than a standard incandescent bulb, but are less efficient than HID bulbs.

Halogen lamps can be used for horticulture, but are very inefficient at generating light that supports plant growth. The spectral output of many halogen bulb is in the orange to yellow range. Cannabis plants grown under halogen lamps tend to have long internode spacings, poor vegetative growth, and low yields. For these reasons, cannabis growers do not commonly rely on halogen lamps to provide photosynthetic lighting. Halogen lamps also produce a dangerous amount of heat that presents a major fire hazard.

High Intensity Discharge (HID) Lamp Family

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 lamp wavelength spikes at 632 nm, 605 nm, 589 nm, and 568 nm light wavelengths. MH lamp wavelength spikes at 674 nm, 630 nm, 583 nm, 564 nm, 540 nm, 497 nm, and 422 nm.

Cannabis plants respond better to HPS lamps, or a combination of HPS and MH lamps than to MH lamps alone. 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.

HPS Grow Lights

High pressure sodium (HPS) lamps use sodium in an excited state to produce light. HPD grow lights are a high efficiency bulb that is preferred for flowering due to large amount of red light. HPS lamps have unbalanced full spectrum light with large outputs of green, yellow, and red light and very little violet, blue, and cyan light. HPS bulbs replace MH bulbs after vegetative growth.

Small-scale growers can have success with 150W, 250W, and 400W HPS grow lamps for cannabis. Commercial growers producing large crops usually prefer 600W or 1000W HPS grow lams for cannabis.

Metal Halide Grow Lights

Metal halide (MH) lamps are a type of HID lamp that produces light by running an arc current through vaporized mercury and metal halide gas. Efficacy of MH lamps range from 75-105 LPW (lumens per watt). Metal halide grow lamps require a minimum of 4-15 minutes to restart after turn-off, this is known a a restrike period.

Metal halide grow lamps are popular for use in vegetative growth phase of cannabis due to very high blue content of light. Common wattages of metal halide grow lights fro cannabis include 250W, 400W, 600W, and 1,000W.

Ceramic metal halide (CMH) lamps are a new variant of metal halide grow lamps that use an extremely hot ceramic tube to ionize gasses and salts chosen for a specific spectral output. CMH are becoming popular with indoor cannabis growers because they produce light that is very similar to natural sunlight. CMH grow lamps also have more red than a standard metal halide lamp, so they deliver more light that can fuel photosynthesis.

Mercury Vapor Lamps

Mercury vapor lamps produce light by arcing electricity through mercury vapor. They are very old members of the HID family that have very low efficacy and do not have the proper color spectrum for plant growth.

HID Grow Lamp Ballasts

HID lamps receive electrical currents through a ballast that makes sure they run on the appropriate voltage from an outlet. The total wattage needed by the ballast and lamp are the input watts. Hoods often integrated with the ballast for HID lamps, some hoods have a reflective lining to help redirect light towards the plants. Reflective hoods are called reflectors or parabolic reflectors.

Magnetic ballasts are a type of ballast with an iron core. They are heavy, inefficient, and produce a buzzing noise. Digital ballasts are more expensive than magnetic but are more energy efficient. Ballast with built in cooling are helpful since HID lamps produce a lot of heat.

Air cooled hoods are sealed reflective hoods that connect to ducts equipped with an exhaust fan to move out heat. Water cooled hoods are more efficient than air cooled hoods and allow cannabis growers to move lamps closer to the plant canopy. Water cooled hoods work by pumping chilled water from a reservoir above the sealed reflective hood which transports heat away from the lamp.

Reflective Materials for Light Dispersal

The reflectivity of a material is its ability to reflect light away without absorbing or diffusing it, or altering its quality, intensity, or spectrum. Reflectors inside bulbs maximize efficiency, but to truly maximize lighting efficiency cannabis growers often line grow rooms with reflective materials like mylar, visqueen, astrofoil, or foylon. Flat white paint with a high titanium dioxide content is also very effective. Small scale cannabis growers may opt for a reflective grow tent.

When light is reflected from all angles of the grow room, lower portions of the cannabis plant can receive more light. If lower branches of cannabis plants do not receive as much light as the top of the canopy, their growth will be reduced and their buds will take longer to mature. Placing plants on a table helps reflected light reach these lower branches and help the plant grow evenly with buds that will all be ready to harvest at once.

Cannabis Lighting Problems

Light saturation is the limit to how much energy is able to be processed by a leaf. Lighting that is more intense than the leaf can handle can result in photoinhibition, where photosynthesis becomes inhibited by the excessive light. Bleaching from light can also occur when there is too much light. Bleaching causes chloroplasts to turn white, so leaves lose their green color. Plants can also get burnt when lights are placed too close to the canopy.

Effect of Grow Lights on THC Production

In a 2012 study focused on determining optimal lighting power, 250–1000W/m2 high pressure sodium lamps were used in growing the cannabis varieties Early Pearl, Hindu Kush, Super Skunk, White Widow, Wappa, White Berry, and G1 by GW Pharmaceuticals. Yields were measured in grams per square meter (g/m2) and grams per watt of light (g/W).

Light and Foliage

In cannabis foliage, this study found that light had no effect on foliage mass or foliage derived THC mass. In cannabis flower, the mass of flowers increased corresponding to increased light strength. Increase light strength also increased the ratio of flowers to foliage. Total quantity of floral derived THC increased with light strength.

Light and THC Production

A THC potency increase of up to 0.192% for each 100 w/m2 increase in irradiance up to 600 W/m2 PAR was observed. THC levels measured were 2.64% at 270 W, 2.53% at 400 W; 2.73% at 600 W. This is likely due to the fact that terpenoid (THC) synthesis requires more energy than synthesis of most other compounds in the plant.

Limits of Photosynthesis in Cannabis

No benefit to plant weight was shown with irradiance levels over 300 W/m2 PAR, this could be the point of light saturation for cannabis. Grams of floral biomass per watt rose to 1.2 g/W when light irradiance was lowered to 400 W/m2 from 600, and to 1.6 g/W with 270 W/m2. The findings of this study supported a 2006 study predicted a floral yield of approximately 505 g/m2 dependent on light range of 300g/m2 to 800 g/m2.

Practical Applications for Growing Cannabis

Overall, this research demonstrated how light quality can impact the quality of the cannabis plant. While irradiance over 300W/m2 do not add to the total weight and volume of plant matter, they can increase THC potency.

When selecting grow lights, growers have many options to choose from and many factors to consider. Lighting commercial cannabis plants in grow rooms is very expensive. It may be more practical to light plants with lower irradiance, like 400W/m2, which have enough intensity to give THC a slight boost, but will have some energy lost that cannot be used for plant growth. If a grower is interested in propagating a particularly potent cannabis plant, they may see more success using 600W/m2 lamps.

References Cited

Chamovitz, D. 2012. What a plant sees. In What a plant knows (9–26). New York, NY: Scientific American/Farrar, Straus and Giroux.

Chandra, S., Lata, H., Mehmedic, Z., Khan, I. A., & ElSohly, M. A. (2010). Effect of Light Intensity on Photosynthetic Characteristics of High Δ9-THC Yielding Varieties of Cannabis sativa L. Planta Medica, 76(05), P11. DOI: 10.1055/s-0030–1251773

When growers do not have the option of growing cannabis outside, they will have to decide on the best cannabis grow lighting for indoor growing. Growers have a challenging task when deciding what…