Types of Lights for Cannabis Growing - Different Marijuana Lighting Sources

Light for cannabis plants

Without light, cannabis plants cannot grow. In the countries in which marijuana grows best, the sun is the source of light. The amount of light and the length of the growing season in these countries results in huge tree-like cannabis plants. In most parts of North America, however, the sun is not generally intense enough for long enough periods of time to produce the same size and quality of cannabis plants that grow with ease in Latin America and other tropical countries.

Sunshine is most intense at the equator where it is closest to the earth. Some of the largest, resin-coated buds in the world grow beneath this blazing sun. So what’s this got to do with indoor growing? Plenty. Light is one of the most basic needs of all cannabis plants and the most often mismanaged.

Even the most experienced growers forget about the basic needs of a cannabis plant, especially light.

The answer to the problem of lack of sun, especially in the winter months, shortness of the growing season, and other problems is to grow indoor under simulated conditions. The rule of thumb seems to be the more light, the better.

The common incandescent light bulb emits some of the frequencies of light the cannabis plant can use, but it also emits a high percentage of far red and infra-red light which cause the cannabis plant to concentrate its growth on the stem. This results in the cannabis plant stretching toward the light bulb until it becomes so tall and spindly that it just weakly topples over.

There are several brands of bulb type. One is the incandescent cannabis plant spot light which emits higher amounts of red and blue light than the common light bulb. It is an improvement, but has it drawbacks. it is hot, for example, and cannot be placed close to the cannabis plants.

Consequently, the cannabis plant has to stretch upwards again and is in danger of becoming elongated and falling over. The red bands of light seem to encourage stem growth which is not desirable in growing marijuana. The idea is to encourage foliage growth for obvious reasons.

Flourescent light tubes range in size from one to eight feet in length so you can set up a growing area almost anywhere. There are two types of flourescent lights; standard and the wide spectrum. They can be used in conjunction with one another, but the wide spectrum lights are not sufficient on their own. The wide spectrum lights were designed as a supplementary light source and are cheaper than the standard lights.

Wide spectrum lights emit the same bands of light as the standard but the standard emit higher concentrations of red and blue bands that the cannabis plants need to grow.

The wide spectrum lights also emit infra-red, the effect of which on stem growth we have already discussed. If you are planning to grow on a large scale, you might be interested to know that the regular fluorescent lamps and fixtures, the type that are used in commercial lighting, work well when used along with the grow lights.

These commercial lights are called cool whites, and are the cheapest of the fluorescent lights we have mentioned. They emit as much blue light as the standard grow lights and the blue light is what the cannabis plants use in foliage growth.


Now we come to the question of intensity. Both the standard and wide spectrum lamps come in three intensities: regular output, high output, and very high output. You can grow a crop of cannabis plants under the regular output lamps. The difference in using the HO or VHO lamps is the time it takes to grow a crop.

Under a VHO lamp, the cannabis plants grow at a rate that is about three times the rate at which they grow under the standard lamps. People have been known to get a cannabis plant that is four feet tall in two months under one of these lights.

Under the VHO lights, one may have to raise the lights every day which means a growth rate of ate least two inches a day. The only drawback is the expense of the VHO lamps and fixtures. The VHO lamps and fixtures are almost twice the price of the standard.

Now that you have your lights up, you might be curious about the amount of light to give you cannabis plants per day. The maturation date of your cannabis plants is dependent on how much light they receive per day. The longer the dark period per day, the sooner the cannabis plant will bloom. Generally speaking, the less dark per day the better during the first six months of the cannabis plant's life.

If your cannabis plants receive 12 hours of light per day they will probably mature in 2 to 2.5 months. If they get 16 hours of light per day they will probably be blooming in 3.5 to 4 months. With 18 hours of light per day, they will flower in 4.5 to 5 months. Its a good idea to put your lights on a timer to ensure that the amount of light received each day remains constant.

Energy Emissions In Arbitrary Color Bands
40 Watt Flourescent Lamps
In Watts and Percent of Total Emissions
    Daylight Cool White Gro-Lux GroLux WS
Light Type Band Watts % Watts % Watts % Watts %
Ultra-Violet -380 0.186 2.15 0.16 1.68 0.10 1.42 0.27 3.16
Violet 380-430 0.832 9.60 0.72 7.57 0.70 9.67 1.07 12.48
Blue 430-490 2.418 27.91 1.98 20.78 1.96 27.07 1.22 14.29
Green 490-560 2.372 27.38 2.35 24.67 1.02 14.02 1.24 14.49
Yellow 560-590 1.259 14.53 1.74 18.27 0.10 1.42 0.83 9.77
Orange 590-630 1.144 13.21 1.69 17.75 0.44 6.05 1.36 15.93
Red 630-700 0.452 6.22 0.81 8.47 2.86 39.55 1.86 21.78
Far Red 700-780 0.130 1.53 0.07 0.81 0.06 0.80 0.69 8.10
Total 8.890 100.0 9.52 100.0 7.24 100.0 8.54 100.0

Grow Light Guide

Horticultural lighting systems allow you to extend the growing season by providing your cannabis plants with an indoor equivalent to sunlight. This is a great advantage for those of you who appreciate having a year-round supply of fresh flowers, veggies and herbs. Artificial lighting is also a great way to jump-start spring by starting your seedlings months ahead of the last frost. There are three main types of horticultural lighting systems.

In summary, marijuana has a lust for light. HP sodium lamps produce 20% more light than super metal halides. Horizontal reflectors yield up to 40% more light than vertical reflectors. Hammer or pebble specular, anodized aluminum or white are the most reflective surfaces for hoods. Mirror finish is the absolute worst reflective surface for a hood. Hang 400 watt lamps at 18-24” above garden, 600 and 1000 watt lamps, 24-30” above garden. Use a light meter. Grow the strongest, healthiest cannabis plants that reach their maximum potential for resin production. Bright light brings big buds.

HID (High Intensity Discharge) Lighting 

HID lighting is the most efficient way to convert electricity into light that is available to the consumer. There are two types of HID grow lights used for horticultural lighting:

HID Light Efficiency

Metal Halide - MH

Metal halide bulbs produce an abundance of light in the blue spectrum. This color of light promotes cannabis plant growth and is excellent for green leafy growth and keeping cannabis plants compact. It is the best type of light to be used as a primary light source (if no or little natural sunlight is available). The average lifespan is about 10,000 cumulative hours. The bulb will light up beyond this time but due to the gradual decline of light, it is not worth your while to wait for the bulb to finally burn out. If you compare their lumen (brightness) per unit of energy consumed, metal halides produce up to 125 lumens per watt compared to 39 lumens per watt with fluorescent lights and 18 lumens per watt for standard incandescent bulbs.

High Pressure Sodium - HPS

High pressure sodium bulbs emit an orange-red glow. This band of light triggers hormones in cannabis plants to increase flowering/budding in cannabis plants. They are the best lights available for secondary or supplemental lighting (used in conjunction with natural sunlight). This is ideal for greenhouse growing applications.

Not only is this a great flowering light, it has two features that make it a more economical choice. Their average lifespan is twice that of metal halides, but after 18,000 hours of use, they will start to draw more electricity than their rated watts while gradually producing less light. HPS bulbs are very efficient. They produce up to 140 lumens per watt. Their disadvantage is they are deficient in the blue spectrum.

If a gardener were to start a young cannabis plant under a HPS bulb, she/he would see impressive vertical growth. In fact, probably too impressive. Most cannabis plants would grow up thin and lanky and in no time you will have to prune your cannabis plant back before it grows into the light fixture. The exception to this is using a HPS light in a greenhouse. Sunlight is high in the blue spectrum which would offset any stretching caused by HPS bulbs.

Common manufacturers of metal halide and high pressure (HP) sodium lights include Philips, General Electric, Iwasaki, Venture, and Osram/Sylvania. Many of the manufacturers buy and use the same components, often manufactured by competitors. Most often the bulbs have the exact same technical statistics.

Lighting Chart - A guide for wattage per square foot.

HID Light Output Primary Growing Area Supplemental Area
100 watts 2' x 2' 3' x 3'
250 watts 3' x 3' 4' x 4'
400 watts 4' x 4' 6' x 6'
600 watts 6' x 6' 8' x 8'
1000 watts 8' x 8' 12' x 12'

HID Lighting Helpful Tips

Hanging height: Due to the heat that is emitted from these types of fixtures, you should hang them according to size. Smaller wattage systems (100 and 250) should be hung about 2 feet from the tops of the cannabis plants. Medium wattage systems (400 and 600) should be hung around 3 feet from the top of the cannabis plants. High wattage systems (1000 and up) should be placed at least 4 to 5 feet from the cannabis plant tops.

How long should lights run?

This depends on the type of cannabis plant. Most cannabis plants and vegetables need about 10 to 12 hours of light to promote growth. cannabis plants that produce fruits or flowers will show improvement with up to 16 hours a day of supplemental light.

Fluorescent Lighting 

This type of light is perfect for starts and seedlings. They are also popular for growing low-light cannabis plants like herbs and African violets. Fluorescent lights are low intensity and need to be placed within 8" (up to 15" for shade loving cannabis plants) of the cannabis plants to be effective. They are a poor light source for flowering and budding primarily because of their low lumen output. 

Incandescent Lighting 

These lights are also good for starts and seedlings and provide an inexpensive alternative to HID lights, because they do not require a ballast. These lights are only good for individual cannabis plants or small groups of cannabis plants because of their low lumen output and limited range.

The Cost to Run a Lighting System

To get the operating cost per hour for a light, take the lights combined wattage, and divide it by 1000 to get the kilowatts used. Then multiply that number by the amount your electric company charges per kilowatt hour. HID lights will use the number of watts it emits per hour, ie; 600w system will use 600 watts per hour (regardless of spectrum).

How the Sunlight Effects cannabis plant Growth

200 - 280 nm UVC ultraviolet range which is extremely harmful to cannabis plants because it is highly toxic.
280 - 315 nm Includes harmful UVB ultraviolet light which causes cannabis plants colors to fade.
315 - 380 nm Range of UVA ultraviolet light which is neither harmful nor beneficial to cannabis plant growth.
380 - 400 nm Start of visible light spectrum. Process of chlorophyll absorption begins. UV protected plastics ideally block out any light below this range.
400 - 520 nm This range includes violet, blue, and green bands. Peak absorption by chlorophyll occurs, and a strong influence on photosynthesis. (promotes vegetative growth)
520 - 610 nm This range includes the green, yellow, and orange bands and has less absorption by pigments.
610 - 720 nm This is the red band. Large amount of absorption by chlorophyll occurs, and most significant influence on photosynthesis. (promotes flowering and budding)
720 - 1000 nm There is little absorption by chlorophyll here. Flowering and germination is influenced. At the high end of the band is infrared, which is heat.
1000+ nm  


Full Spectrum Fluorescents are ideal for starting seeds or cuttings or for low light cannabis plants. They are the most energy efficient grow lights available.

Although more efficient than incandescent lights, fluorescents are still much less efficient than High Intensity Discharge (HID) light systems. Larger wattage systems will cover larger areas, and since these bulbs produce greater light intensity they are superior for growing taller cannabis plants.

Metal Halide (MH) and High Pressure Sodium (HPS) are the most common HID grow lights. MH bulbs emit a blue-white spectrum, which is more conductive for vegetative growth or starting seeds and clones. HPS bulbs are red-orange in the spectrum, which is ideal for the fruiting and flowering stage of a cannabis plant’s development.

Marijuana Lighting Tips

Although natural sunlight is the best form of light for growing cannabis plants, their life cycle can be controlled more effectively with artificial lighting

The life cycle of the cannabis plant is determined by the daily photoperiod (hours of light per day).

A young cannabis plant tends to grow faster if it is grown under continuous light for the first two months of it's development.

Although the lamps should be kept as close as possible to the growing leaves, they should never be allowed to touch the cannabis plants.

The best light for cannabis plant growth emits high intensities of the blue and red bands of the light spectrum.

Fluorescent light is one of the most effective source of artificial light readily available to home growers.

Metal Halide lamps are better than most fluorescents, because they radiate more light in the blue and white bands of the light spectrum.

If your lights are too hot to touch with your hand, they are probably also too hot to be close to the growing leaves of cannabis plants.

The amount of light you supply your cannabis plants depends on the size of the garden, but at least 20 watts per square foot is recommended.

Although Metal Halide lamps are generally considered a better light source for marijuana growth, they use a lot more electricity than fluorescents..

Under artificial light, a cannabis plant can grow from three to six feet tall in three to four months.

cannabis plants entering the vegetative stage enjoy up to twenty two hours of light per day, while twelve hours or less is required for flowering.

The more light you give your cannabis plants, the faster they will grow.

Lamps should be fixed to the roof in such a way that they can easily be raised as the cannabis plant grows in height.

The use of reflectors ensures that the light is spread more uniformly around the cannabis plants.

To increase the humidity of the air in a small, enclosed garden, place a small bowl of water in the grow room.

Use a fan to cool down your cannabis plants if the lights in a small grow room raise the temperature too much.

cannabis plants grown in conditions of varying daily light patterns will not settle into a regular life cycle and will grow poorly.

If there is insufficient light in the grow room, the cannabis plants will often grow tall and thin, in search of the light.

Do not "wake up" your cannabis plants by switching on the lights if they are in the dark period of their daily light cycle.

cannabis plants grown with a long daily photoperiod, will require more frequent watering than those grown with less light.

Although incandescent, or screw-in light-bulbs are usually not suitable for good growth, they can be used in conjunction with fluorescent tubes.

If the edges of the leaves near the lights and growing tips start curling up, it usually means that they are getting too hot.

A cannabis plant will grow with as little as six hours of light per day, but requires more than twelve hours per day for good strong growth.

A cannabis plant grown with insufficient light may sometimes develop disfigured leaves with only one or two blades per leaf.

Paint the walls, floor and roof of your grow room white for better reflection of light onto your growing cannabis plants.

If you have a few cannabis plants growing unevenly under one light, you could raise the pots of the smallest cannabis plants by placing them on a few bricks.

Although easy to control, the daily light photoperiod is the single most important factor influencing the growth of your cannabis plants.

cannabis plants require less than twelve hours of light per day for at least two weeks before you can expect to see the first signs of flowers appearing.

Never increase the daily light photoperiod to more than twelve hours, once your cannabis plants have started flowering.

A cannabis plant grown indoors with lots of light will often grow larger and more potent than a similar cannabis plant growing outdoors in natural light.

To increase the amount of light available to the lower branches, you could mount extra fluorescent tubes vertically, onto the walls of your grow room.

Lights for marijuana Growing