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
Flourescent light tubes range in size from 0,3m to
2,4m 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
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 1,2m 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
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
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 approx 0,4m to 0,5m above
garden, 600 and 1000 watt lamps, 0,6m to 0,7m 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 -
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
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 Osram, Philips, General
Lighting Chart - A guide for wattage per square meter.
HID Light Output
Primary Growing Area
0,6m x 0,6m
0,9m x 0,9m
0,9m x 0,9m
1,2m x 1,2m
1,2m x 1,2m
1,8m x 1,8m
1,8m x 1,8m
2,4m x 2,4m
2,4m x 2,4m
3,6m x 3,6m
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 0,6m from the tops of the
cannabis plants. Medium wattage systems (400 and 600)
should be hung around 1m from the top of the
cannabis plants. High wattage systems (1000 and up)
should be placed at least 1,2m to 1,5m from the cannabis
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.
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 0,2m (up to 0,4m 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.
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
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
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.
WHAT TYPE OF GROW LIGHT SHOULD I USE?
Full Spectrum Fluorescents are ideal for
starting seeds or cuttings or for low light cannabis
plants. They are the most energy efficient grow lights
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
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
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
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 0,9m to 1,8m 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
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
Use a fan to cool down your cannabis plants if the
lights in a small grow room raise the temperature too
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
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
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
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
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.