Light Intensity for Marijuana Plants

What is the Optimal Intensity? How to Choose the bulb?

Table showing Lumens levels depending on the distance from the light bulb

Hi there and Welcome (back) on Free the Tree!
In today’s article we’re going to cover light intensity for Marijuana. We’ll look at its importance, lumens (luminous intensity) levels, the inverse square law and much more!
Hopefully this will help you get some nice buds during you’re grow 😉

Light Intensity – Table of Content

Let’s get into it!

Why is Light Intensity Important for Cannabis

As we all know, light is the way for the plant to make its food through photosynthesis, so without light there’s no plant.
That said, too much light and the leaves will be saturated and burn up, too little light and you’ll have long and stretchy plants, neither is really optimal. Meaning that finding the optimal light intensity within your grow room is something crucial if you want to optimise the final yield of your grow.

What is Light Intensity

Light intensity is the magnitude of light energy per unit of area. It is at its greatest near the bulb and diminishes rapidly as it moves away from the light source.
For example, plants that are 2 feet from a lamp receive 1/4th of the amount of light received by a plant 1 foot away from the lamp! Crazy no?
An HID (High Intensity Discharge) lamp that emits 100k initial lumens will only yield 11 111 lumens 3 feet (90cm) away from it.

Couple this with a poorly designed reflective hood and your beautiful buds will suffer badly!

Table showing Lumens levels depending on the distance from the light bulb

Reduction of Light Intensity the further the source is

High Intensity Discharge Light Bulbs

HID’s emit large amounts of light and lumens levels, if properly managed these bulbs will strongly increase the yield per watt of your plant.
That said there is one big, big, problem with HID’s and it’s the heat generated. If the Apex’s get too close they’ll cook up

The Inverse Square Law

The relationship between light emitted from a point source (ie: light bulb) and distance are defined by the inverse square law. The law affirms that the intensity of light changes in inverse proportion to the square distance.
The Formula : I = L/D²
I being the Intensity, L the light output and D the Distance.

Quick example: A 1000W standard MH bulbs emits from 80k to 110k Initial Lumens.
If we take the high end of the stick, and let’s say you’re canopy is on average a foot away from the light, they’ll be receiving about 27000 lumens.
Here’s a good source to read up more

Relationship of light intensity and the distance from its source

ISL dictates that light intensity and distance are correlated

What is the Optimal Distance of the Light source for Marijuana.

To avoid any misunderstanding here we’re are talking of HID bulbs and not LED. Furthermore these are averages, we advise you to calculate yourself the optimal distance taking into account your bulbs characteristics.

HID Bulb Wattage (w)Grow SpaceDistanceLight Intensity
175w2×2 foot (60×60 cm) gardenUnder a foot (30cm)Between 1200 and 2000 tops
250w3×3 foot (90×90 cm)Between 12 and 18 inches (30-45 cm)1500 to 3000
400w4×4 foot (1.2 x 1.2 m)From 12 to 24 inches (30-60 cm)1400 to 3000
 600w4×4 foot (1.2×1.2 m)18 to 24 inches (30-60 cm)3300 to 4800
 1000w6×6 foot (1.8×1.8 m)24 inches (60 cm) minimumAt least 5500

If you’re thinking, ok i’m going for the 600 or 1000 watt, read on 😉

Maximum Light Requirements for Cannabis

The value below will give your marijuana plants all the light they need to form nice and dense buds, although it is important to note that this may vary a little depending on the strain.
Less light will often cause looser, less compact buds.

Growth StageFoot Candles (Lumens)LuxHrs of Light
Vegetative stage250027 00018
Flowering Stage10 000107 50012

Benefits of using Lower Wattage Bulbs

You might think, high wattage = more lumens, thus it’s best to go for the higher wattage, but in reality it’s better to favor multiple smaller bulbs than one main one, here’s some reasons followed by graphics

Why use multiple lower wattage bulbs

  1. There are more points of light for your plants
  2. The light will be more evenly distributed over your canopy
  3. Easier to adjust the light distribution
  4. Better light penetration
  5. Stronger light intensity
  6. Lower wattage = less heat generated
  7. You can place the lights closer to the Apex’s (less heat)
  8. Better coverage (up to 40% more!)
One 1000w HID lamp vs 3 400w bulbs

3 400W bulbs have have better lumens levels

Two 1000w bulbs vs 3 600 watt HID light bulbs

Many Light sources cover more ground and can be place closer to the Apex’s

Lamp Spacing

Light intensity virtually doubles for every 6 inches (15cm) closer an HID is to the Canopy of a garden. As you can see in the image on the right, intensity is also stronger right under the bulb than on the sides.
If you’re using multiple lights, keeps enough space between them in order to maximise the light cross to make up the light loss of plants that aren’t right under the bulb.

By tuning the spacing between the lamps you’ll be able to have a much better lumens per watt average with lower wattage bulbs.

Dispersion of light intensity of an HID bulb

Light intensity is the brightest right under the HID

Rotating your Cannabis Plants

Rotating your plants will help ensure an even distribution of light upon each plant. Rotate them every day or two by moving them one-quarter to one-half of a turn. This well help promote even growth and fully-developed foliage. The idea is to move them so they each receive as much light as possible, while not favoring one specific plant.

If you have an un-even canopy, move the smaller plants toward the center and the taller ones toward the edges. The size of the plants will compensate a little for the disparity in size and the smaller plant should catch up since the lumens levels are higher.
A good way to organize your plants is in a concave shape (stadium method); this will ensure that all the plants receive the same amount of light.

You can also take advantage of the differences in light intensity by:

  • Placing seedlings and cuttings on the perimeters since they don’t require as much light intensity
  • Place the vegetating or flowering plants in the center.

Alright folks that’s it for this one! We’ll update the 3 missing sections in the very very near futur
Until next time, Be safe and grow easy!

Couple words about the grower

This article was written by Plant Lover, Free the Tree's founder and a passionate grower of Cannabis, as well as all other plants.
Mostly from the city, I work on my computer all day so about 4 years ago, on my off time, I started growing to have some more green in my life.

I never imagined I would fall in love with botany and discover so much about this plant. Since then I keep learning everyday, meeting new awesome people of our community and grow constantly

You can always get in touch with me on our Instagram!

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