How 4 Basic Factors of Light Affect the Growth of Your Plants

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Light is extremely important for growing high quality, high yielding cannabis plants indoors. Since exposure to light is what triggers plant photosynthesis, not getting the proper type or amount of light will result in stunted growth for your plant.

Written by Stefanie Chan from Grobo

Generally speaking, the more light your plants get, the better and stronger they grow, and the more yield they produce. However, just shining random light bulbs at your plants isn’t enough to help them grow properly. There are four main things to consider when it comes to light:

  1. The distance of your lights
  2. The intensity of your lights
  3. The colour spectrum of your lights
  4. Your lighting schedule

Distance of Light

Light distance is extremely important to the growth of your plants. Too far away, and your cannabis won’t get all the light that it needs, however, if the light is too close, you risk giving your precious plants light burn.

Light distance while growing
Light distance while growing

Having your lights the right distance away is crucial in your plant’s seedling stage. One of the biggest mistakes that growers make is keeping their lights too far away. This results in “leggy” or stretched stems that flop over and can’t support the weight of the plant. These stretched stems occur as the seedlings reach up in attempts to get more light.

The actual optimal distance of your light will vary depending on the type of light you use, and your growing space. Here are some basic guidelines for you to follow:

 Light distance from plant canopy
Grow LightClosest~ SunlightFurthest
150W5″ (10cm)7″ (18cm)11″ (28cm)
250W6″ (13cm)9″ (23cm)13″ (33cm)
400W8″ (15cm)12″ (30cm)19″ (48cm)
600W9″ (20cm)16″ (41cm)25″ (64cm)
1000W11″ (26cm)21″ (53cm)31″ (79cm)

*Chart is based on HID lights (MH, HPS, etc), and not LED lights.

Intensity of Light

Plants that get more light tend to grow better and output higher yields – it’s a fact. However, it is easy to over saturate your plants with bright light and cause light burn. It is also easy to set your lights too dim, and thus your plants receive inadequate amounts of light and they will “stretch” or have stunted growth.

Intensity of Light
Intensity of Light

Light intensity, or brightness, can be measured in both lumen (lm) and lux:
Lumen – measures the flow of light which is emitted from a source. The higher the lumen, the brighter the source of light.
Lux – the measurement of light intensity that falls on a surface. Because plants only pick up the light that falls on its surface, growing guides will usually measure light levels using lux.

Similar to light distance, the optimal light intensity will also depend on the type of light that you choose to use in your setup. Below are some guidelines to help you get started:

Life StageMaximumGoodMinimum
Vegetative~70,000 lux~40,000 lux~15,000 lux
Flowering~85,000 lux~60,000 lux~35,000 lux

Colour of Light

Many people don’t realize that the colour of light can affect the growth of your plants. Visible light actually behaves as a wave, and it displays varying properties depending on the length of its particular waves. For example, a light with a wavelength of 400nm will be detected by the human eye as being purple in color.

Certain types of lighting solutions display a specific color of light. For example, MH produces a predominantly blue colour light, where as LED lights can isolate and display a variety of colours.

Colour of Light
Colour of Light

In terms of plant growth, blue light is best used during seedling and veg stage, as it influences the formation of chlorophyll, a chemical that makes plants grow faster and stronger. MH lights are often used for this, as its blue light is designed to mimic the summer months when the sun is high in the sky. Conversely, HPS lights mimic the end of the summer, with the sun’s rays passing through more of the earth’s atmosphere, causing it to display a red spectrum, which is great for flowering cannabis plants.

Lighting Schedule

The final important component to proper lighting is the actual amount of time your plants receive light. Just like humans, plants need their sleep and cannot normally be hit by sunshine for 24 hours a day.

The only time that plants can use 24 hours straight of light is when they are in their seedling stage, as they are still babies and need lots of light to grow. For the duration of their vegetative stage, they will enjoy 18 hours of light, and 6 hours of darkness. Once you are ready for your plants to flower, you can trigger its flower stage by switching the lighting schedule to 12 hours of light, and 12 hours of absolute darkness.


Getting your lighting system perfected is crucial to great yield, and it can take many tries to get it right. If you don’t want to experiment with your supply of medicine, an alternative is to grow your plants with an automated grow box that has an optimal lighting schedule and all light requirements built into it. Otherwise, break out the charts and spreadsheets, because there will be a lot of testing and light tracking involved on the road to optimal lighting!

Here is a quick chart to help distill all the information that has just been thrown at you:


How many weeks this stage lasts for

Colour of light

Lighting schedule 

Distance of light

Intensity of light (brightness)


1-2 weeks


24 hours of light

Depends on light (see chart above)

Depends on light (see chart above)


3-5 weeks


18 hours on 6 hours off


7-10 weeks

Red-far red

12 hour on 12 hours off

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    5 comments on “How 4 Basic Factors of Light Affect the Growth of Your Plants”

    1. My rule of thumb is to replace clear bulb to sodium once into pure water last aprox two weeks

    2. N.W.,

      Well I would first advise you to join out support forum.

      IN regard to using light through a window. You can do this but, you cannot possibly get full Sun throughout the entire day through a window; Can you?

      Spend a couple hundred bucks and get a 400-600 watt lamp system.

      All in all any natural light you can expose your plants too is great. I just have to wonder whether or not you can give them optimum light under these circumstances.

      Happy growing.

    3. OK, I have a question, and I wish I would have been reading and asking questions all along. About four months ago, I decided I was going to “grow my own”! So I acquired a few seeds (train wreck) and away on this journey I started…. And I was totally in the dark. First of all, let’s just say my thumb is not green, in fact it’s probably pink! Well, low and behold, because I really didn’t know what I was doing, I should have gotten feminized seeds, and so to make a long story short, I panicked when I thought I saw seeds and destroyed all but one, and I was positive the last one standing was female, which it was, however, I know now that what I destroyed were not male, they were hermy. Yep, feeling pretty dumb right now, but it is what it is. So my question is, and maybe it’s actually a statement, but anyhow, I started all the seedlings out under red and blue lights, actually looks purple, and when they began flowering, I switched to natural sunlight (yes I live sorta remotely so I do have that option, it is indoors, but sits right in the east window, gets all that glorious morning sun! But would it even be more beneficial for me to put it in the west window in the evening?

    4. I bought the white widow seeds a while back, I lost all but two . I’m going to wait till this summer to buy more,