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.
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 aren’t enough to help them grow properly. There are four main things to consider when it comes to light:
- The distance of your lights
- The intensity of your lights
- The color spectrum of your lights
- Your lighting schedule
In this article, we’ll discuss everything related to the correct height of your grow lamps.
Table of contents
Determine the light distance for seedlings
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.
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 Light||Closest||~ Sunlight||Furthest|
|150W||5″ (10cm)||7″ (18cm)||11″ (28cm)|
|250W||6″ (13cm)||9″ (23cm)||13″ (33cm)|
|400W||8″ (15cm)||12″ (30cm)||19″ (48cm)|
|600W||9″ (20cm)||16″ (41cm)||25″ (64cm)|
|1000W||11″ (26cm)||21″ (53cm)||31″ (79cm)|
*Chart is based on HID lights (MH, HPS, etc), and not LED lights.
Grow Light Distance Chart
There’s a sweet spot when it comes to distancing grow lights from your plants.
Hanging them too near to your plants is not only a fire hazard, the outer areas of your grow space probably will miss out on those delicious rays.
Hanging them too far off and energy will be wasted before they can reach your plant’s leaves.
This graphic helps determine the best height for your HPS lamps to hang from.
Setting up and using your MH/HPS lights
When setting up and adjusting your HID grow lights in your indoor setup, there is one main thing to consider: distance from your marijuana plants. You can never perform too many “hand tests” to ensure that it is not too close to your plants, as it may then cause them to burn (and will therefore negatively affect your yield).
In order to check this, simply hold your hand underneath the lights for ten seconds near where the tops of your marijuana plants are. If you can stand the heat, this is too hot for your plants and you should move the grow lights up, further away from them.
The following distances will be useful when you are initially estimating what the distance should be:
You should also think about how much space these lights are going to cover. Check the following measurements to know exactly how many lights and how strong of lights you will need:
|150W||area of 2×2 feet|
|250W||area of 2×2 to 2.5×2.5 feet|
|400W||area of 3×3 to 3.5×3.5 feet|
|600W||area of 3.5×3.5 to 4×4 feet|
|1000W||area of 4×4 to 5×5 feet|
Moving the lights up further will cover a larger area, but the light intensity will also be decreased. Remember this when setting up your light system.
Some growers invest in a lux meter to measure how bright the light is from different distances. If you have one of these, keep in mind that for a healthy vegetative stage, it should measure between 15,000 and 70,000 lux, with 40,000 being ideal. During the flowering stage, it should be between 35,000 and 85,000 lux, with 65,000 being ideal. If you use LED lights, don’t bother getting a lux meter because it won’t measure it accurately.
Distancing HID grow lights
Positioning lights and marijuana plants
In this section, we’ll run through some basics on positioning your marijuana plants and lights, as well as covering some tips and techniques for avoiding stretching in your plants.
This section will help you maximize the growth and bud efficiency of your plants, so pay attention.
The leaves of the Cannabis plant uses light for photosynthesis, and the plant will orient itself towards the light it needs.
Outdoor, it’s easy to see this phenomenon in other plants, and you’ll see it yourself in indoor growing operations.
If you put the Cannabis plants too close together, they will prevent each other from getting light and less photosynthesis will take place.
Obviously this will cause major issues for your plants. Make sure that every plant gets plenty of light.
You also want a fairly uniform amount of light cast onto the plants themselves.
It’s important that the buds on your plants all get the exact same amount of light, even if they are of different sizes.
To do that, you need to make sure that each plant gets plenty of space.
You don’t want any plants to hog all of the light and prevent its smaller relatives from growing.
Giving your plants plenty of space from one another will also help air circulate, which is extremely important for maintaining stable levels of humidity.
Let the plants touch each other only a little, or you risk buds malforming. This could encourage bacterial or fungal growth.
If you aren’t growing a lot of huge plants, you can keep them together with small gaps.
Trimming isn’t the best solution, but sometimes growers need to make do with limited space.
Above all, your plant needs reliable and uninhibited access to light in order to thrive. Don’t let the plants crowd each other out!
An easy mistake for many growers is placing their Cannabis plant too far or close to their lighting fixtures.
Obviously if a plant is too far from a light, it won’t be getting the energy it needs to produce a healthy crop.
These sorts of plants will grow up to be too thin and spindly, lacking the structural integrity to bear heavy yields.
The opposite is true for lighting that is too close.
It will be too hot, and plants can dry out and even die if they are badly burned by a powerful light source.
Distance between light and plant
Below we have a convenient list for growers to use in checking how far apart they want their plants and lights to be positioned from one another.
Make sure the temperature never exceeds 80 degrees.
|Fluorescent light||2 to 4 inch (5-10 cm)|
|Low-energy light bulb||6 to 6 inch (10-15 cm)|
|Metal halide lights 250-Watt||8 to 12 inch (20-30 cm)|
|Metal halide lights 400-Watt||12 to 16 inch (30-40 cm)|
|HPS standard 250-Watt||8 to 12 inch (20-30 cm)|
|HPS 400-Watt||12 to 16 inch (30-40 cm)|
|HPS 600-Watt||20 to 24 inch (50-60 cm)|
|HPS Agro light 400-Watt||12 to 16 inch (30-40 cm)|
|HPS Agro light 600-Watt||20 to 24 inch (50-60 cm)|
One thing to remember in regards to light and positioning is that you want each plant to receive the maximum benefit possible from the lights.
That means smaller or shorter plants should be put closer to a light source than big ones so that they receive equal amounts of light.
Although this seems simple, it’s a common oversight for new growers (and sometimes old ones.)
Utilizing this technique also offers a grower the advantage of keeping smaller plants out of the shadows of the larger ones, an issue that would compound the lack of light.
If your plants are of similar sizes, it’s not a bad idea to move them or the lights around periodically to help ensure that they are all getting equal shares of the bright stuff.
Other grow light factors
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.
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:
|Vegetative||~70,000 lux||~40,000 lux||~15,000 lux|
|Flowering||~85,000 lux||~60,000 lux||~35,000 lux|
Download my FREE marijuana grow bible and start growing high-quality marijuana strains.
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
Distance of light
Intensity of light (brightness)
24 hours of light
Depends on light (see chart above)
Depends on light (see chart above)
18 hours on 6 hours off
12 hour on 12 hours off
Be sure to check out my free Grow Bible to learn more about setting up your lights.
My rule of thumb is to replace clear bulb to sodium once into pure water last aprox two weeks
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.
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?
Thanks for the helpful advise
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,