Getting The Optimal Distance Between Lights And Plants
3 min

Getting The Optimal Distance Between Lights And Plants

3 min
Growing Seedshop

Simply rigging up grow lights isn’t enough if you want to get the most out of your cannabis. To really push it to the limit, you need to figure out the most efficient height for your lights, and here is why.

Although easy to do, growing cannabis is hard to master. The distance between your lights and your cannabis is one of those simple, yet often overlooked factors that separates the novices from the pros, and getting it right can make all the difference to your crops.

So what is the deal here? Surely as long as your plants are getting a good coverage, everything should be ok? Well, at the end of the day, the closer the lights are to your cannabis, the better quality of light they are going to receive. However, you need to make sure that they are not so close that they will burn your crop, and at the same time, not so far away that the quality of light is diminished.

Artificial lights are subject to the Inverse Square Law. This is the principle that light diminishes exponentially in energy as the distance it travels increases. What this basically means when growing plants is the further the light has to travel, the less energy it is going to be able to give your cannabis – irrespective of how well you can see it, or how bright it may appear.

For example, you may have a light that produces a PAR rating of 1000 when 10 centimetres from your light, but 250 when 20 centimetres away. PAR stands for photosynthetically active radiation, and is a good measure of available energy given to your plants. This makes it extremely important to get the optimal distance essential for your lights when growing indoors.

Although fairly simple to work out once you know your stuff, there is no set standard as to how far your lights should be away from your cannabis. In order to work it out, you need to weigh up the individual factors.

Weighing Up Your Factors

The first condition that needs to be met is light coverage. You need to ensure that the entire canopy of your crop is receiving light, and that you maintain this no matter what.

Once this has been set as a parameter in your mind, you can start working on optimal light height. You need to get your lamps as close as you can without compromising the above condition. How you do this is going to depend on your set up. Are you using HPS lights that pump out a ton of heat? Do your lights have a dedicated exhaust to reduce heat? Are you using LEDs?

HPS Lights

Getting it right with HPS lighting is pretty easy. For this you can use the hand test as a measure. Hold your hand about 2 centimetres above the top of your cannabis. Now start lowering your lights until you begin to feel heat from the bulbs. When it begins to get warm to the point of discomfort, you have found your starting point. Keep an eye on your plants and see if the begin to beach or show signs of heat stress. If so, lift the lights up slightly. If there are no signs, and you are getting s good coverage, then you have found your optimal distance. Measure this, and keep it constant as your plants grow. You should be able to use this for all similar grows – just make sure to keep an eye on your plants for any signs of stress.

CFL and Florescent Lights

This is even easier. When you use CFL and florescent tubes to light your plants, it is nearly impossible to burn or bleach them – however, it is worth noting that this type of lighting is seen as inferior in quality to HPS. In this situation, 5-8 centimetres is usually around the optimal distance from your crop – just make sure you have full coverage.


LEDs are a bit trickier. It will largely depend on the wattage of diodes you are using. The lower wattage the diode, the closer it can be – as lower wattage diodes put out less light. However, generally speaking, for quality, you want diodes of a reasonably strong wattage. Also, as LEDs don’t really put out much heat, you can get them very close with little risk of burning –just keep an eye out for bleaching.

The best way to go about it with LEDs is to start your lights 60 centimetres from the top of your cannabis, and slowly lower them over the coming days. When you start to see signs of bleaching, raise the lights by 5 centimetres. If the bleaching stops, this is the optimal distance you should maintain with these lights.

Once you have the optimal distance down, it is super easy to maintain in all future grows. It may sound like a lot of work, but it is really simple, and will dramatically increase the quality of all your grows – making it well worthwhile to learn.

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