Light Height – How low can you go?

“How high should my light be above my plants?” is a very common question that we get in the shop. It depends a lot on the type of light that you are using. An array of fluorescent tubes or a CFL should only be about 6 inches or so above the tops of the plants.

However, the answer gets a little more complicated as we go into the realms of HPS, MH and Plasma lamps.

HPS (High Pressure Sodium) and MH (Metal Halide) lamps (along with a few others such as Mercury lamps) are collectively known as HID (High Intensity Discharge) lamps. As the name implies, HID lamps produce a high intensity of light. However, they also produce a lot of heat.

When answering the question: “How high should my light be above my plants”, there are 5 things that we need to consider.

1) Why it’s important not to put your light too far from your plants

There is something called the inverse-square law which describes how light intensity goes down as a square of the distance from the source (the lamp). The diagram below shows how this works:


Illustration by Borb

As the illustration shows, as your light is moved twice the distance from your plants, the energy from it is spread over 4 times the area. This means the intensity will be a quarter (1/4) of that at the original distance.

At 3 times the distance from your plants, the energy is spread over 9 times the area. This means the intensity is one ninth (1/9) of that at the original distance.

Basically, this means that the light intensity drops of more and more quickly the further from the lamp you get.

Up to a certain point, the amount of photosynthesis that a plant can perform is directly related to the amount of light that the plant receives. Plant growth and flower/fruit production (and therefore final yield) is proportional to photosynthesis. By using the diagram above, this tells us that if a lamp is twice as far from your plants that it needs to be, your yield will be a quarter of what it could have been if the lamp had been at the correct height.

2) Saturation point for photosynthesis

As I mentioned a moment ago, photosynthesis is proportional to the amount of light a plant receives, but only up to a certain point.

This point is where the plant simply cannot make use of any extra light that it receives. This is called the “saturation point”. There is also a point above this where extra light actually becomes detrimental to the plant. In other words, too much light can cause physical damage to the plant by what is often called “light burns” and “light bleaching”.

μmols is a much more accurate way of measuring light for plants than lumens. However, most people do not have access to an expensive quantum meter that measures μmols. It is also a relatively new term for most people. However, most growers have come across the word lumens and will far better understand what I am talking about if I use this term of measurement instead. For this reason, I shall explain the following in terms of lumens and lumens per square foot.

With gradually increased exposure, plants in sunlight will generally achieve light saturation at about 5000 lumens per square foot. HPS lamps emit a much narrower spectrum with no UV and plants will achieve saturation at about 6000-7000 lumens per square foot, depending on the type of plant. At and beyond light intensities of 8000-10000 lumens per square foot, there is real risk of damage to plant tissue.

Here is an example of how a plant might react to different levels of light intensity:

As can be seen from the graph, there is a point where the on-going damage caused by too much light causes a net reduction in the amount of photosynthesis.

3) Temperature

Regardless of getting the light height right regarding, it is of the utmost important that the temperature at the top of the plant canopy never goes above 28C in the absence of CO2 supplementation, and never above 32C-35C with CO2 supplementation (the exact temperature depending on whether the CO2 supplementation is sub-optimal or optimal).

The best way to measure the temperature at the top of the plant canopy is with a thermometer with an external probe which is attached to the tallest part of the plant directly under the lamp.

Quite often, growers will use the “back of the hand” method. To do this, you simply place your hand at the height of the top of your plants, with the back of your hand facing the lamp for 20-30 seconds. If, after this time, the back your hand becomes uncomfortably hot, then the lamp needs to be raised until you can comfortably keep your hand at the height of the tops of the plants.

Obviously, this only really gives an indication, this is because different people will be able to withstand different temperatures. To be completely sure that the tops of your plants are not getting too warm, make sure you get a thermometer with an external probe and attach the probe to the tallest part of a plant which is situated pretty much directly under the lamp.

4) CO2 Level

As mentioned in 3), the maximum temperature that you can allow at the tops of your plants is somewhat dependent on whether you supplement with CO2 or not.

If you are not using any CO2 supplementation then the maximum temperature that any part of any of your plants should be subjected to is 28C

If you are using CO2 supplementation at about 1200-1500ppm (optimal), then the temperatures can be pushed up to as far as 34C, and maybe even 35C.

As the highest temperatures are usually experienced by the highest tops of the plants that are more or less directly beneath the lamp, this temperature is usually directly related to how high from your plants that your lamp is.

5) Light Height, Light Spread and your reflector

All reflectors cast their own footprint, or light pattern, onto the plant canopy. There’s quite a bit of choice out there. There are small-domed parabolic reflectors that cast a narrow footprint with the light concentrated in a small spot directly below the lamp. There are large wide reflectors that spread the light wide and more evenly over a huge area!

It is important to match your reflector to your area that you need it to cover and the lamp that you want to use it with. It is also important to use it at an appropriate height. There is no point using an enormous Magnum XXXL (which has a very wide spread) at a height of 12 inches to cover a 0.8m square grow tent. The bigger reflectors with a wider coverage should be used with larger lamps (1000 Watters usually) to cover a 1.5m square area and they should be hung at around 24 inches.

Fortunately, most reflectors fall between these 2 extremes. Unless you go for a specialist spot-reflector, or a huge Magnum XXXL, you will find that most reflectors that will nicely cover a 1m to 1.2m square area fairly evenly when they are place at 18-24 inches from the top of the plants.

If the reflector is placed much lower than this height, then the plants at the edges of the grow-area will not receive enough light. If the reflector is placed much higher, then you will not be making use of the intensity of light achievable by placing it at the correct height {see 1)}.


  • Use the right reflector (wide-throw ones for 1000 Watters, standard-throw ones for 250, 400 and 600 Watters). Consider an adjustable one for versatility.
  • For a 400W and 600W lamps used in a 1.0m to 1.2m square tent, start off placing the lamp and reflector at about 18″-24″ from the tops of your plants. If the temperature is above 28C in the centre of the plant canopy after half an hour, move it up.
  • If you are adding CO2, the temperature can, and should, be at about 34C
  • Having your lamp and reflector too low is actually worse than having it too high. Damaged plants due to the light being too intense will take some time to recover.

All in all, it’s a bit of a Goldilocks situation. The light should not be too high, nor too low, but just right.