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Plants need a lot of light from nearing the end of vegging and right through flowering to harvest. The yield at the end will be directly proportional to the amount of light the plants receive during this period. This guide will help you understand how high to hang your grow light for maximum productivity and yield.

Whatever your chosen lighting system the principal is the same; hang your light as close as possible to the plants so they receive as much light as possible. The trick is ensuring the light is not too low as to cause damage directly under the light fixture, where the light and heat are most intense.

Let’s look at some of the reasons why this is the best method to optimising your setup and show you how to adjust your own lighting system.

Why is it best to hang the grow light as low as possible?

Grow lights with reflectors (HPS, CMH etc) and boxed LED grow lights spread light wide. So, even when the grow light is hung low over the canopy, some light is going to go over the plants and hit the side wall of the grow tent.

A 600W HPS system hung at optimum hanging height loses a lot of light from reflective losses.

A PAR light output from a 600W HPS hung at only 40cm above the plant canopy reduces 27% due to reflective losses

The optimum hanging height for a 600W HPS system is only 40cm or 16″. Even at this low height a lot of light will shine on the side walls and have to be reflected down onto the grow. The reflective materials will not reflect all of the light and there will be some light lost (heating the walls).

The higher the light is hung the more light will shine on the walls and the greater the reflective losses.

A test result of the PAR output to the grow canopy of a 600W HPS system in a 4x4 grow tent. The light was hung at different heights to see what the optimum hanging height is

The PAR intensity maps show how much PAR light (PPFD) falls on the grow canopy at different hanging heights

 

We have tested a 600W HPS grow light at different hanging height to demonstrate the benefit of fine tuning. At each hanging height we measured the light intensity in PAR (PPFD) across the grow canopy with the following results:

  • The optimum hanging height is 40cm.
  • At 10cm lower the PAR intensity is too high in the centre and plant damage will occur.
  • Hanging at 50cm height and you lose 8% of the PAR light onto your canopy. This would translate directly into a 8% yield reduction.

So hanging the light as low as possible really improves yield but how low is too low?

What’s the lowest you can can hang your grow light over your plants?

There are two factors to consider when ensuring the plants are not damaged by hanging the grow light too low:

  • Light intensity (PAR intensity)
  • Leaf temperature

If the light intensity is too high or the temperature of the leaves gets too high the plants will be damaged. The difficult part is how to tell when the PAR intensity or leaf temperature is at the limit without damaging the plants in the process.

How much PAR light is too much for my plants?

A graph of PAR (Photosynthetically Active Radiation) intesity V's Photosynthesis or growth rate for Medicinal Herbs

A graph of PAR (Photosynthetically Active Radiation) intensity V’s Photosynthesis or growth rate for Medicinal Herbs

Typically medical herb growers will target a PAR level (PPFD) across their plant canopy of 500 to 1,000 ┬Ámols/m2/sec.

PAR levels of 1,200 ┬Ámols/m2/sec and above are likely to stress the plants and result in bleaching of the leaves and buds. This damage will kill off healthy bud growth and leave the plant susceptible to disease.

The only way to detect the PAR intensity (PPFD) level on the plant canopy is to use a PAR meter or Quantum Sensor. This is an instrument that is calibrated to measure the light level in PAR for horticultural applications (not a Lux meter for measuring lumens for humans). We use an ‘Apogee SQ 500’ PAR meter in our tests and experiments and it is quite expensive. There are cheaper PAR meters such as the one from ‘Hydrofarm’ that are very good value for money and as accurate as you need for this task.

If you are a committed grower I would highly recommend the investment as you will benefit hugely from the improved knowledge and resulting yields!

PAR intensity map for a 600W HPS grow light hung 40cm over the plant canopy

The PAR intensity under a single grow light will be highest directly underneath and reduce as you move away to the edges of the grow area

Where do I measure the maximum PAR under my grow light?

The closer you are to the grow light the higher the PAR intensity will be. The shortest distance from the grow light to a flat plant canopy is directly underneath. That is the point on the canopy where the PAR intensity will be greatest. As you move to the side of the plant canopy the distance from the grow light increases and the PAR intensity reduces.

If you lower the light until the centre point is at the maximum PAR intensity then the rest of the canopy is going to be receiving reduced and therefore safe levels of PAR light.

How can I adjust for maximum PAR if I don’t have a PAR meter?

IR thermometer

You can use a Infra Red Thermometer to measure leaf temperature

The other method of determining the lowest possible distance the grow light can be from the plants is to use leaf temperature as a guide. The best way to do this is to use a infra red thermometer.

Use a Infra Red Thermometer to measure leaf temperature

The leaves respire or ‘sweat’ and should lose their surface heat faster than it is absorbed from the grow light. If the leaf receives too much heat it will not be able to evaporate the heat away and the leaf will curl up, brown and die. If the leaf is not receiving too much heat the leaf temperature will be below the room temperature around it. For example, if the temperature of the environment is 25 Degrees Celcius the leaf temperature should be 23 – 25 Degrees.

Lower the grow light to where you think it is suitable and, after a few minutes, test the leaf temperature. If the temperature is below the surrounding air temperature then lower the light a little. Test again until the leaf temperature is the same as the surroundings. Raise it a few centimeters above this height and this should be the optimum hanging distance. As the plants grow adjust the height up regularly to avoid damage to the upper parts of the canopy.

Arm test of grow light heat

Testing where the heat from the grow light becomes uncomfortable

Detect the temperature using the back of your arm

Put your forearm, underside up, underneath the grow light and test at what height your skin becomes uncomfortably warm. This will be approximately the minimum distance you should have to the top of the canopy. Add a few centimeters to the distance and adjust the height of the grow light above the canopy by this much. Check a few hours later to see if there is any leaf curling or bleaching. If there are signs of damage, such as leaf curling, raise a few centimeters until you find a height that is suitable and maintain this distance as the plants grow.

Trial and error

Set the distance from the underside of the light to the top of the canopy of about 60 centimeters. Check a few hours later to see if there is any leaf curling or bleaching. If there are no signs of damage lower the grow light five centimeters, leave for a while and check for damage again. Repeat until you detect signs of heat damage. This will be approximately the minimum distance you should have to the top of the canopy. Add a few centimeters to this distance and adjust the height of the grow light above the canopy accordingly. Maintain this distance as the plants grow.

Maintain the optimum distance throughout the grow

Once you have established the optimum hanging height for your grow light system it is essential you constantly raise the light as the top of the canopy gets higher.

If you are using a net it is easier to manage grow light height. You can tuck pieces of plant that stick up through the net back underneath and have a fairly flat and static canopy. This means you will have less adjustment of the grow light height to make.

If you are not using a net the top of the canopy will be irregular in height. You will have to adjust to suit the tallest parts of the plants. Alternatively you can arrange the plants in a ‘stadium shape’. This means putting the taller plants around the edges of the grow light coverage and the smaller plants directly underneath. This will compensate for the higher light intensity of the light in the centre of the grow and lower intensity at the edges.

 

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