Photos and Autos | The Recommended PAR levels for Photos and Autos

For photoperiod plants on a 12 hour flowering cycle we recommend an average PAR intensity of 900 µmols/m²/second for home growers and up to 1,400 µmols/m²/second  for expert and commercial growers. Autoflowering plants on a 20 hour light cycle require a lower PAR intensity of about 550 µmols/m²/second.

LED grow lights allow growers to push for higher growth rates

Growers have been increasing the light intensity or PAR intensity in the grow room thanks to the increased efficiency of LED grow lights. Increasing efficiency means the fixtures run cooler and allow the use of higher wattage LED grow lights in grow tents than were used before.

Recommended PAR intensity for photoperiod plants is increasing

Growers are looking to increase the PAR intensity in their grow rooms. Grow light manufacturers are now recommending about 900µmols/m²/second average PAR. For home growers this is the level with the best return in terms of growth rate per watt.

You can increase PAR intensity up to 1,400 µmols/m²/second without increasing CO2 and still get increases in grow rate and yield. However this is a very high performance level which requires a lot of skill to support this level of plant growth with environment control and feeding systems. This level is most suitable for commercial and expert home growers.

Recommended PAR intensity for photoperiod plants

For the novice to experienced grower applying PAR intensity levels of about 800 to 900 µmols/m²/second for Photoperiod plants will achieve very good yields from your grow space. To calculate what this means in terms of grow light wattage here is a table to convert this PAR intensity to consumed power for different types of grow light systems:

Maximising the Daily Light Integral DLI

The average PAR intensity of 900 µmols/m²/second over the 12 hour period equates to about 40 DLI (Daily Light Integral, the amount of PAR reaching the plant over the total light cycle).

This is an important measure because, although plants can absorb PAR intensity up to at least 1,500 µmols/m²/second for a period of time, plants are limited as to how much light they can absorb over the total light cycle. 40 DLI is generally accepted as the upper limit for plants to absorb in one day.

Recommended PAR intensity for auto flowering plants is lower

Autoflowering plants do not require switching to a 12 hour light cycle to stimulate the plants to flower. For Auto flowering plants the typical light cycle is 20 hours on and 4 hours off. This maximises the time the lights are on while still allowing a few hours darkness for the plant to 'sleep' and metabolise.

Because the day is longer the PAR intensity can be lower to reach the same maximum 40 DLI. Here is the table for the recommended grow light system wattage to achieve our recommended PAR intensity of 550 µmols/m²/second for Autoflowering plants.

There are caveats to consider of course.

If you go over the maximum I have recommended in these tables you should still get an increase in growth rate, it just may not be a '1 for 1' increase i.e. 10% more PAR intensity = 10% more growth, for example it may be 5% more growth for 10% more PAR intensity

It is strain dependent, some are much more suitable to high light intensity than others

Grow room conditions such as temperature, nutrients, humidity, CO2 levels etc can restrict the growth potential from your plants if not optimised