January 19, 2022

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

By Shane Torpey
Photos and Autos | The Recommended PAR levels for Photos and Autos

Growers have been increasing the light intensity or PAR levels in the grow room thanks to the increased efficiency of LEDs but how does this apply to Autoflowering plants with their longer lifecycle?

In the past the limiting factor to increasing light intensity for indoor growers was heat and Photoperiod plants (must be on a 12 hours lights on cycle to flower)were the only game in town .  Typically the largest HPS fitted in a 4ft x 4ft (1.2m x 1.2m) grow tent was a 600w system and would deliver about 550 µmols/m²/second. Increasing to 1,000W HPS was not possible because of the heat output of the lighting system was too much to treat with extract fans.

Now we have LED grow lighting systems which are almost twice as efficient as the best HPS systems available and so growers are increasing the PAR levels in their grow rooms to 1,000 µmols/m²/second and higher. However PAR intensity levels of 1,000 µmols/m²/second and greater require raised CO2 levels which require closed systems and expensive CO2 cannisters, sensors and controls.

For the novice to experienced grower applying PAR intensity levels of about 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:

Recommended grow light wattage for photoperiod plants


The average 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.

For Autoflowering 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.

Recommended grow light wattage 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

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



grow light spectrum grow room setup