What size grow light do I need?

What size grow light do you need?

For photoperiod plants we recommend LED grow lighting of between 30 watts and 45 watts per square foot (375 to 500 watts per sq.m.) depending on your experience level. Autoperiod plants have a longer light cycle so 20 to 30 watts per square foot (250 to 350 watts per sq.m.) is the maximum plants can absorb in one day.

Grow Light Wattage for Photoperiod plants

For home growers we recommend a PAR intensity of 900 µmols/m²/second. 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.

To achieve this PAR intensity with new LED grow lights it is necessary to size your grow light wattage as follows.

Grow light wattage for Autoflower plants

The light cycle for photoperiod plants is 12 hours per day in the flowering stage whereas the light cycle for autoflower plants is up to 24 hours per day.

This means up to twice as much PAR can be delivered to autoflowers than photoperiod plants for the same PAR intensity. To allow for this we recommend a lower PAR intensity or 550 µmols/m²/second for autoflower grows.

Grow light output is measured in PAR (Photosynthetically Active Radiation)

The units we use to measure the light intensity for growing plants are called PAR (Photosynthetically Active Radiation) and are measured in µmols/m²/second. A wide range of PAR intensity can be used to grow plants indoors and there are positives and negatives in using either low or high PAR intensity.

Recommended PAR intensity for all stages of growth

Young plants from seedling to a few weeks old generally require lower PAR intensity of about 150 to 250 µmols/m²/second as they can be stressed if the light intensity is too high. From two or three weeks old plants can utilise much higher PAR intensity and it can be increased over week or two to about 800 µmols/m²/second for the VEG stage and into the Flowering stage.

In late Veg and flowering you can use PAR intensity from about 300 to 1,500 µmols/m²/second and the growth rate will vary accordingly.

However the green line represents growing in a normal environment without enhanced CO2 levels and at PAR intensity of about 800 µmols/m²/second the rate of increase in growth reduces as the PAR intensity increases. This is why most grow light manufacturers recommend this level for their fixtures.

Is it worth using CO2 in a closed system grow room?

If you run a closed system i.e. run CO2 and Air conditioning together to maintain artificially high levels of CO2 in the grow room, then the plants can utilise the higher PAR intensity more efficiently and growth increases at a higher rate above 800 µmols/m²/second. It is recommended to run the CO2 levels to the same parts per million (PPM) as the PAR intensity. For example at 1,200 µmols/m²/second you should increase CO2 levels to 1,200 ppm (normal atmospheric levels are 400ppm).

However this system is expensive and complex to run. The CO2 cylinders are heavy, hard to source and expensive. You also need CO2 regulators, CO2 controllers and alarms to protect you from CO2 poisoning. The total CO2 system could cost $1,000 to $2,000 to setup. You would also have to supply an split AC system to control the temperature and humidity in the grow room (you cannot use extract fans as they will remove the CO2 and it will not build up to the required levels.

If you are very limited for grow space and an expert grower this may make sense to implement but if you wish to increase your growing capacity a much simpler option is to add more area to your grow room or get a bigger or additional grow tent.