Grower's Choice strives to be your source of information to help weed through the barrage of info you can find online and particularly when researching lighting for cannabis. First, let's go over the correct unit of measurement for lighting a plant. This would be moles. In the US the more common forms of measuring light are lux and foot candles, but these units are an immediate measurement of the intensity of light that is visible to the human eye-- mainly between 500-600 nanometers. For crops, photosynthesis happens using a broader spectrum of light wavelengths referred to as PAR (Photosynthetic Active Radiation) which is the range from 400 to 700 nanometers and actually stimulates photosynthesis. The measurement of the mole is the number of actual photons over a square meter over a time period (photo period).
Most cultivators know the term micromoles ( µmol) and also have a number in mind of how many micromoles they wish to grow. This number is typically between 850-1000 for indoor crops. Let's take a minute to discuss where this number stems from. First, what is a micromole? A micromole is simply one millionth of a mole (micro). A micromole measurement of the PAR for your crop is based on the amount of photons being delivered per square meter per second. It is with this measurement of PPF (Photosynthetic Photon Flux) that we can then calculate total moles produced each day. It is actually this number-- total moles-- that is important for your crop to produce.
"If 'moles' are the important number for determining how much light to put on my crop, why does everybody refer to micromoles?" Because a micromole is a per second, instantaneous image of your light level at that moment in time we can measure this with a par sensor. This is why you see so many requests for 900 micromoles on a marijuana plant.
The reason marijuana is such a large number is because of the photoperiod requirement of this plant. To start flowering of the marijuana bud we have to allow for 12 hours of darkness. Also, presently cannabis is a higher profit margin plant than the majority of produce, so although the production curve in regard to amount of light starts to level off in the 30-- 40 mile range, many growers will push it out as much as possible. The micromoles needed for the crop are dependent on the full photoperiod of your day. It is not always that cannabis utilizes more lighting than other fruit bearing plants, it is that you just have 12 hours to gather that amount of light.