Critical factors for inducing medical cannabis to flower

The simple answer to the question above is: ‘Yes, medical cannabis will flower when grown with LED lighting’. How much and how fast is a bit more complex. Medical cannabis plants are triggered to flower according to their photoperiod, which simply means the number of hours of light they receive. In general terms, plants like medical cannabis need to be in full darkness for at least 12 hours and sometimes longer, every day.

For plants like chrysanthemums and medical cannabis, we even talk about the ‘critical night’. They must go through a dark cycle to guarantee flowering. We’ve known this about medical marijuana since the 1910’s, thanks to studies carried out by Julien Tournois¹ and B. Thomas² on the light-darkness effect with hop and cannabis plants. In the 1970’s, Salisbury and Roos³ also documented their results with growing cannabis plants.  It doesn’t matter what kind of light – sunlight, high pressure sodium (HPS) or LED lighting –  the medical marijuana plant receives during the day, where the plant is in the growth cycle or, what light intensity medical marijuana plants are exposed to during the daytime, they will flower if they receive at least 12 hours of full darkness.

Set-up of trials

I investigated several trials that are being conducted to grow medical cannabis crops under LED lighting in a greenhouse and in a vertical farm. Flowering is the third growth stage for medical cannabis. This is arguably one of the most important stages, because it triggers the production of buds and cannabinoids or terpenes. So, there would be no point in growing medical cannabis with LED lighting if it couldn’t flower.

The pictures below show crops from the trials. Each trial used different lighting spectra, but all plants flowered if their 12-hour ‘nights’ were respected.