As medical cannabis or medical marijuana becomes legalized in more regions around the world, this represents an exciting new market for growers. But how do you get the best results when growing this crop in a greenhouse under LED lighting? Signify Plant Specialist Sabrina Almeida de Carvalho is carrying out several trials with research partners. In this blog, she answers the question based on research in collaboration with Wageningen University & Research: Will medical cannabis flower with LED lighting?

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.

Flowering occurs with both HPS and LED, if the plants get at least 12 hours of full darkness

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.

Images from trials which all had different spectra. All situations flowered and achieved similar to higher yields compared to conventional set-ups.

What I find the most exciting about these results is that the impact of light is the definitive factor and that it doesn’t matter whether it’s HPS or LED. The great benefit of using LED though, is the fact that LEDs are more energy efficient and allow higher intensities of lighting to be applied to crops compared to HPS lighting.  The trials also show that growers who use LED lighting can expect to produce 1 extra crop cycle a year for medical cannabis, while using energy very efficiently.

In our long-term research collaboration with Wageningen University & Research (WUR), we are investigating how LED grow lights can influence compound production, quality and plant characteristics of cannabis crops grown for medical purposes. The research results will support growers in creating the ideal growing conditions for medical cannabis. Read more about the scope of the research in our press release and follow the Horti blogs from Signify on growing with medical cannabis to read results as they become available.

1. Tournois J. Études sur la sexualité du houblon. Annis. Sci. Nat. (Bot.). 1914;19:49-191.
2. Thomas B, Vince-Prue D. Photoperiodism in Plants. Academic Press. London. 1996.
3. FB Salisbury, CW Ross. Plant physiology, 2nd ed., Wadsworth Publishing Comp., USA. 1978.