A study from Clemson University has found that adding far-red light to red-blue light treatments significantly boosts shoot production and growth in marijuana plants grown in vitro.
Researchers studied two hemp varieties—’BaOx’ and ‘Cherry 1’—in controlled growth vessels under varying red-to-blue light ratios, with and without the addition of 5% far-red light. Plants were harvested every two weeks over five cycles, using a hedging and fed-batch method that involved replenishing nutrient media after each harvest.
Results showed that plants exposed to far-red light produced more shoots and longer growth than those under red-blue light alone. Shoot count increased from 15 to 28 over three cycles with far-red light, compared to just 18 shoots without it. Shoot length also improved—growing from 19 mm to 25 mm under far-red conditions—while those without it averaged around 15 mm in the same timeframe.
Interestingly, shoot dry mass was highest during the first harvest cycle under the most blue-intensive light conditions, with ‘BaOx’ reaching 7 mg and ‘Cherry 1’ hitting 6 mg. However, this mass dropped by half by the third cycle and then stabilized.
The study also found that 68% of shoot tips rooted successfully in greenhouse conditions, regardless of their prior in vitro treatment. These findings suggest that far-red light may be a valuable tool for improving early-stage marijuana propagation in controlled environments.
