The effects of photoperiod and light quality on yield parameters in medicinal <em>Cannabis</em>

Tyson Peterswald

doi:10.25918/thesis.581

The cultivation of Cannabis sativa for medicinal purposes has expanded rapidly in response to rising global demand for high-quality, standardised products. Despite advances in controlled-environment agriculture, research into lighting strategies to optimise the balance between product yield, quality and energy efficiency remains limited. Conventional practices, such as maintaining a 12-hour light/12-hour dark (12L:12D) flowering cycle and using LED systems that provide far less far-red (FR) light at dusk and dawn than natural sunlight, originate largely from illicit growing methods rather than evidence-based optimisation. This thesis addresses the need to identify improved lighting regimes and determine whether these responses are genotype dependent. Using one high-CBD and two high-THC commercial varieties, the two research chapters investigate (1) the effects of different photoperiods and (2) the effects of FR light supplementation on biomass yield and cannabinoid production in medicinal Cannabis. In the first chapter, nine flowering photoperiod treatments, including static 10L:14D, 12L:12D, and 14L:10D schedules, as well as combinations where plants were shifted to longer or shorter photoperiods mid-flowering, were tested. Results demonstrated marked genotype-specific differences, showing that the standard 12L:12D schedule is not optimal for all cultivars. Extending the photoperiod to 14 hours more than doubled cannabinoid yield for the high-CBD variety through increases in both biomass and cannabinoid concentration. A 14L > 10L strategy produced similar yield gains without increasing total daily light hours. Results for the high-THC cultivars were less promising; although 14L > 12L improved yield in 1 variety by about 40%, it required two additional hours of light per day compared with the 12L control, and no benefits were identified for the other high-THC variety. The second chapter examined whether FR supplementation could maintain or enhance productivity while shortening the photoperiod to a 10-hour light cycle. Here, one high-THC line showed a 70% increase in total cannabinoids when FR was applied during the dark period after 10L while reducing energy use by 5.5%, demonstrating clear economic and environmental advantages. Together, these experiments show that lighting responses in medicinal Cannabis are strongly genotype-specific and that photoperiod and FR treatments must be tailored to each cultivar. If done correctly, small adjustments in light duration or spectral quality can produce large yield gains, in some cases even while reducing energy use. These findings provide practical guidance for commercial cultivation and emphasise the need for genotype-specific lighting protocols and screening systems that can help growers group compatible cultivars for optimal outcomes.

The effects of photoperiod and light quality on yield parameters in medicinal Cannabis

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