Optimal Harvest Timing: Quantifying Temporal Changes in Dry Mass and Tissue Cannabinoids for High-CBD Cannabis sativa Grown in Controlled Environments

Harvest timing is a crucial production decision that affects the yield and quality of horticultural crops, yet little research-based information is available to guide this decision for cannabis (Cannabis sativa L.) production. The objective of this study was to quantify temporal changes in dry mass and tissue cannabinoids under an inductive photoperiod to track yield and identify optimal harvest timing. This information can be used to guide production scheduling. Two experiments were conducted that flowered cannabis ‘Southern OG’ under a 12-h photoperiod for 8 weeks (Expt. 1) or 10 weeks (Expt. 2) with weekly destructive harvest of whole plants beginning on the fourth week. Two distinct trends were revealed as flowering progressed: reproductive dry mass increased linearly each week and the cannabinoid concentration increased to a maximum and then gradually declined. On a per-crop basis, harvesting at 9 or 10 weeks resulted in maximum cannabidiol (CBD) yield or tetrahydrocannabinol (THC) yield, respectively. On an annual basis, reproductive dry yield was greatest when harvesting at 10 weeks, while cannabinoid yield declined when harvesting after 8 weeks. These results demonstrate that optimal harvest timing depends on the final product of interest (e.g., dried inflorescences or tissue extracts) because the maximum yield of reproductive mass and cannabinoids occurred in different harvest timing treatments. Commercial growers must also consider production costs and product value. Longer crop cycles are desirable to reduce variable costs, such as containers, substrate, and cuttings, but the accompanying decline in cannabinoid concentration with longer crop cycles adversely impacts the perceived quality and economic value of dried inflorescences. Therefore, optimal harvest timing depends on the interactions of yield, production cost, and product value.