Growth dynamics and yield formation of Cannabis (Cannabis sativa) cultivated in differing growing media under semi-controlled greenhouse conditions

Knowledge on the impact of growing media (GM) composition and management on plant growth and quality parameters in the production of medical cannabis is still scarce. This study compared the most commonly used GM in the cannabis industry, namely rockwool (RW), peat-perlite (PP), and coconut-coir (CC), and their impact on plant growth, CBD yield, assimilate partitioning, and nitrogen (N) uptake of a chemotype III medical cannabis strain in pot cultivation, under GM-specific fertigation schedules. GM were tested in two experiments: i) over the whole cultivation cycle, including a flowering period (FP), and ii) an extended vegetative growth period (VP). In FP, common linear regressions were sufficient to describe the relation between dry inflorescence and CBD yield over growing degree days for all GM. Likewise, the effect of GM was negligible for CBD concentration. Comparisons of height, cumulative side shoot lengths, leaf area index (LAI) and biomass accumulation of leaves, stems and roots were based on parameter comparison of fitted growth functions over growing degree days. In FP, the fitted logistic growth functions indicated, that LAI and biomass accumulation of leaves and stems were impaired in PP when compared with RW and CC. In VP, the same parameters were described by expolinear growth functions, which, except for height, estimated consistently higher growth rates for plants grown in RW. Measured N concentrations in different plant organs were consistently at the upper end or above typical ranges, provided by previous studies. However, N uptake was further improved in RW, except in inflorescences, where this effect was not visible. The effect of GM on assimilate partitioning between vegetative plant organs was marginal but significant. We argue, that the reported differences could be mainly explained by the differences in water-holding capacities and irrigation management between GM.