Plant density and nitrogen fertilization affect agronomic performance of industrial hemp (Cannabis sativa L.) in Mediterranean environment

The renewed interest in industrial hemp (Cannabis sativa L.) is due to its large number of applications and for the wide range of agro-environmental conditions under which it can be cultivated. Two-year field experiments (2007 and 2008 growing seasons) were carried out in a Mediterranean environment of central Italy with the aim of assessing the impact of genotype, plant density and N fertilization on hemp yield, in terms of stems, inflorescences and seeds. The treatments consisted in: (a) seven genotypes (Epsilon68, Fedora17, Felina32, Ferimon, Futura75, Santhica27, and Uso31); three plant density (40, 80, and 120 plants m−2); two N fertilization levels (50 and 100 kg of N ha−1). Physiological parameters, plant height, stem weight and diameter, inflorescence yield, seed yield and the characteristics of hemp and weed aboveground biomass were recorded. High plant density resulted in shorter plant height compared with low plant density (−41%) as the hemp plants tended to reach the reproductive stage early at high density. At full flowering, stem yield ranged from 3.4 to 8.0 t ha−1 of dry matter and was positively correlated with the duration of vegetative phase, which tended to be high in the intermediate flowering genotypes (Epsilon68, Futura75 and Santhica27). Stem diameter was inversely correlated with plant density (6.7, 5.8 and 5.2 mm at 40, 80 and 120 plants m−2, respectively). Conversely to stem yield, inflorescence and seed production proved to be higher in the early flowering genotypes (Fedora17, Felina32, Ferimon and Uso31) and increased as plant density increased. High N fertilization level had a positive impact on stems rather than inflorescence and seed yields (on average +28%, +17% and +4% in 100 kg of N ha−1 compared with 50 kg of N ha−1 fertilization level, respectively). Farmers should consider making a dual-purpose production of stems and inflorescences or stems and seeds, even if it is clear that yield is related to the choice of genotype. Further research should be carried out to find various genotypes as well as flexible agronomical practices that are able to improve both traditional (stems) and innovative (inflorescences and seeds) hemp yields under Mediterranean conditions.