How the interplay between harvest time and climatic conditions drives the dynamics of hemp (Cannabis sativa L.) field retting

Dew retting, the selective biodegradation of industrial hemp (Cannabis sativa L.) stems after harvest, is a key field process for plant fiber use. The stage of maturity of the crop and the weather conditions during the subsequent dew retting process differ with hemp harvest time, which depends on the hemp valorization scenario. In this work, the aim was to investigate and rank the important factors that drive hemp retting, such as hemp harvest date, weather during retting, soil type under hemp mulch, and their interactions under agricultural field conditions. To this end, an experimental field trial was set up in the northeastern area of France (Champagne region), with two hemp harvest scenarios and two types of soil common to the agricultural area. Physicochemical hemp characteristics and climatic conditions were dynamically monitored during the retting periods. Dynamics of stem characteristics determined at the biomass level (dry matter loss) and the outer tissue level (surface analysis, chemical and thermal analysis) showed that the kinetics of retting followed similar patterns regardless of soil type for both harvest date scenarios. The kinetics were strongly and linearly related to air temperature, expressed as the number of normalized days at 15 °C, and to cumulative radiation during retting periods. The cumulative amounts of rain and dew explained the kinetics less than temperature and radiation, regardless of the retting scenario considered. The strong relationship observed between colorimetry and infrared spectroscopic data for characterizing the retting progress for both harvest scenarios paves the way for the development of tools to monitor changes in hemp stem quality.