Irrigation Management Strategies for Medical Cannabis in Controlled Environments

Medical cannabis production is a new industry in Canada and represents a challenge for the production of a repeatable and standardized product for medical use. A reliable and reproducible environmental control strategy can contribute significantly to meeting this challenge. Irrigation management and control of plant water status is one of the key environmental control elements. To assess the effects of various irrigation management strategies this study deployed in situ stem psychrometers to measure the water status of plants. As a routine feedback device for irrigation control these devices are not ideal for large-scale production so correlation with the key environment variable representing the aerial demand for moisture (vapour pressure deficit) was assessed. By establishing a relationship between cumulative water potential (cWP) and cumulative vapour pressure deficit (cVPD) an irrigation management strategy that predicted plant water status based on measurements of cVPD could be employed. Three treatments; control (irrigation events every 1-2 days), mild-stress (irrigation events every 2 days), and moderate-stress (irrigation events every 3 days) were tested. The effects of flushing were also investigated to determine whether it had the intended effect of reducing nutrient concentrations within the dried bud. Through the use of psychrometers, water status (cWP) thresholds were correlated with humidity (cVPD) thresholds and reduced irrigation frequency resulting in water use reductions up to 45.7% which had negligible impacts on yield and cannabinoid profile. Flushing was found to be ineffective in removing any significant amount of nutrient from the bud.