Cannabis sativa is a known hyperaccumulator of heavy metals (HM), and testing of hemp medicinal products is required for HM contaminants including arsenic (As), cadmium (Cd), and lead (Pb). The objective was to evaluate how the timing of HM applications impacts hemp growth. A plant experiment was conducted where 40 Cannabis sativa ‘Wife’ hemp plants were grown in deep water culture systems in a growth chamber. Aqueous nutrient solutions including combined As, Cd, and Pb were applied directly into the hydroponic solution on a weekly basis, with HM concentrations of 0.0, 0.5, and 1.0 mg HM/L. During the first six weeks, plants were grown under long days to promote vegetative growth, and 15 plants were harvested at the end of this vegetative growth phase. The remaining 25 plants were grown under short days to initiate flowering, with harvest 6 weeks later. Total dry mass (P < 0.05), shoot dry mass (P < 0.05), and root dry mass (P < 0.01) were significantly affected by HM application timing. The total and shoot dry mass was the highest for the control group, followed by the plants that received HM at 0.5 or 1.0 mg/L during the reproductive phase, and the least growth occurred in plants that received HM at 0.5 or 1.0 mg/L during the vegetative phase. Root dry mass decreased as HM concentration increased (P < 0.05). Flower dry mass was not significantly affected by HM application timing or concentration. Results highlight how exposure to HM at 0.5 to 1.0 mg/L markedly reduce biomass accumulation, especially when exposure occurs early in the production cycle.
