Impact of Industrial Hemp (Cannabis sativa L.) Extracts on Seed Germination and Seedling Growth: Evaluating Allelopathic Activity Across Various Extraction Methods

The noticeable reduction in plant species abundance near industrial hemp (Cannabis sativa L.) highlights the need to investigate its potential allelopathic effects on selected cultivars' seed germination and seedling growth. Industrial hemp of the "Helena" variety was used to obtain aqueous extracts by conventional (macerate, hydrolate, and post-distillation residue) and green methods (ultrasonic and microwave extracts) in order to treat thirteen most commonly cultivated plant species, including lettuce, kohlrabi, onion, tomato, carrot, pepper, savoy cabbage, rocket, alfalfa, white mustard, pea, sunflower, and parsley. This is the first time that the allelopathic effects of seven different hemp extracts were tested simultaneously on thirteen different species. The extracts were applied at 10, 25, 50, and 100% concentrations. The seed germination percentage and root/shoot length results for all tested plants, except peas, clearly demonstrated an inhibitory effect of higher concentrations of hemp extracts. This effect was observed regardless of variations in chemical composition (CBD, THC, and total polyphenols), suggesting that different extracts have varying impacts on different species. The weakest inhibitory effect on the germination and seedling length for the majority of the tested plant species was noted for PDR, while the strongest inhibitory effect in terms of seedling length was observed in the case of MAE700.