Heracleum sosnowskyi Manden is a highly toxic invasive plant of the Apiaceae family that spreads rapidly in the temperate climate of Europe and Asia, causing significant damage to biodiversity and ecosystem functioning. The aim of this research was to quantitatively assess the influence of key factors on the effectiveness of herbicidal control of Heracleum sosnowskyi, and to determine optimal timing and methods of treatment. Field studies were conducted during 2020–2025 in natural phytocoenoses of the Western Forest-Steppe of Ukraine on plots of 25 m² with three replicates. Three herbicidal treatments were studied: Slash (1.5 L/ha), Roundup Max (4.0 L/ha), and a tank mix of Elumis (2.0 L/ha) + Roundup Max (2.5 L/ha), compared to an untreated control. Results showed that herbicide effectiveness strongly depended on the plant developmental stage. Roundup Max achieved over 92 % plant mortality at the cotyledon stage, but only 39 % at the eight-leaf stage. Slash showed a similar trend with lower values, from 89.6 % to 36.7 %. The tank mix of Elumis + Roundup Max was the most effective, ranging from 98.3 % at the cotyledon stage to 58.9 % at eight leaves, providing prolonged suppression of growth and regenerative capacity. Morphometric analysis indicated that plant height, stem diameter, leaf area, and the number of regenerated shoots were lowest under the tank mix, and correlation analysis confirmed an inverse relationship between herbicide effectiveness and morphometric parameters (r = –0.88–0.94, p ≤ 0.05). ANOVA showed that plant developmental stage accounted for 83.3 % of the variation in control effectiveness, herbicide type 15.1 %, and the stage × herbicide interaction only 1.6 %. These results highlight the critical importance of timely herbicide application at early growth stages and demonstrate the advantage of using tank mixes for maximal suppression of Heracleum sosnowskyi growth and regeneration, which is crucial for adaptive management of invasions and maintaining ecosystem stability.

Key words: Heracleum sosnowskyi, herbicidal control, effectiveness, morphometric parameters, plant developmental stage, invasive species.