An HPLC-MS-DAD analysis of phenolic compounds was carried out on the extracts of staghorn sumac (Rhus typhina L.) and Himalayan balsam (Impatiens glandulifera Royle). This study focuses on the influence of solvent type and extraction time on the phenolic extraction efficiency from both invasive plants. Methanol extraction resulted in a 4.2 times higher content of hydroxybenzoic acids, a 3.7 times higher content of hydroxycinnamic acids, a 3.2 times higher content of flavanols, a 9.6 times higher content of flavanones, and an 8.7 times higher content of flavonols in the Himalayan balsam extract compared to aqueous extraction. Anthocyanins were only detected in the alcohol-based extraction. In comparison, the aqueous extraction from staghorn sumac resulted in a higher yield of total hydroxybenzoic acids, hydroxycinnamic acids, and flavonols, while the methanol extraction resulted in a 1.4-fold lower total polyphenolic content compared to the aqueous extraction. The type of solvent had no significant effect on the total content of flavanols in staghorn sumac. Extraction time considerably affected the total phenolic content in both plant extracts. After 84 h of extraction, the staghorn sumac extract showed a 3.5-fold increase in the total phenolic content compared to the initial measurement. In contrast, the Himalayan balsam extract showed a 40% decrease in the total phenolic content after 84 h. The Himalayan balsam extract reduced the germination of perennial ryegrass seeds by 55%, while the staghorn sumac extract reduced it by 80%. Both extracts also inhibited shoot and root growth of perennial ryegrass, although the Himalayan balsam extract at a concentration of 0.125 g/mL stimulated root growth of perennial ryegrass. The strategic use of invasive alien plants could be an effective approach to control their spread in the environment, potentially reducing management costs. The effectiveness of this approach depends largely on the type and content of allelochemicals present in the invasive plants.
