The aim of our study was to determinate an antimicrobial activity of a shell of the root of Japanese knotweed (Fallopia japonica), Giant knotweed (Fallopia sachalinensis) and Bohemian knotweed (Fallopia x bohemica). Ethanol extracts with a concentration of 600 mg/mL were prepared and tested for antioxidant capacity (AOC) with stable DPPH• radical. Total phenolic content was determined by modified Singleton and Rossi method (1965) while individual phenols (catechin, epicatechin, polydatin and resveratrol) were detected by high-performance liquid chromatography (HPLC). Japanese knotweed extracts proved to be an important source of resveratrol and polydatin while the highest AOC and total phenolic content ware determined in Giant knotweed extract. Antimicrobial activity of extracts was evaluated with Listeria monocytogenes, Escherichia coli and Candida albicans. Minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of extracts were determined by combination of broth microdilution and plate-count methods. Giant knotweed extract with concentration of 37 mg/mL was the most effective in inhibition of C. albicans. Japanese knotweed was the only taxon with bactericidal activity against E. coli (75 mg/mL). We can conclude that knotweed taxes with higher AOC and total phenolic content had higher inhibition and/or bacteriocidal activity against selected microbial strains.