Every year, experts and laic community are more aware of the problems posed by non-native species. It is increasingly necessary to look for ways to inhibit the colonization or spread of non-native species, as they do not benefit our own native species. Thus, the main goal of my diploma thesis was to find out if the extract of brittle willow (Salix fragilis) affects the growth and development of Japanese knotweed (Fallopia japonica). In the previous study, the plants of Japanese knotweed were watered with 0.1 % and 1 % essence of aqueous extract of brittle willow and distilled water. I took some samples of leaves and roots from each group of plants and prepared them forspetrofotometric measurments. I measured the concentrations of chlorophyll a, chlorophyll b, and xanthophylls in each sample. I also measured the fresh and dry weight of the samples, and calculated the water from these data. I calculated the mean and standard deviation, and then further processed the results using Student's t-test. Despite stresss is reflected in biochemical and physiological changes, in our thesisextract of brittle willow didn't reflect on japanese knotweed.