The aim of our experiment was to evaluate phytotoxicity of three knotweed species (Reynoutria japonica, R. sachalinensis, R. x bohemica). The tests examined suppression of germination in the seeds of two crop plants (Leucosinapis alba, Brassica napus) and two weed plants (Chenopodium album agg., Echinochloa crus-galli) using dried knotweed leaves mixed with soil. Data processing by two-way ANOVA has shown that the type of seed tested was a more important factor affecting germination than the type of knotweed leaves used to contaminate the soil. The tested crop plants were more sensitive than either weed plant. The highest phytotoxicity was found for crop plants cultivated in soil contaminated with R. japonica leaves (wherein seed germination for L. alba was 35% and for B. napus 43%). Reaction of weed plant was stimulatory (seed germination for E. crus-galli in soil contaminated with R. sachalinensis leaves was 191%). This response is probably plant hormesis.