The aim of research was to determine the effect of different concentrations of water extracts of cornflower on germination capacity and early growth of winter wheat (Triticum aestivum L) and rye (Secale cereale L.) seedlings. The experiment was conducted in laboratory conditions with the use of modified test of germination and early growth of seedlingsPHYTOTOXKIT™. Investigation was carried out in 3 independent experimental series, in three replications for each of them. Experimental series took place in two-week intervals. In the experiments there was evaluated germination capacity of cereals grains on the basis of the quantity of normally germinating seeds after five days from their sowing. Additionally there was measured the length and weight of roots and shoots. Water extracts from Centaurea cyanus fresh leaves did significantly inhibit roots growth of winter wheat and rye, irrespective of extract concentration. However, hypocotyls proved to be more tolerant to the examined extracts. Inhibitory effect of cornflower extracts on length and weight of shoots was observed only after applying the highest concentration. Research results reported in this work prove that water extracts from C. cyanus fresh leaves can have a negative, allelopathic effect on germination of T. aestivum and S. cereale.