Fallopia japonica and Fallopia bohemica, the both of them invasive neophytes, represent an ecological problem due to their characteristics of growth and their regenerative capability. In this work, the growth rate of Fallopia japonica was measured. The aqueous extracts of leaves and rhizomes of Fallopia bohemica were analyzed on allelopathic effects in a cress test. Additionally, methanolic extracts of leaves and rhizomes of Fallopia bohemica and Fallopia japonica were analyzed in terms of potentially allelopathic compounds. The growth rate of Fallopia japonica was 5,2 cm a day averaged during the main period of growth. Aqueous extracts of rhizome and leaves of Fallopia bohemica showed allelopathic effects on seeds of Lepidium sativum. Secondary metabolites with known allelopathic effects were detected in the rhizome of Fallopia bohemica by using HPLC and in the rhizome and roots of Fallopia japonica by using TLC.