Japanese knotweed (Fallopia japonica (Houtt.) Ronse Decraene) is considered the most problematic non-native invasive plant species in Europe. It is known primarily for its ability to spread aggressively through vegetative reproduction. However, because it also has a strong allelopathic effect, we were interested in how extracts from the aerial parts of Japanese knotweed affect the germination and growth of buckwheat and incarnate seeds and what phenolic substances the extract contains. We analyzed 63 phenolic substances from 7 different phenolic groups in the extract of Japanese knotweed, and flavanols predominated in terms of content. We found that methanol is a more effective solvent for the extraction of phenolics than water. The phenolic content of the extract is also affected by the time of sampling of the plant material. We confirmed that the phenolic content of the aqueous extract is also significantly affected by the concentration of the extract. Aqueous extracts of Japanese knotweed had no significant effect on the germination of buckwheat and crimson clover seeds, but they had a significant effect on seedling growth. Lower extract concentrations resulted in elongation of crimson clover seedlings, while higher concentrations inhibited their growth. No elongation was observed in buckwheat, but in some cases even a lower concentration of the water extract of Japanese knotweed resulted in growth inhibition.
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