The tree of heaven (Ailanthus altissima) is an invasive, non-native plant species that threatens native plant species through its rapid growth. In this study, we investigated whether extracts of tree-of-heaven have allelopathic effects and how they can be used to inhibit the germination and growth of certain plants (white clover, sweet white clover and phacelia). In the experiment, we observed the changes in phenolic content depending on the extraction time and the chosen solvent. The samples were analyzed by HPLC analysis. We analyzed 52 phenolic substances in the extract of the tree of heaven, which we divided into 7 different phenolic groups. Through the experiment, we found that methanol is a more successful solvent for the extraction of phenolics than water. We found that different extraction times significantly influenced the success of extraction of phenolic substances. A longer extraction time influenced the higher content of hydrolyzable tannins, flavanols and hydroxycinnamic acids in the leaf extract of the tree of heaven. The contents of flavanones and flavones were highest with the shortest extraction time. Time had no effect on the extraction efficiency of flavonols, hydroxybenzoic acids and total phenolics. All aqueous extracts of tree of heaven inhibited the germination of white clover, sweet white clover and phacelia seeds. No significant differences in the germination percentage of sweet white clover and white clover were observed between the different concentrations of the extracts. The lowest percentage of phacelia seed germination was achieved with medium and high concentrations of tree of heaven extract. Higher and medium concentrations of tree of heaven extract were most successful in inhibiting shoot and root growth of the plant species studied.
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