Invasive plants are one of the biggest threats to biodiversity. The invasive plant species have a high allelopathic potential, which helps them spread to new areas. The allelopathic effect of invasive species on different plants varies upon to the phenophase. Erigeron annuus is an annual invasive alien species, which we can find in Romania. This paper investigated the allelopathic effect induced by aqueous and hydroalcoholic extracts obtained from Erigeron annuus (L.) Desf. subsp. annuus on crop species. The extracts were obtained using dried plant material from the aerial parts of the plant: stems, leaves, inflorescences. For each type of extract, we used two dilutions: 1:10 and 1:100. We used the seed of three of the most important crop plants: wheat (Triticum aestivum L.), cucumber (Cucumis sativus L.) and pea (Pisum sativum L.). After hydration, the seeds were immersed for one hour in extracts, and then put in Petri dishes in the dark. The parameters which were determined after four days of the beginning of the experiment are the length of the root and stem, fresh and dry biomass. The influence of extracts was more pronounced at the roots than at the stems of the seedlings, regardless of species. The effect varies upon the type of extract and upon the species. The hydroalcoholic extract diluted 10 times inhibited growth, especially in the case of dicotyledons.
