Nowadays, introduced alien tree and shrubby species have become integral components of the green infrastructure in cities and suburban artificial forest plantations around the world, and their participation in regional dendroflora is steadily increasing. Along with autochthonous species, tree and shrubby introducents perform a wide range of ecological functions and ecosystem services, but, with successful naturalization and increased invasive activity, they can cause negative biological, environmental, social, and economic consequences. The potential for the evolution of adventive species in secondary habitats in the direction of increasing allelopathic activity and competitiveness compared with native species determines the need for assessment of allelopathic potential as a possible predictor of their successful invasive activity. The allelopathic activity of the species was determined by the bioassay method based on the effect of water extracts of different concentrations obtained from dry biomass of leaves on the germination of testing culture seeds of Raphanus raphanistrum subsp. sativus (L.) Domin and biometric indicators of seedlings of Lepidium sativum L. It was determined that in the conditions of the northern steppe Dnieper region, naturalized tree and shrubby introducents Celtis occidentalis L., Morus alba L., Lonicera tatarica L., Mahonia aquifolium (Pursh) Nutt., Prunus virginiana L., Prunus serotina Ehrh., Rhus typhina L. have significant allelopathic potential, which can be a predictor of increasing their competitiveness compared with native species and increasing invasive activity within the region. Water extracts from dry leaves of the studied tree and shrubby species had an inhibitory effect on seed germination and biometric indicators of testing cultures seedlings; the intensity of this effect varied depending on the plant species and extract concentration. Assessment of the allelopathic potential of the alien tree and shrubby species used in urban and suburban green planting is a necessary measure for preliminary identification of potentially invasive species, which will contribute to the increase of efficiency of green infrastructure management.