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Plant protection with natural products is a new trend in environmentally friendly agriculture. Nepeta rtanjensis Diklić et Milojević is an endemic and critically endangered plant species in Serbia. We explored the phytotoxic potential of a water emulsion of Nepeta rtanjensis essential oil (NrEO) with high amounts of trans,cis-nepetalactone on five weeds. The most sensitive was Stellaria media (L.) Vill., as NrEO (from 0.013% to 0.1%) completely inhibited germination. Germination of Amaranthus retroflexus L. and Artemisia vulgaris L. was completely inhibited at the highest applied concentrations of NrEO (0.1%), while Ambrosia artemisiifolia L. germination was reduced to 48% at the same concentration of NrEO. The most tolerant species was Cephalaria transsylvanica (L.) Schrad. ex Roem. & Schult. as the final germination rate at the highest applied concentration of NrEO (0.1%) was 81%, like the control (82%). To our knowledge, this is the first time the interaction of essential oils on the germination and growth of A. vulgaris and C. transsylvanica is reported. The germination dynamics of S. media in pots with soil were significantly inhibited when the soil was initially treated with the highest applied concentration of the NrEO water emulsion (1%). Spraying S. media seedlings with NrEO significantly inhibited growth parameters (shoot height, shoot fresh weight, and the number of nodes) at the higher applied concentrations of NrEO (0.5% and 1%). Therefore, the water emulsion of the essential oil of N. rtanjensis could be potentially developed for use in the control of invasive and allergenic weeds. [ABSTRACT FROM AUTHOR]