Peppermint (Mentha ×piperita L.) rootstock cuttings with 9–11 internodes were planted on April 10, 2014 in rows 50 cm apart and with 25-cm distance in the row, on well fertilized muck-peat soil containing 82.1% of organic matter with a pH of 5.9. Peppermint plants were sprayed once with an activated EM-1 preparation, then on two or three further occasions as follows: at 10 cm height (May 10), at branching stage (May 29), and during rapid growth (June 19). EM did not affect peppermint growth or yield. Yields of the fresh and dry herb were high (means: 15,563 and 2,661 kg ha−1, respectively) and characterized by a medium (1.85–1.90%) essential oil content in the dry herb. Twenty-nine compounds were identified in the oil and its main components were menthol (53.1–58.5%), menthone (14.6–16.8%), isomenthone (6.3–6.7%), menthyl acetate (4.0–5.0%), germacrene D (2.3–3.4%), ß-caryophyllene (1.8–2.4%), viridiflorol (1.5–2.3%), and 1,8-cyneole (0.3–3.7%). EM did not affect the content of essential oil in the dry herb or the oil composition (except for 1,8-cyneole). Thirty-four days after planting, 22 weed species grew in the experimental plots and the dominant were common meadow grass (Poa pratensis L.) accounting for 20% of total weed population, annual meadow grass (Poa annua L.) 17%, common chickweed [Stellaria media (L.) Vill.] 20%, creeping yellowcress [Rorippa sylvestris (L.) Besser] 8%, hairy galinsoga [Galinsoga ciliata (Raf.) S. F. Blake] 7%, gallant soldiers (Galinsoga parviflora Cav.) 6%, Canadian horseweed [Conyza canadensis (L.) Cronq.] 6%, common groundsel (Senecio vulgaris L.) 5%, and annual nettle (Urtica urens L.) 5%. Other species occurred sporadically. The total number and fresh weight of weeds growing on 1 m2 were 412 and 246 g on plots treated with EM and 389 and 227 g on control plots, respectively, but the differences were not statistically significant
