Juncus bufonius L. (Juncaceae) is recognized by the US Department of Agriculture as a weed or invasive plant. Recently, we reported on J. bufonius L. var. bufonius associated with the native vascular plants Deschampsia antarctica and Colobanthus quitensis in the environs of the Polish Arctowski Station, King George Island, in the Maritime Antarctica. In this study, we evaluated the developmental stages and morphological characteristics of J. bufonius plants cultivated in controlled conditions beginning with seeds obtained from plants of the Antarctic population. Germination occurred at 3 weeks and the germination percentage was low (22.5%). The average time between the anthesis and seed formation was 7 weeks, similar to that reported for other species in the Juncaceae. According to data reported in the literature, Antarctic individuals were significantly smaller than their relatives growing in other conditions, except for the number of inflorescences. The morphological characteristics of a species vary according to its distribution and the edaphoclimatic environment where it occur; cosmopolitan plants shuch as J. bufonius also have reduced stature in cold environments. The low percentage germination may have been due to water availability in the plant chamber in which the study was conducted. J. bufonius is intolerant of dry environments, and once it suffers hydric stress its recovery is very low; thus, a moister environment could be beneficial. J. bufonius has become established amongst native vegetation near Arctowski Station and without careful control or eradication; it may have the potential to spread far beyond the site, as has happened with the alien grass Poa annua as human disturbance and climate warming increase.