Abstract: The early development of estuarine vegetation is a key aspect in the establishment of new individuals in invasion processes. The aim of this research was to characterize experimentally the survival capacity and early growth of the invasive Baccharis halimifolia and the native Juncus maritimus, the main native species displaced by the invasive species in European estuaries, with a complex experiment that recreated the multifactorial dynamics of the estuarine environment (controlled conditions of salinity, immersion time and interspecific competition). Seeds of both species were sown in soil cores subjected to a periodic immersion of saline water. The results show that the survival capacity of B. halimifolia is significantly affected by salinity, while the survival capacity of J. maritimus is significantly affected by immersion and the interaction of salinity and immersion. Interspecific competition did not affect neither species. The main finding of this research is that the survival rates of B. halimifolia were considerably lower than the germination rates measured in other published laboratory univariate experiments. This might indicate that in reality, the species is more sensitive to small changes in environmental conditions, and thus, not as capable of establishing as successfully as expected. The knowledge acquired might help to identify those areas in which this invasive species is prone to appear more accurately, based on its real capacity for establishing in a dynamic environment such as the estuary.