The Wastewater Treatment Plant (WWTP) based on horizontal subsurface flow constructed wetland (HSS) achieved its full operation after two years of implantation. Removal efficiency of this WWTP is function of seasonality, type of species plant and maturity period of the system. The objective of this work is evaluating of the influence of season and plant species Phragmites australis and Schoenoplectus californicus in removal of organic matter and nutrients in HSS units during start up operation phase. To this end, four HSS units were implemented in Mediterranean Austral weather conditions. Two HSS units were planted with Phragmites australis, and the other two HSS units were planted with Schoenoplectus californicus. This paper shows results from start up operation phase which cover the first 193 days. It was evaluated the influence of plant species and season (winter and spring), in the removal efficiency of organic matter (5-day Biological Oxygen Demand, BOD5, and Chemical Oxygen Demand, COD) and nutrients (nitrogen (NH4 +-N, NO3 - -N and TN) and phosphorus (PO4 -3-P and TP)). The HSS units planted with Phragmites australis showed better early adaptation than HSS units planted with Schoenoplectus californicus. However, removal efficiencies were similar in all units. Seasonality, it can evidence differences for COD (winter: 18 to 30%, spring: 45 to 55%) and TN (winter: 25 to 65%, spring: 25 to 35%).