The Cd-inhibition effects on Phragmites australis germination characteristics, early seedling growth, and the antioxidant activity response of Guaiacol peroxidase enzyme (GPOx) in seedling organs were investigated under various cadmium (Cd) levels. The final germination percentage (GP%), germination index (GI), mean germination time (MGT), and time to 50% of germination (T50%) indices indicated that Cd inhibited the germination percentage and delayed the start of germination in a dose-dependent manner. A slight increase in the order of parts per billion of Cd negatively affected the rate and speed of germination. This high susceptibility partly explains why the common reed depends mainly on vegetative propagation. The root and shoot length indices (RLI and SLI) measured at 10-day intervals for 30 days showed that shoot growth was susceptible during the first ten days of germination, whereas root growth was more affected after 30 days. It is most likely related to the damaging effects of Cd sequestered in roots, as P. australis is an excluder species. The GPOx antioxidant response was found to be dose and organ-dependent. The leaf/root GPOx ratio was >1 in control, and seedlings grew under 10, 30, and 100 ppb. In contrast, the leaf/root GPOx ratio was <1 in seedlings grown under 100, 500, and 1000 ppm, where some phytotoxicity symptoms such as necrotic roots, yellowing, and abnormal leaves were observed. The pattern of changes in GPOx activity observed in the current study indicates that P. australis may have a mechanism to regulate its GPOx activity response, not only according to the intensity of the Cd dose and plant organ sensitivity but also according to the stage of seedling development, probably for optimal nutrient uptake and growth.