We studied the relationship between genome size and ploidy level variation and plant traits for the reed grass Phragmites australis. Using a common garden approach on a global collection of populations in Aarhus, Denmark, we investigated the influence of monoploid genome size and ploidy level on the expression of P. australis growth, nutrition and herbivore-defense traits and whether monoploid genome size and ploidy level play different roles in plant trait expression. We found that both monoploid genome size and latitude contributed to variation in traits that we studied for P. australis, with latitude being generally better predictor of trait values and that ploidy level and its interaction with monoploid genome size and latitude also contributed to trait variation. We also found that for four traits, tetraploids and octoploids had different relationships with the monoploid genome size. While for tetraploids stem height and leaf water content showed a positive relationship with monoploid genome size, octoploids had a negative relationship with monoploid genome size for stem height and no relationship for leaf water content. As genome size within octoploids increased, the number of aphids colonizing leaves decreased whereas for tetraploids there was a quadratic, though non-significant, relationship. Generally we found that tetraploids were taller, chemically better defended, had a greater number of stems, higher leaf water content, and supported more aphids than octoploids.