Competitiveness of the Exotic Silphium perfoliatum against the Native Urtica dioica: A Field Experiment

Figure 6

Number of living shoots (a) and number of capitula (b) per plant of S. perfoliatum depending on the treatment: control without surrounding species (white box), surrounded by S. perfoliatum (S around, yellow box), and surrounded by U. dioica U. dioica (U around, green box). Precipitation is given as the sum of April to August of the respective year. Capitula were counted from the stage of full flowering on (=fully expanded ray florets or later stages). n = 9 (except S. perfoliatum U around n = 8). Different letters indicate significant differences in the respective year (Tukey’s post hoc test on GLMs with Poisson-distribution).