Fecundity and germination of the invasive aquatic plant, Senegal tea (Gymnocoronis spilanthoides (D. Don) DC.)

Abstract

Senegal tea, Gymnocoronis spilanthoides (D. Don) DC., (Asteraceae) is an invasive perennial herb of aquatic and wetland habitats. In this study we investigated fecundity (per cent florets setting seed per inflorescence) of two populations in south-eastern Queensland (SEQ) and germination responses to light and temperature. Proportions of capitula that had set seed varied significantly between Ithaca Creek and Strathpine populations (Ithaca = 6.0%, Strathpine = 19.0%) (P <0.001) and both values were greater than previous unpublished estimates (mean number of seeds per capitula; Strathpine = 11.9 ± 1.3; Ithaca = 4.0 ± 0.96). Seeds largely failed to germinate under continuous darkness. Under a 12 h light:12 h dark regime, germination, occurred between five and 20 days after initiating the experiment. Mean germination reached a maximum of 83% under an alternating 12 h at 20°C and 12 h at 30°C regime and was significantly greater than under the other temperature treatments tested (P <0.05). Germination was still relatively high (63%) at lower (10/20°C) temperatures, suggesting that Senegal tea is capable of germinating over most seasons in SEQ. Results suggest that production of germinable seeds is high at the Strathpine infestation and that a seed bank is likely to exist at this site. The light requirement for germination suggests that Senegal tea recruitment would be favoured by soil or canopy disturbance that exposes seeds to light. We suggest that future management would be aided by an understanding of seed bank persistence and the relative importance of clonal and sexual reproduction in the population dynamics of this species.