Germination ecology and seedling growth of invasive Ageratum species and allied native Adenostemma lavenia

Abstract The success of invasive plants can be better understood by comparing their traits with closely related native species. This study compared germination ecology and seedling growth of invasive Ageratum houstonianum and Adenostemma conyzoides with co‐occurring and allied native Adenostemma lavenia . Seeds were germinated under a different light (12 h photoperiod/complete dark), temperatures (low: 25°C/15°C day/night, and high: 30°C/20°C), and different levels of water stress (−0.1, −0.25, −0.5, −0.75, and −1 MPa). Seedlings were grown to determine biomass allocation, relative growth rate (RGR), plant height, and number of leaves. The seed mass and size of native A. lavenia were higher than those of invasive species. Seeds of all species were positively photoblastic. At low temperature and in all levels of water stress, all measured parameters except mean germination time were the highest in A. houstonianum . However, at high temperature, there was no significant difference in germination percentage between A. houstonianum and A. lavenia . No germination of A. conyzoides and A. lavenia was recorded beyond −0.5 MPa water potential, but seeds of A. houstonianum germinated up to −0.75 MPa. A. houstonianum had higher root mass fraction, root‐to‐shoot ratio, and number of leaves than the other two species. Stem mass fraction and the height of seedling were highest in A. conyzoides . The RGR was 1.6 times higher in invasive Ageratum species than the native species; it was slightly higher in A. houstonianum than in A. conyzoides . Overall, the results suggest that germination traits and seedling growth performance can be used as predictors of a species' invasiveness.