Invasive congeners are unlikely to hybridize with native Hawaiian Bidens (Asteraceae)

• Premise of the study: Invasive plant species threaten native plants in multiple ways, one of which is genetic assimilation through hybridization. However, information regarding hybridization between related alien and native plant species is generally lacking. In Hawaii, the invasive Central American species Bidens pilosa and Bidens alba have colonized natural areas and often grow alongside the native Hawaiian Bidens species, a clade representing an adaptive radiation of 27 endemic taxa, many of which are threatened or endangered.• Methods: To assess the risk of hybridization between introduced and native Hawaiian Bidens (which will readily hybridize with one another), we undertook crosses in cultivation between the invasive species and nine native Bidens taxa.• Key results: The majority of the crosses formed no viable seed. Although seed did mature in several of the crosses, morphological screening of the resulting seedlings indicated that they were the result of self-pollination.• Conclusions: This result suggests that B. alba and B. pilosa are incapable of hybridizing with these Hawaiian Bidens taxa. Further, we found that B. alba in Hawaii was self-compatible, despite self-incompatibility throughout its native range, and that the tetraploid species B. alba and the hexaploid species B. pilosa were cross-compatible, although pollen fertility was low.