An expanded study of non-native plant species in eastern Wisconsin Natural Areas resulted in a total of 64 exotics being found. The most abundant were Rhamnus cathartica, Taraxacum officinale, and Berberis thunbergii. Sites suffering the greatest degrees of invasion were the oak openings, while the more mesic areas appeared relatively resistant.
Leitner, L.A. 1990. Alien plant species in Eastern Wisconsin Natural Areas. Field Station Bulletin 23(2): 8-17.