Invasive plants threaten not only natural habitats but also the native weed associations of the agricultural land. The segetal (arable) weed vegetation in Hungary has changed greatly in recent decades, mainly through the decrease of diversity of arable field flora. This study deals with the role of invasive weeds in transforming the segetal weed communities in South Hungary. It can be stated that these invasives have a serious transforming effect on the natural arable weed associations. Some of the investigated plants and their frequency in the examined region are common ragweed (84%), horseweed (22%), annual fleabane (14%), johnsongrass (36%), and Italian cocklebur (7%). These plants were all frequent and dominant on the studied area, both on arable land and in vineyards. Some of the weed associations already bear the names of these community-forming species.
Nomenclature: Annual fleabane, Erigeron annuus (L.) Pers. #3 ERIAN; common ragweed, Ambrosia artemisiifolia L. # AMBEL; horseweed, Conyza canadensis (L.) Cronq. # ERICA; Italian cocklebur, Xanthium italicum Moretti # XANIT; johnsongrass, Sorghum halepense (L.) Pers. # SORHA.
Additional index words: Arable weed flora, impoverishment, invasive weeds, segetal weed vegetation.