Alien species represent one of the causes of biodiversity loss, both in natural and anthropic environments. This study contributes to the assessment of alien species impact on Western Po Plain rice field cultivations, referring to different agricultural management practices and ecological traits. Flora and vegetation were studied (the latter through the phytosociological method), and α-biodiversity was estimated through Shannon and Simpson Indices. Results highlighted a significant floristic contingent depletion and increase in therophyte and alien components, compared to pre-existing studies (1950s); higher α-biodiversity levels in organic farms, compared to conventional farms, but also a higher invasive alien species percentage. The high deterioration of the territorial–landscape context appears to play a major role in shaping these patterns. Some of these alien species are particularly aggressive (e.g.,Murdannia keisak), as confirmed by two experimental rice field plots which were left unharvested, continuously flooded, making it possible to assess the competitiveness between weed species. The detected weed vegetation is attributed to theOryzo sativae-Echinochloetum cruris-galliassociation, already described for Southern Europe, with two different ecological and floristic variants. Future studies, by including other sites and framing their territorial–landscape context, may further complement this overview on the alien species distribution and behavior in rice fields, hence facilitating their strategic management.