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Rainfed paddy cultivation predominates in north‐east Thailand. Direct dry seeding is replacing transplanting rice cultivation to increase the frequency of successful plantings and to save labor. The present study clarified differences in weed vegetation of direct dry‐seeded and transplanted paddy fields under rich, medium and poor water conditions from agricultural and ecological viewpoints. A large number of paddy fields from a wide range of climatic, topographic, soil and hydrological conditions were used. A survey was conducted every three weeks, and data regarding paddy cultivation, weeds and water conditions were collected. Expansion of direct dry seeding may increase species with in‐between water adaptability to hygrophytes and mesophytes, and decrease hydrophytes and mesophytes. This characteristic was remarkable under poor water conditions. Rice cover was similar in either planting method under rich water conditions. However, cover of direct‐seeded rice was significantly smaller than that of transplanted rice under medium and poor water conditions. Species diversity was higher in direct dry‐seeded paddy fields than in transplanted paddy fields under medium and poor water conditions. Impact of direct dry seeding in the fields with rich water conditions on the vegetation is small in the context of maintaining rice production by transplanting and species diversity. However, direct dry seeding is mainly adopted in fields with poor water conditions. Impact of the changes in cultivating methods is large on rice productivity, species composition and species diversity under such conditions. Therefore, how to best use resource‐poor fields may be the key to maintaining rice production and regional‐level species diversity.