A field survey was taken up in Coimbatore, Tamil Nadu state to know the dominant weed species competing in the lowland rice ecosystems to identify the major weeds and study the biology and phenology of the weed species.  Barnyard grass (Echinochloa crus-galli), blistering ammannia (Ammania bacciffera), false daisy (Eclipta alba) and Viper grass (Dinebra retroflexa) were found predominant in lowland and were selected for the study. The seeds of selected weeds were collected along with the inflorescence from the field pot culture and field studies. It was found that among the four lowland weeds, Echinochloa crus-galli and Eclipta alba germinated  6 days after sowing both in the pot as well as field study.  Ammania bacciffera was germinated earlier in pot culture (7 days) and  Dinebra retroflexa germinated earlier (7 days) in field condition. Weeds grown in field condition came to 50% flowering earlier than grown in pot culture. Total dry weight per plant at flowering was higher in Dinebra  retroflexa  both in field  and pot culture and at maturity, it was higher with Echinochloa crusgalli.  Total dry weight at maturity was 8-12 times more than at flowering for all the weeds. The numbers of seeds/plant was higher with Echinochloa crus-galli in both in pot culture and field study with 650 and 850 seeds/ plant respectively.