Floristic composition and abundance of rice‐field weeds in four low‐country Wet Zone districts of Sri Lanka

One hundred and thirty six weed species, belonging to twenty nine plant families were recorded from rice weed surveys carried out in four major rice‐growing districts of the low country Wet Zone of Sri Lanka, during two consecutive ‘Yala’ seasons in 1984 and 1985. In terms of abundance and distribution Moncotyledon weeds of the Poaceae and Cyperaceae, were found to be far more important than dicotyledon weeds. Isachne globosa (Thunb.) Kuntz., and Ischaemum rugosum Salisb. of the Poaceae, and Fimbristylis miliacea Vahl Cyperus iria L. and Cyperus pilosus Vahl, of the Cyperaceae, were the most abundant, widespread and troublesome rice weeds in all four districts surveyed. Of the dicotyledon weeds, Ludwigia hyssopifolia (G. Don) Exell emerged as the single most widespread and abundant species in all four districts, and a new weed Ludwigia decurrens Walt. was found to be spreading rapidly in the area surveyed. Some indications of district to district differences in the floristic composition and abundance of the more important weeds were obtained, apparently due to factors such as local distribution of weeds, management practices, and cultivars of rice grown.