Rice-field weeds in the Colombo and Gampaha districts of Sri Lanka

Abstract

Two rice-growing districts in the low-country Wet Zone were surveyed for the wced-flora in the ricefields. A comprehensive list of the spccies, including grass, sedge and broad-leaved weeds found in the ricefields surveyed, is presented. An estimation of the frequency of occurrence of the major weeds was made in order t o evaluate the a1)untlance of the species in the rice agroecosystem. A method of scoring based on visual estimation of cover was used t o evaluate the level of infestation of the wecds. l*imbristylis rniliacca, Isachvre globosa and Cypems iria were the most abundant monocotyledonous weed species found in the ricefields of the two districts. Among thc broad-lcaved weeds, Ludwigia byssopifolia was by far the most abundant spccies with high levels of infestation in most of the fields surveyed. A second species of I.udwigia, yet unidentified and probably a relatively new weed record for Sri Lanka, was also found to occur in high frequency in the Colombo district. Thc survey revealed that a rich weed-flora occurs, both within the ricefield habitat and on the raised earthen hunds in the areas studied.