Although paddy fields in Lao PDR maintain rich biodiversity, agricultural modernizations have been steadily increasing. The installation of irrigation facilities, in particular, has allowed farmers to cultivate rice even during the dry season. It is expected that the life cycle of plants that are in harmony with the cycle of traditional rice cultivation could be upset by the double-crop system. This study aimed to determine the effects of double cropping on weed vegetation in the paddy fields of southern Laos.

The field survey was carried out at 2 villages in southern Laos. Twenty sample paddies were selected from each village. One quadrat (1 m²) per sample paddy was selected and the coverage of each plant species was recorded in each quadrat. The survey was conducted in September 2010 and March 2011.

There were no significant differences in the number of species per quadrat and the Shannon’s index between irrigated and rain-fed paddies at the rainy season. The irrigated and rain-fed paddies at the rainy season were divided into different groups at the result of TWINSPAN classification. These results suggested that the double cropping influenced the species composition rather than species diversity.