The competitive effects of pale smartweed (Polygonum lapathifolium L.) were studied on cabbage (Brassica oleracea L. ‘K-Y’) which was transplanted into rice (Oryza sativa L.) fields either 3 to 12 days before or 5 days after rice was harvested. Competitive effects were determined at weekly intervals from the time beds were formed around the cabbage at 5 days after rice harvest. Where cabbage was transplanted before rice harvest, at a time before pale smartweed germination, cabbage yields were reduced when weeds were allowed to grow for more than 4 weeks from the time the beds were formed. Where cabbage was transplanted at 5 days after the rice was harvested at a time when parts of pale smartweed had emerged, cabbage yields were reduced when weeds were allowed to grow for only 2 or more weeks after the beds were formed. Also, where weeds were removed at the beginning but allowed to grow at a later date, yield was reduced when cabbage was not kept weed-free for a minimum of 4 weeks.