ABSTRACT An experiment about of the effect of initial popula- tion and time of emergence of grass (Cyperus rotundus, L.) on growth and yield of onion (Allium cepa fa ascalonicum, L.), had been done since May 20th until August 8th, 2004 in plastic house at Dumpoh Village, Potrobangsan North Magelang District, Magelang City. The altitude is 350 m, kind of soil is latosol and pH 6,5. …