SUMMARY
In the cultivation of oil palm plants, there are several obstacles that are always faced, including weeds. Especially in Indonesia, which is classified into the tropics with a climate that is very supportive of the growth of plants and weeds. Weed vegetation analysis on oil palm plantations is to find out what weeds are dominant in oil palm fields. This study aims to determine the vegetation and density of weeds in ecology and determine the types of weeds that grow dominantly in oil palm fields and determine their characteristics. This research was carried out in PT Perkebunan Nusantara IV. (Bah Birung Ulu, Pabatu, and Adolina) North Sumatra Province. The research was carried out from February to July 2018. Bah Birung Ulu Estate has an elevation of 800-1,200 meters above sea level, with a temperature of 15-22 oC, and the humidity of 85%-92%. Pabatu Estate has an elevation of 300-500 meters above sea level, with temperatures of 25-27 oC, and humidity of 70%-80%. Adolina Estate has an elevation of 3-30 meters above sea level, with a temperature of 29-33 oC, and humidity 65% -77%. The highest diversity of weeds in the Bah Birung Ulu Estate in oil palm plantations aged 9 years was Cyperus kyllingia (23%), in plants aged 12 years was Axonopus compressus (41%), and in plants aged 21 years was Elephantopus scaber (37%). In Pabatu Estate the highest diversity of weeds in plants aged 12 years was Cyperus kyllingia (29%), in the plants aged 13 years also Cyperus kyllingia (45%), and Cyrtococcum acrescens (29%) in plants aged 17 years. In Adolina Estate, Asystasia intrusa (73%) dominates in plants aged 7 years, in plants aged 12 years was Cyperus kyllingia (36%), in plants aged 22 years dominated by Cyrtococcum oxyphyllum (65%). The diversity index measurement from the three estates shows: moderate diversity, quite a productivity, the condition of the ecosystem is quite balanced, moderate ecological pressure.