Competitive Effects of Three Perennial Weeds,Cynodon Dactylon (L.) Pers., Cyperns Rotundus L. and Sorghum Halepense (L.) Pers., on Young Citrus

One-year-old sour orange seedlings were grown with the perennial weeds Cynodon dactylon, Cyperus rotundus and Sorghum halepense in adjacent containers, which were either separated or linked. In the separated containers no effect due to aerial competition was apparent on citrus after ten months. In the linked containers citrus growing with weeds showed, after 3-4 months, severe reductions in plant height, stem diameter and leaf number. The addition of nitrogen slightly reduced the competitive effects of the weeds. The order of competitive capacity was C. dactylon<S. halepense<C. rotundus. When weed tops were removed, the citrus plants recovered within six months from the most severe damage. It is suggested that competitive effects are caused, at least in part, by phytotoxic substances produced by the weeds.