AbstractSeedlings ofRumex obtusifoliusL. were grown in gaps (11 cm diameter) in an established sward atLolium perenneL., Root competition from the surrounding sward was controlled by PVC tubes and shoot competition was controlled by cutting the sward frequently. All combinations, with and without root competition and with and without shoot competition, were used.Plants received either 40 or 80 kg N ha−1month−1and 0 or 300 kg K2O ha−1and were harvested 39 d after emergence; a second harvest was made 12 d later, after shading to 20% of full sunlight.The shoot dry weight per plant ofR. obtusifoliuswas much more affected by root competition than by shoot competition, especially at low nitrogen applications. Potassium supply had no effect.More dry matter was allocated to the leaf petioles under shaded conditions and the specific leaf area was greater. This morphological plasticity enabledR. obtusifoliusto grow well under shading and to be affected only slightly by shoot competition fromL. perenne.The results suggest that competition for nitrogen was the main factor limiting the initial grown ofR. obtusifoliusin a ryegrass sward and that the morphological characteristics ofR. obtusifoliusmake it less susceptible to competition for light in the early stages of its develo