AbstractThe role of ecological factors in the invasion of fescue tussock grassland by Hieracium species was investigated in a box experiment. Soil fertility and cutting frequency effects on interference between the introduced flat weeds Hieracium pilosella, H. praealtum, and Rumex acetosella and the native bunch grass Festuca novae‐zelandiae established on a denuded soil were examined using a model and technique involving stress and disturbance gradients. The weed species showed marked biomass and phenological responses to increased availability of mineral nutrients compared to F. novae‐zelandiae, which had reduced yield at the highest level of soil fertility. At high soil fertility, R. acetosella suppressed both Hieracium species and F. novae‐zelandiae, probably because it competed more effectively for light. Cutting frequency also influenced the pattern of interference between the species by effects on competition for light, vegetative spread, and partitioning of biomass to parts of the plants above and near or below ground. Characteristics that have enabled Hieracium species to be successful invaders of tussock grasslands include markedly phasic growth coincident with the time of year when temperature, water, and nutrients are least limiting, ability to respond rapidly in vegetative and reproductive growth to pulses of mineral nutrient availability, and growth habits that enable them to invade and then hold on to sites they have occupied. Once they have occupied inter‐tussock spaces it appears that hawkweeds can competitively exclude tussocks. The pattern of Hieracium root distribution and seasonal periodicity of the activity of their root systems may be important in the process of competitive exclusion of tussock grasses.