An hypothesis, that the invasion of hawkweeds (Hierucium species) into South Island high country was triggered by fertiliser application from about 1950, is examined. Reference is made to the historical occurrence of hawkweeds in New Zealand, volumes and patterns of fertiliser application to high country, and experimental and observational information about the response of hawkweeds to soil fertility. Both mouse ear (H. pilosella) and king devil (H. praealtum) hawkweeds show marked and rapid increases in vegetative growth and flowering when available soil nutrients are increased. The response of hawkweeds to soil fertility is much greater than that of fescue tussock (Festuca novae-zelandiae) but appears similar to that of most introduced grasses, legumes and weeds. A specific P effect is inferred to explain why sheep's sorrel (Rumex acetosella) became an important weed of tussock grasslands before hawkweeds. Keywords: fescue tussock, hawkweeds, Hieracium, high country, sheep's sorrel, soil fertility,
