Response of hawkweed (Hieracium floribundum) patches to NPK fertilizer in an abandoned pasture

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Abstract
Fertilizer (336 kg ha−1 year−1 NPK) was applied to an abandoned pasture in southern Ontario to investigate the impact of changing soil fertility on the process of patch formation by hawkweeds (mainly Hieracium floribundum Wimm. and Grab.: Compositae). A single application of fertilizer was sufficient to increase the grass standing crop, reduce the hawkweed standing crop, and thereby temporarily halt the process of patch formation in areas initially dominated by grasses (mainly Poa compressa L.). Repeated (i.e., annually for 4 years) fertilization simply increased further the predominance of grasses in these areas. In dense, well-established hawkweed patches annual fertilizer treatment was essential to maintain the increased standing crop of grasses. In these dense patches even repeated fertilizer application had no effect on the standing crop of hawkweed. The experimental results clearly showed that soil fertility could influence the outcome of grass–hawkweed interactions in this pasture. Presumably soil fertility is an important determinant of the rate of patch formation by hawkweeds.