Boron application to kill mouse-ear hawkweed in snow tussock grassland in Central Otago: does this have toxic effects on the native flora?

Abstract

ABSTRACTPilosella officinarum is a serious adventive weed in New Zealand. It has also proven to be highly invasive in other countries. The areas infested are typically extensive rangelands for which expensive control measures would be hard to justify economically. However, it has been known for many years that species of this genus are particularly sensitive to the concentration of boron in the soil. The application of boron fertilisers is now a common practice to control P. officinarum infestations in the extensively grazed tussock grasslands in Central Otago. The farmers seek to exploit the differences in boron tolerance between P. officinarum and the exotic pasture species which farmers would like to establish in its place. However, there are large tracts of infested land for which the aim is the restoration of the native vegetation for water harvesting, aesthetic or conservation reasons. This study examines the effect of boron application (as ulexite) on the native species of the tussock grasslands flora. At the application rates recommended for P. officinarum control none of the native species was adversely affected by the added ulexite. Doubling the recommended application rates affected some but not all of the native species. The results suggest that the use of boron in this manner is unlikely to have any adverse effect on conservation values in tussock grassland, whether grazed or retired.