Persistence of residual herbicides for preventing establishment of broom (<i>Cytisus scoparius</i>) seedlings

Abstract

A bioassay tested how long after application that residual herbicides used in forestry continue to suppress or kill broom (Cytisus scoparius) seedlings Eleven herbicide treatments were applied to small plots of Tokomaru silt loam soil near Palmerston North on 4 December 2008 Soil samples were taken fortnightly until herbicide residues no longer affected broom seedlings The samples were placed into pots and sown with scarified viable broom seeds within a heated glasshouse Herbicide effects were determined by scoring the severity of damage to seedlings and measuring seedling dry weight; these were compared with broom seedlings established at the same time in untreated soil The most persistent residues came from triclopyr/picloram applications which killed broom seedlings for 5 months after application and then suppressed growth of seedlings for a further 7 months Broom seedling death also continued for 45 months for hexazinone application for 2 months with clopyralid and terbuthylazine residues and for less than a month after triclopyr and metsulfuron treatments With all herbicide treatments there was a period of several weeks or months after the time that seedlings could germinate successfully when the residues were still sufficiently high to stunt development of new broom seedlings