The City of Stirling’s report into the incident said the council normally used Roundup every six months to control weeds in sumps. But a decision was taken in 2004 to spray once a year with a longer-lasting herbicide, partly because sumps in the neighbouring City of Joondalup presented well and were sprayed annually.
But problems occurred six months after Stirling’s herbicide was changed to the chemical hexazinone in May last year. “City officers and residents started noticing deterioration in the condition of established trees and shrubs in and adjoining the sumps,” the report said.
Stirling’s investigation found that trees and vegetation in sumps sprayed with hexazinone were dead or in severe decline. “Though replacement planting could be undertaken using native vegetation, a number of the dead and declining trees were of a significant age class that categorised them as historic,” the report said.
In all 136 sumps had been sprayed, 74 of which required tree lopping or removal of dead vegetation, including dead or dying material in private properties adjoining 10 of the sumps. The report recommended that herbicides such as hexazinone should not be used in sumps.