Farmers and officials have come up with a plan to try to stop black grass putting down roots in Canterbury.
A PGG Wrightson truck spilled black grass seeds along State Highway 77 throughout mid-Canterbury in a biosecurity incident in July.
The grass is detested by farmers in Europe, where it is resistant to herbicide, spreads rapidly and competes with crops, reducing their yields.
Federated Farmers grain and seed vice-chairman David Clark said the Ministry for Primary Industries had approved an eradication plan put together with the Foundation for Arable Research.
The plan included spraying the roadside three times a year for three years with specific sprays and visual monitoring up to nine times a year for up to five years. The plan had been approved for two years but it was intended it would run for longer, Mr Clark, an Ashburton cropping farmer, said.
It was hoped the black seed had spilled only on the roadside and had not spread into neighbouring paddocks.
"One thing we are looking at is the wind direction the day the truck drove off the road and looking at any potential risk of it blowing over (into) the neighbouring paddocks, and looking at paddocks that may have been cultivated in the time between it being spilt and now," he said.
"We'll also be monitoring down water courses for any seed that may have been spilled into drains and washed downstream."
However, efforts would initially be primarily focussed on the roadsides, Mr Clark said.