Canterbury farmers are being told to look out for any signs of black grass sprouting up after 2000 seeds of the pest were spilled from the back of a truck.
The seeds were among a 28kg bag of other seeds imported from Denmark that spilled across 40km between Methven and Ashburton in July.
The bag was being sent to a factory so the black grass seeds could be removed after the Ministry for Primary Industries discovered them at the border.
David Yard from the ministry said the seeds are a low biosecurity risk.
He told Checkpoint not many are likely to germinate because they are immature and a lot landed on barren ground.
Listen to David Yard