This follows the weed pest being found on a Canterbury farm last year and previously being detected in 2013, 2016 and 2021.
So far black grass has only been found in isolated patches with each response relying on rapid reporting, strict movement controls, and repeated surveillance to prevent it from becoming established.
The weed is ranked high on the undesirable list by Seed and Grain Readiness and Response (SGRR) — a biosecurity group set up to respond to incursions.
No further plants have been found during the scouting of paddocks related to the 2025 incursion and paddocks still under surveillance from the 2021 incident have also been pest-free.
Last year’s incident on a single farm was found in a seed dressing sample during a pre-export inspection of commercially grown perennial ryegrass.
SGRR chairman Ivan Lawrie said the source of the incursion was unconfirmed so far and would probably be difficult to identify.
"We haven’t found any other contamination in any of the other paddocks that had the same seed line. We didn’t actually find any black grass on the lines that were imported, so it could be a small amount of seed got through in only one bag that went to one destination. But the important thing is that it was detected and contained."
He said growers looking for unusual plants and reporting them deserved praise.
The farm remains under active surveillance.
Cleaned and inspected seed produced from the seed line and other farms has been exported, while straw bales were destroyed with the cost covered by the government and SGRR through a biosecurity levy.
Black grass can be found in many European countries as well as parts of North America’s Pacific Northwest and other countries.
Mr Lawrie said the 2025 incident was completely unrelated to the 2021 incursion.
He said the ongoing incident at two farms in North and Mid Canterbury in 2021 remained active, but was coming to an end.
"There has been nothing detected since late-2022 and early-2023 that season. There were two years of detection in the paddocks and then there was a remnant paddock that appeared the following year all related to the same incursion. Nothing has shown up thankfully and we have found no black grass since then."
The first paddock was growing linseed and the second sighting was a paddock in cereal crop which had been in linseed the previous year.
He said the containment after a follow-up programme and herbicide treatments had been successful.
"We were very lucky. Black grass is something that will continue to appear sporadically because we import and export seeds. Obviously, as an industry it’s really important to understand that these processes are more like a set of sieves and screens rather than a wall."
The 2013 outbreak was from a spinach line of suspected black seed and was followed by an unrelated incident when it was detected in a paddock in 2016 — traced to an importation of seed in 2007 when black grass was not on the weed pest list.
He said good control measures were in place with the series of "sieves", but it was important to remain vigilant as the weed pest could be expected to slip through the system.
Every single kilogram of seed entering the border would have to be sampled to prevent it entering again, but this was not feasible, he said.
"The system seems to work because people are reporting anything they find. The key message is everybody in the industry related to growing a crop — growers, representatives, field inspectors, seed cleaners, agronomists — needs to be keeping an eye out for anything unusual, however obvious it may seem."
Black grass can go from a pale colour to a purplish tone when it matures and growers should be vigilant during November to January when seed heads are likely to be at or above the crop canopy.
This is the critical window to find plants before they reach the point where seed can shed.
The weed spreads quickly in crops and overseas it has reduced yields, disrupted rotations, and developed herbicide resistance to the extent that growing cereals has become unviable in the worst infestations.
Some countries refuse to accept seed where black grass has become a problem.
Growers should take photographs of suspected plants and report it immediately to the Biosecurity New Zealand Pest and Disease Hotline after marking the spot.
