Northland farmers and stock food growers are warned to look out for the danger weed velvetleaf which has taken root in imports of fodder crop seeds.

The contaminated seed has been sown on more than 400 properties from Southland to Waikato, and is linked to beet seeds imported from Italy.

Ministry of Primary Industries and regional councils throughout New Zealand initiated an urgent response to the problem in March.

Northland Regional Council (NRC) biosecurity officer Sara Brill said there had been no reports of velvetleaf (Abutilon theophrasti) in Northland but all agricultural growers should be on the alert.

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The invasive, broad leafed, prolifically seeding plant is capable of reducing a host crop's yield by more than 30 per cent and spreading further in the environment.

It grows to about a metre high, has very short-lived buttery yellow flowers, distinctively segmented seedpods and large heart-shaped or circular soft leaves that have an unpleasant odour when bruised.

Each plant can produce thousands of seeds that stay alive in the ground for years.

Massey University weed expert Dr Kerry Henderson said that once velvetleaf is here, "the horse has bolted".

People finding suspected velvetleaf should photograph it, mark the location and contact MPI or NRC staff who will remove the plants. Landowners or growers should not pull up plants or graze stock in infested crops.

The highly competitive plant was first found in New Zealand growing amongst fodder crops on a Waikato farm in 1978, having arrived in contaminated imported seed, but was contained to only a few sites. This latest heavy incursion is far more widespread.

MPI has directed grain and seed retailers who have sold the Kyros and Bangor beet seed to hold any remaining seed in stock and to instruct customers to return any unsown product. Growers who have unsown seed should return it to the retail outlet.

Velvetleaf has been grown in China and northern India for thousands of years for its jute-like fibre, and edible seeds.