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Up to 20 Otago Regional Council staff have so far inspected 1800ha of farmland in the region and confirmed the presence of the invasive weed on 38 properties.
Velvetleaf has been described as one of the world's worst cropping weeds, affecting crops by competing for nutrients, space and water.
A further 76 properties, covering about 1100ha of possibly contaminated crops, were now being inspected.
Input from about 40 volunteers, including farmers and students from Lincoln University's Telford campus, would be ‘‘welcome cavalry'' for staff who had been working long hours, the council's director environmental monitoring Scott MacLean said.
The focus was now on South and North Otago. Most inspections in Central and West Otago had been completed.
It was ‘‘a race against time'', as farmers wanted to start using their fodder beet crops for stock feed, but the inspections should be completed by the end of the week, thanks to the involvement of volunteers, Mr MacLean said.
In recent weeks, ORC staff had been supported by staff from the Hawkes Bay Regional Council and Hawkes Bay environmental consultancy Habitat Biodiversity and Pest Management Ltd, and botany students from the University of Otago.
Velvetleaf seed could remain active in the soil for up to 60 years. Every time it was disturbed, there was a risk of spreading seed which might have dropped from the plant.
That meant farmers needed to remain vigilant over the coming years, even if the velvetleaf plants had been removed, Mr MacLean said.
Removing velvetleaf had to be done by hand, rather than spraying, as the crops would soon be used for winter grazing.
The Ministry for Primary Industries has developed a farm management plan to assist landowners with affected paddocks and restrict its spread.