The arrival of warm, humid days means velvetleaf and other nasty weeds will be popping up, prompting a plea for landowners and rural contractors to be on the lookout.

Helping to sniff out velvetleaf in Waikato region has been Rusty and his handler John Taylor.

The Southland-based pair visited 12 high-risk farms, finding plants sprouting on eight of them. They'll be returning to carry out further surveillance work in late January and early February ahead of the harvest.

Waikato Regional Council's biosecurity pest plants team leader Darion Embling said it was the right time to be looking out for pest plants and acting to remove them.

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"Pre-emergence spraying has been carried out by most farmers, but we're getting reports from those which have previously had confirmed velvetleaf infestations that seedlings are pushing through," Mr Embling said. "This is a critical time for control of pest plants, and in particular velvetleaf, with hand-pulling seedlings and post-emergence spraying essential to get on top of them.

"Landowners and rural contractors should be looking around gateways and the first 3-4 rows of crops for signs of velvetleaf. And if they spot this plant, they need to notify us for advice to avoid crop loss."

The leaves of velvetleaf are heart-shaped and velvety to the touch, and have a distinctive smell when crushed. It grows up to 2.5 metres tall and has buttery-yellow flowers.

It damages crops by competing with them for nutrients, space and water, and its seeds can persist for decades, even surviving digestion and silage processes.