People in Manawatu, Rangitikei and Whanganui are being warning about an invasive weed, gunnera, which is finding its way from gardens into drains, river systems, and cliff and coastal areas.
The regional council says it is as aggressive as Old Man's Beard.
Environmental management officer Neil Gallagher says the weed can totally modify areas by displacing native seedlings in damp areas, and shading out established natives.
He is appealing for people to report if they have the weed on their property so it can be destroyed by the council.
Gunnera has umbrella-sized leaves, is covered in rubbery prickles, and can grow up to 2 metres high.