Wild ginger threatens native forests

Kahili ginger flowers from January to March. A single plant can cover many square metres.

Valuable native forests on the Bay of Plenty's east coast are under serious threat from infestations of wild ginger.

The rampant weed is spread over more than 600ha - and on 250 properties - between Torere, 24km northeast of Opotiki, and Cape Runaway.

"It poses possibly the greatest environmental threat to the coast," says Environment Bay of Plenty pest plant co-ordinator John Mather.

Kahili ginger invasions were centred round all the small communities dotted along the east coast and were particularly serious at Waihau Bay, Whanarua Bay and Te Kaha.

Control of the pest plant was critical, he said. It had spread steadily from original seaside plantings to deep inside the bush.

"Without intervention, the situation will eventually become unmanageable."

Unchecked, Kahili ginger had the capacity to spread many kilometres from the coast well into the hinterland, said Mr Mather.

It already "seriously compromises" the high biodiversity and landscape values of the area, which had the largest tracts of coastal native forest in the North Island.

Feral ginger, clearly visible in some places, formed an understorey to the forest canopy.

"This destruction of the natural process will lead to the complete collapse of parts of the forest as old trees die," he said.

Wild ginger was a progressive control pest plant in Environment Bay of Plenty's regional pest management strategy, which meant landowners or occupiers must control infestations on their properties. But the regulation was difficult to enforce because of fragmented land ownership and a sparse population, said Mr Mather.

"There is a small number of people pitted against a large area of ginger. Individuals and voluntary groups, including marae committees, are making a valiant effort.

"Everyone is genuinely concerned but equally genuinely unable to muster the manpower and resources to deal with such a large problem."

Kahili ginger was first officially recorded as growing wild in New Zealand in the 1940s.

It has caused major problems in conservation areas round Auckland and further north, in the Waikato and the Coromandel Peninsula, and on the West Coast of the South Island.

The once widely popular garden plant is now on a national list of plants banned from sale, distribution and propagation because of its invasive abilities.

Stock, possums, insects and pathogens seem to avoid it and the two main control methods are physical removal and herbicides.