Along the narrow road to Mount Cook, between golden tussocks and rivers of ice, there's a battle being fought.

It's happening beyond the farm gates, just out of view.

It's a battle to save a landscape hundreds of thousands visit each year, one found nowhere else in the world that is under serious threat.

JOHN BISSET/FAIRFAX NZ Marion Seymour, right, at Ferintosh Station with newspaper delivery man Mark Bonnington, left, and husband Gilbert Seymour.

"A lot of people drive past this place and say it doesn't have a problem," says Marion Seymour, who lives on Ferintosh Station.

"Oh, it's got a problem."

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LINZ/SUPPLIED Wildings on the shore of Lake Pukaki.

The problem are wildings, invasive trees from North America which have taken over her farm along with many others.

As far as infestations go, it's a beautiful one. Many people have a wilding tree in their home, Christmas ornaments dangling from its branches.

But the wildings have gone rogue. They are fundamentally changing an iconic landscape, turning Mackenzie into Montana in a process becoming more and more difficult to reverse.

MYTCHALL BRANSGROVE/FAIRFAX NZ Experimental research on fire behavior of wilding conifers at Pukaki Downs.

Seymour has fought for decades, well past retirement, to save her property from the trees. She's not winning.

At their current rate, wildings will cover 22 per cent of the country within two decades, an area the size of Canterbury and Marlborough combined.

Those fighting the lonely battle on their farms beneath the mountain are determined for that not to happen.

JOHN HAWKINS/FAIRFAX NZ Spraying pines at Mid Dome.

"I wouldn't know any woman who has spent anything like the time I have slogging against wildings," Seymour says.

"It did consume us, definitely. They've ruined our life and they have for years."

WILDING PINES SUPERCHARGED BY NZ CLIMATE

PETER MEECHAM Wilding Pines on the Mid Dome in Southland, one of the worst affected parts of the country.

They have scientific names, of course. Pinus contorta is the most common wilding species in New Zealand; others include Corsican pine and Douglas fir.

The blanket term, wildings, is easier to spit off the tongue, as is often the case in the high country where they've caused the most grief.

Wildings are from North America, where they're dotted across the great plains, in states such as Montana and Wyoming.

There are about 10 wilding species now spreading across New Zealand.

The wildings here are supercharged, emboldened by the climate. They can grow about five times faster than they do in their homeland.

They sail on the wind and take root, sucking the ground dry and growing tall, spindly and unavoidable, often on steep rock faces or above the snowline, clinging to life in the most improbable places.

One tree can spread thousands of seeds. It takes one seed to start a forest.

If left unchecked, they could cost the economy about $1.2 billion, according to a Ministry for Primary Industries estimate.

"20, 30 years ago the word wilding wasn't known much at all," says Nick Ledgard, a retired forestry researcher.

"People hadn't really given it much thought. They sort of had visions of triffids marching up the hills - it wasn't really well understood.

"Now, without a doubt, the word is well understood."

There's nothing inherently wrong with wildings, Ledgard says, and he's not opposed to exotic species in general.

But when wildings spread, they strangle the life out of the land. No other species may grow, no animals may graze.

They push out native species, and grow so thick they turn the landscape into a monotone. In the Mackenzie, they take over the famous golden tussock, permanently changing how it looks.

It's about having the right species in the right place: and wildings, infamously, refuse to stay put.

EXTRAORDINARY FINANCIAL AND EMOTIONAL TOLL

For more than 30 years, Marion Seymour and her husband, Gilbert, have fought the wildings. They were among the first.

They roamed the farm every day, ripping the young seedlings from the ground, using their dusty old Land Rover as a bulldozer.

Marion would climb up steep hills, into the unlikely corners where wildings thrive. The farm has several major creeks, meaning the trees can't be sprayed; they had to be toppled by hand.

She has become an amateur botanist, identifying wilding species on sight, keenly aware when a new species arrives with the prevailing wind.

She skipped weekends to clear wildings; her hand has stiffened from the labour.

"Man, have I been bone-weary some years. I've been past muscles and into bone-weary. I think we've put in the biggest battle of anyone and anything I know.

"We've put our heart and soul into it. Other people have been driving around in new cars while we were socially out of it."

It's taken an extraordinary financial and emotional toll, too.

There is no exact amount, but it's likely several hundred thousand, she says.

There were intangibles too; the weekends worked, opportunities missed, the 2000 acres which had to be given to the wildings because they had grown too quickly.

After a valiant effort almost a decade ago, the Seymours fully cleared one section of trees, but "now you wouldn't ride a horse through it," Marion says.

She looks up at a steep hill on the farm, now riddled with Rowan, a new type of wilding that floated in from the north. It's a tough one.

It clings to gullies and creeks, largely hidden from view.

"I just look up our hill face and think, I slogged my innards out on all the other species and now we've got something that's going to be very hard to control.

"It's really, absolutely disheartening."

She still goes out to tackle the wildings, albeit less vigorously than before.

There was never any thought of giving up the farm. It had been in the family since the 1930s, and the infestation wasn't their fault.

By containing it, they're limiting its spread elsewhere – if nothing else, it may save someone else the grief.

"We were doing what was best for our country, and after all that's what we're here to do, look after it. We've certainly tried to do our best.

"It's heartbreaking what we've put into it and to end up with what we've got."

PROBLEM 'TOO BIG FOR LANDOWNERS'

The snow has melted around Lake Pukaki and the sun is shining, which means one thing; it's time to kill wildings.

At Pukaki Downs Station, wildings are controlled with military precision.

For several months of the year, the farm hires and trains a dedicated crew to contain the trees, which have taken over nearly half of the property.

"We've put more money than probably any other landowner in the country into containing the problem," says George Ormond, one of the farm's managers.

"We're committed to doing something and have been seriously tackling the problem for many years."

It costs the farm about $400,000 each year to keep on top of the wildings, Ormond says.

Over the Summer a crew roams the property, destroying the seedlings wherever they pop up to limit the spread.

The farm has taken an unusual approach to pay for the control.

It has left a large wilding forest mostly untouched, entering it into the Emissions Trading Scheme (ETS), allowing the farm to sell the carbon credits to fund control elsewhere on the property.

It would replace the wilding pines in the forest with a non-spreading timber species, key to its admittance into the ETS.

The wilding problem was one the government needed to step in and address, quickly.

"It's a problem too big for the landowners on their own to deal with... For some landowners, basically the problem has become so big they've given up on it. We understand that very much.

"They say a stitch in time saves nine; In this case it's a stitch in time saves 99, with ten zeroes on it.

"If you don't deal with it now, you've got a huge problem in the future. It's millions and it will move into the billions."

Much of the cost at the moment is being picked up by landowners themselves. For some, it has come at a bad time.

On a nearby dairy farm, the cost of wilding control has been enormous.

The manager says he doesn't really want the publicity; wilding control can be a touchy subject in the Mackenzie. But he had spent more than most.

"We've spent a heap of money on it. With the downturn in the dairy industry, it's sort of caused me a bit of grief," he says.

"But we'll survive to keep fighting the battle."

PLANNED PLANTINGS NOW A 'GIANT NIGHTMARE'

The role of government in wilding control is an awkward topic; the government, some say, largely caused the problem in the first place.

From the 1950s, it was looking for ways to control soil erosion, particularly in remote parts of the high country.

It decided to plant trees to anchor the soil; pinus contorta was chosen in most places for its hardiness. They didn't realise it would spread.

Ledgard himself planted some of the trees while training for the Forest Service.

The problem wasn't the wildings themselves, he says; it was that authorities ignored the problem for too long, letting them spread too far.

"In some situations, for a long time nothing was done, so it was getting to the stage where you're trying to shut the door after the horse had bolted."

He says the government has come around. In the last budget, $16 million over four years was set aside for wilding control, which showed they were taking it seriously.

"They're [Treasury] not interested in motives and aesthetics, they just want to know a cost/benefit judgment. They accepted that business case and put money towards it, and I think that's the big success."

He firmly believes wildings can be defeated.

"They're very predictable. And anything predictable, almost by definition, is more manageable.

"The fact is, wildings can be eradicated."

At Pukaki Downs Station, the infestation was a government one. It had planted wildings on the shores of Lake Pukaki, after construction of a dam.

While not being bitter about it, it was good to see the government now acknowledging its complicity in causing the problem, Ormond says.

"Hindsight's a wonderful thing. It was planted intentionally, a lot of it by government agencies, and it's turned into a giant nightmare."