Weeds: Aliens in our backyards

ARC biosecurity officer Mary Stewart and colleagues visit control sites several times a year to clear weeds and check for new growth. Photo / Greg Bowker

Labour Weekend is when denial must end - especially if the weather forecasters are correct and rain won't save us.

Yes, it's time to prise open the back door and rediscover the garden you last visited as the autumn leaves and chill descended. You know it's in there somewhere, beneath the tangled mess of vines and crawlers and wandering willy which have thrived in the long wet winter.

Reclaiming the garden may seem a backbreaking chore for doubtful reward - few of us have the green fingers or knowledge to achieve our botanical visions. But you can take the pain out of weeding by making it virtuous. Tell yourself you're not labouring just to spare your blushes when you host your first barbie - you are saving the countryside. Gain strength from the thought that you are part of an underground army, digging for victory.

The warnings coming from some scientific quarters are enough to make us look at weeds in a fearful new light: the language is of space invaders, ticking timebombs and slow-burning fires. Thousands of weeds are biding their time, apparently, waiting to leap the fence and attack our native forests, birdlife and valued landscapes.

Weeds are defined as plants which are capable of taking over habitats; plants which grow too well. Offenders currently raising national concern include wilding pines, which are transforming the tussock slopes of central Otago and the Mackenzie basin, and giant rhubarb, which is running rampant along the Taranaki coast and threatening scenic spots on the West Coast.

Weeds threaten nine endangered plant species; they transform wetlands and block waterways. They smother and outcompete natives and put native birds and insects at risk by destroying nesting and feeding areas.

In the north, the threat to conservation and regional parks from wild ginger, woolly nightshade, moth plant (milk weed) and tradescantia (wandering willy) has been well publicised. They leapt the garden fence many years ago, requiring long and expensive campaigns to prevent them consuming whole forests. Tradescantia forms a dense matting over the forest floor, smothering native seedlings and changing the balance of the forest. Only large trees, such as karaka and kohekohe, tend to regenerate so the balance of the bush is altered. Climbers such as the moth plant can topple canopy trees.

Many other introduced plants are capable of wreaking similar havoc - so-called sleeper weeds which have yet to get out of control. They came in before biosecurity was taken seriously but there can be a lag of up to 40 years before they run amok. It is these that conservation agencies want ordinary gardeners to wage war on, before it is too late.

More than 20,000 exotic plants have been introduced to New Zealand and about 2200 have become established in the wild. About 500 are now controlled as pest plants; a couple of hundred are capable of causing severe economic damage.

That leaves an enormous reservoir of potential weed pests - and one is escaping into the wild every 39 days on average, say experts.

Humans are the worst spreaders, and not just those living near the bush who dump garden waste down the nearest gully. Birds and wind can disperse seeds from urban gardens up to 60 km away, including on offshore islands. Auckland, it turns out, rivals Honolulu as the weediest city in the world.

Scientists predict the cost of fighting weeds - currently about $1 billion when farming and forestry is included - will snowball as sleeper weeds run wild, and climate change is expected to help them to flourish.

Landcare Research ecologist Dr Peter Williams has spent his career battling weeds. He says weeds which have become naturalised relatively recently are a major threat. "Their numbers and distribution in the wild don't reflect their potential."

Nelson-based Williams' work has included looking at the influence on birdlife of exotic food sources. A study found most endemic species such as tuis prefer native vegetation such as coprosma or five finger berries to exotic alternatives - just as exotics attract introduced species such as blackbirds. "It suggests that weed species offer an inferior food source for endemic birds, though it's extremely doubtful that any New Zealand bird species has become extinct through weeds replacing native plants."

His research has helped convince agencies that the weed problem is too pervasive to get on top of; that we need to identify what we most value and target resources into saving them.

Weeds have the potential to completely transform the landscape," says Williams.

"Without exaggeration, pine trees could cover the lower mountain slopes of the eastern South Island from Blenheim to Foveaux Strait and the Volcanic Plateau - any land where grazing is below a certain threshold, with devastating impact on native fauna and flora."

He points to other transformers: heather in the Tongariro National Park and hyraceum which can grow above the tree line in the South Island.

"You lose your unique landscapes and you start to lose your national identity. The only way you know whether you are in New Zealand or Argentina is if you hang on to your native stuff."

The farming and forestry sectors have invested millions to get on top of their weed threats, but the problem of environmental weeds is too big for taxpayer funding to solve, says Williams. It's individual actions which will make a difference from here.

"People tipping green waste into sand dunes - that's behaviour that can be changed for free.

"[But] the public need to be clued up about what it is they are planting because what they plant in their gardens is going into the bush."

He says the problem of potential threats being sold in garden nurseries has largely disappeared since the 1980s, with most nurseries now signed up to a national accord and seeking sustainability.

"It's the small gardener who swaps plants at country fairs, or visiting granny who says 'take one of these to your bach' - swapping stuff that's hard to control. A lot is sold by private collectors on the internet, often in ignorance."

Those responsible for weed control range from state agencies, chiefly the Department of Conservation, through regional and local councils to private landowners.

The Auckland Regional Council spends about $500,000 a year fighting weeds. It has a list of 34 potential threat species which have yet to run rampant which it calls total control plants; 11 staff are working to eradicate them at 960 sites. Many are on private sections bordering conservation parks such as the Waitakeres. Most are under control - but weeds can lie dormant for up to 50 years.

The ARC has other lists - who knew that, in many areas, the common or garden moth plant (milk weed) must be removed if you find it in the garden, along with woolly nightshade?

Many have attractive flowers and features which appeal to serious gardeners, such as the scrambling lily, a vigorous climber, or lantana, a shrub with clusters of yellow/pink or red/orange flowers which can invade coastal and forest margins and suppress native growth. Hackles were raised a few years ago when the ARC added agapanthus and the phoenix palm to its surveillance list. Even the pink-flowering balsam shrub, an impatiens variety, is in the firing line for its potential to prevent regeneration of native seedlings.

Jack Craw, the ARC's biosecurity manager, is the northern variety of Dr Williams: weeds have been in his sights for 30 years. He says weeds run possums and rats a close second in terms of their effect on wildlife in our forests. But animal pests are easier to bring under control than weeds.

"You can kill possums and rats selectively but weeds are very difficult to get rid of. Herbicides would also kill native plants and is much more expensive on a per hectare basis."

Craw says as technology develops, agencies will win the battle against pest mammals, not least because there are no new species. "But there are two to four thousand species of plants we don't know much about. We know about 5 per cent will become weeds - that's a couple of hundred new species of weeds. Currently about nine or 10 a year are taking off.

"It's pretty daunting. In 50 years time we will be spending a hell of a lot more money on weeds than we now spend on animal pests."

Auckland's temperate climate makes it a paradise for weeds, says Craw.

The region is damp for much of the year and gets neither the extreme cold nor heat that would kill off many imports.

"The good news is we think we've won the battle for the hearts and minds of the community. Problems we used to have with people dumping garden refuse have abated a lot even though most [tips] charge for garden waste. The public have come a long way and farmers are really switched on."

Craw says there are many ways to kill a weed.

"You don't have to spray, you can stump paint, stem inject, pull it up, you can manipulate the habitat. We are also improving the ways we use herbicides. And virtually any weed can be composted."

Scientists scored a notable victory over the mist flower early this decade. By the early-1990s, the shade tolerant menace was knee-high in forests in Auckland and Northland. "It forms a complete carpet preventing any recruitment of native seedlings," says Craw. "It dooms the forest to the life of the canopy."

It was dramatically brought under control after Landcare Research scientists unleashed biological warfare on two fronts - the white smut fungus and the mist flower gall fly. The host-specific fungus, introduced in 2000, reduced mist flower coverage in the Waitakeres by 74 per cent in the first two years alone; the gall fly, a Mexican native which burrows into the plant stem to lay eggs, stymied the regrowth.

But biological approaches need cautious research - to ensure any introduced organisms do not do more long-term harm than good - and are notoriously hit and miss. On their own, they cannot defeat what Craw calls the "hundreds of species in hundreds and thousands of gardens" which can be spread by birds or wind dispersal. Some scientists depict these as a ticking timebomb. Doubling the risk is that scientists usually can't predict the menace until it is already widespread.

Peter Williams is 65; battling weeds has been his life's work. Given the predictions of some of his colleagues, has it all been in vain?

"People say 'how do you go to work every day' but I hate being pessimistic.

"In total there isn't going to be an explosion - it's more like a slow-burning fire which we can't put out.

"In the very long term we are really struggling against the odds. Take pines in the high country - it's a multimillion-dollar job. It's very difficult to see how we're going to get a grip on all the pines in the South Island. For some ecosystems in some places I'm pretty pessimistic.

"You can try to kill all the possums and all the weeds everywhere, in which case you are going to run out of money, or you can pick specific places that you want to keep the weeds out of and work on those. Those we are winning.

"As I see it, what we are going to finish up with in 100 years' time is islands of conserved landscape surrounded by landscape which has gotten out of control.

"We're going to have to focus our efforts on those bits of landscape and reserves which we value the most. We can't control all the mar ram grass in all the sand dunes."

Weed hunters patrol 900 sites in Auckland

With its purple lantern-like flowers, cathedral bells was a popular ornamental with Auckland gardeners - until authorities persuaded garden centres to stop selling it. Now it is one of 34 exotic plants listed as total control pest plants which the Auckland Regional Council is trying to eliminate at more than 900 sites.

ARC biosecurity officer Mary Stewart (above) is responsible for sites in West Auckland covering both private and reserve land. She and colleagues visit the control sites several times a year to clear weeds and check for new growth. She is pictured at a site on a reserve bordering private properties near Scenic Drive in the Waitakeres. Cathedral bell was found there 10 years ago and has been gradually brought under control.

"But we don't like to leave things to chance," she says. "We'll come back several times over summer."

Though attractive, cathedral bells is a shade-tolerant climber capable of suppressing native seedlings and smothering large plants.

"It probably originated in someone's garden."

Stewart says most landowners are quite happy to have pest plants removed from the sites.

"People often tell us of potential problem weeds. At field days when we hand-out pamphlets, people often recognise them as something they have at home."

Officers are also alerted to problem areas by local council staff, by "weed spotters" in the community and through surveys.

ROOTING OUT THE CULPRITS

DoC and regional councils back community-led initiatives such as Weedbusters to target weeds in parks and reserves and on offshore islands. One local initiative is at Purewa Creek, behind Selwyn College in Kohimarama, where woolly nightshade, gorse, pampas, acmena and wandering willy have overtaken an expansive but sadly neglected stand of native bush.

Local resident Martin Heffer teamed up with Roy Clements, a teacher at the college's refugee education centre, to retrieve the bush. With a small dedicated bunch, the pair have spent most Sunday mornings for the past seven years clearing weeds and replanting with natives. Their work has received funding and other support from the Auckland Regional Council and Auckland City Council.

Clements says the long valley, which runs from Hobson Bay to St Johns, has the potential to become an important native bird corridor and bushwalking area in the heart of Auckland. It is near the well-preserved Kepa Rd bush reserve.

Clements led an earlier effort to restore bush and a stream behind Mt Albert Grammar School, where a walkway is named after him. He believes the Purewa gully's shared ownership - several state and local agencies have a stake - and its designation for the now-shelved south-eastern motorway contributed to its neglect.

"Transit was waiting for the day they would concrete the whole thing over and I suspect now it has slipped from their consciousness."

Kohimarama locals have proved generally "too busy" to get behind the initiative but Clements found pupils at the refugee centre more than happy to help. About 2ha was initially cleared and replanted near the school and the project has moved further into the valley.

What you can do:

* Remove pest weeds from your garden. Check the ARC's pestsearch website: www.arc.govt.nz/plantsearch
* They are all there, climbers, ground covers, shrubs, grasses, aquatics and you can see pictures without knowing their names. The list includes species in the Auckland Regional Pest Management Strategy, the National Pest Plant Accord and some other common weeds. www.arc.govt.nz/plantsearch
* Join or form a Weedbusters group. Working bees in Auckland this weekend include Rangitoto Island and Little Shoal Bay on the North Shore. www.weedbusters.co.nz
* Form a neighbourhood group under the ARC's pest plant community initiative programme - 27 plants qualify under the programme, which needs the agreement of 75 per cent of landowners in an area. www.arc.govt.nz/environment/biosecurity/
* Virtually any weed can be composted.

EVIL AND INSIDIOUS: JACK CRAW'S WORST WEEDS

EVERGREEN BUCKTHORN
"A very insidious, evil weed." This red-berried tree which invades coastal areas and smothers native plant growth can be mistaken for native coprosma. Its seeds can be spread by birds to Gulf islands; an exclusion zone applies from Long Bay to Howick. It is a major problem on Rangitoto and other islands.

MOTH PLANT
"Just about pure evil in the plant world. It's unbelievably invasive and very difficult to pull out - it is toxic and causes severe itching."

Has "reasonably attractive" white flowers in February but its choko-like pods contain a mass of thistledown-like seed which can be spread 60km to 70km. It is highly versatile - climbing fences on bare industrial land - and shade-tolerant, capable of crushing forests by climbing canopy trees. "Even in really dense forest like in the Waitakeres or Hunuas we have moth plant coming up in the middle."

WANDERING JEW/WILLY
"Very insidious and cunning." It can flourish from a single piece dropped on the ground. It forms a dense carpet on forest floors, preventing all but the seedlings of tall trees from pushing through. "Visitors think the bush is light and attractive because all that comes up are the tall canopy trees - but that forest is doomed."

PHOENIX PALM
"It has gone out of fashion since it was banned but is shade-tolerant and a real nuisance in the bush. It is very hard to kill. We say to people 'if you've got a specimen plant, fine, but it's better if it's a male'."

FIVE DEGREES OF CONTROL

TOTAL CONTROL PEST PLANTS
Weeds which largely have yet to jump the fence to become rampant nuisances but which would smother and displace natives in conservation parks and bush. The ARC hopes to eradicate them before they do, with campaigns at around 900 sites, often bordering native bush. The weeds include several tussock grasses, vines including cathedral bells and balloon vine, scrambling lily, purple loosestrife (which invades wetlands), lantana and several aquatic plants.

CONTAINMENT PEST PLANTS
Weeds which you must treat if growing on your land (though some are limited to specific areas). They include: woolly nightshade, moth plant, yellow and kahili ginger, rhamnus (evergreen buckthorn), bushy asparagus, smilax.

SURVEILLANCE PEST PLANTS
Eighty-two pest plant species identified as being of potential risk in the Auckland region. Most are already widespread and some are stalwarts in urban gardens. Their sale, propagation and distribution is discouraged. They include: arum lily, agapanthus, wild blackberry, blue morning glory, castor oil plant, chilean rhubarb, jasmine, tradescantia (wandering willy/jew), himalayan honeysuckle, sweet pea shrub, mexican daisy, phoenix palm, privet, sweet broom and water hyacinth.

RESEARCH PLANTS
These are not yet declared pests but are flagged for more research. They include dwarf forms of agapanthus, bangalow palm, brazilian rattlebox, guava, loquat, wild tamarind, marram grass, chinese fan palm, chocolate vine, camphor laurel, feral olives and queen of the night.

NATIONAL PEST PLANT ACCORD
Plants banned from sale in nurseries, including giant rhubarb, white bryony, brazilian pepper tree and yellow flag iris.

Source: ARC