A “silent invasion” threatens our native flora, birds and insects, but a top-level review finds our response to the weeds menace is a tangled mess. By Jane Clifton.

If New Zealand's exotic weeds could talk, they'd give similar glowing accounts to those of any happy immigrant family.

In their native country, the environment was harsh, survival a daily struggle. Lack of food and water, little shelter, wild animals apt to have them for dinner. Many in their community perished, and no one cared.

New Zealand, with its high rainfall, mild climate and benign fauna, has been a paradise destination for many exotic plants since European settlement. But in many cases, there has followed an evil version of "togs, togs, undies". Yesterday's cherished garden plant has become today's rampant weed, not just an unsightly nuisance but a conservation and even an economic threat.

A recent stocktake by the Parliamentary Commissioner for the Environment, Simon Upton, has found that although New Zealand is doing plenty to combat weeds, it has almost no idea whether it's making any progress.

Given this country's two main land masses are among the world's most exotic-weed-inhabited islands, there's little question New Zealand's existing climate is hopelessly hospitable to exotic plants – and with global warming, set to get more so.

One study on conifer spread says plantation and self-naturalised pines could occupy a quarter of New Zealand's land mass in as little as 15 years unless seedling eradication is drastically improved.

As Upton's pest-weed report, "Space Invaders", warns, climate change will exacerbate the weed threat in ways we can't even predict. Scientists in Europe and North America are already reporting that warming has weakened the health of some of their native plants, threatening longstanding ecosystems.

In New Zealand, where people love to boast that "natives are tough", the assumption can no longer be justified with confidence. Global warming could affect even robust-seeming native species, while benefiting many exotics. Although this country has had some success using introduced plant-specific predators and fungi, warming could also alter the efficacy of such biological controls.

It's not as though no one is doing anything. Weed eradication has never been more extensively undertaken. But like the weeds themselves, New Zealand's weeders are literally all over the place, "Space Invaders" concludes. Upton has challenged the multiple state and local agencies responsible for land management to urgently align and co-ordinate their mismatched weeding efforts – but admits it's a tough sell in a crowded agenda of policy priorities.

The muted media, public and political response to his report underlines the problem. Weed invasion is a huge, complicated and not terribly sexy problem. Native fauna is much easier to get people worked up about. But as Upton says, the "silent invasion" of weeds is one of our birds' and insects' worst threats – and a fast-vaulting one at that.

Upton can chronicle his own battle with his large country garden over decades, quoting Irish ecologist Yvonne Buckley that wherever humans use land, they create unsustainable associations and conditions that invite unwanted interlopers. "Weed-shaped holes," is Upton's shorthand.

His report laments that state, local government and volunteer squads have been beetling over the weed problem for years, but with patchy application of science and little data on what works and what doesn't, and work in one area can be undermined by lack of work in an adjacent area.

Conservation Minister Kiri Allan says the Department of Conservation (DoC) is already working on meeting the commissioner's concerns, and believes the public will be supportive. "I guess a fluffy bird is far more attractive than, say, old man's beard. However, if weeds like that are not dealt to, then long term that fluffy bird isn't going to have a tree to nest in. I think most people understand that."

Allan raises two main difficulties with Upton's proposals. One is that any changes DoC makes to weed management have to be co-ordinated with several other government agencies, regional councils, Māori and the scientific community. As for monitoring, that's a careful trade-off. "Yes, there is more that DoC can do around surveillance of weeds, but it is expensive and complex. It needs to be sized appropriately to ensure it isn't undertaken at the expense of on-the-ground delivery."

Allan says the department's weed work has usually been coupled with predator control, planting and other big conservation projects, so it's difficult to report specifically on it. There's already extensive co-ordination with other agencies, as urged by the report, notably the eight-agency Desert Road crackdown on gorse, broom and lupins.

She says DoC will continue to run its Weedbusters support programme while it re-evaluates its focus for the next four years. It does know it needs more "domestic intelligence" on invasive species' spread, and to look for new biological control tools and new-generation pesticides and herbicides. It also acknowledges it must build greater pest-management "professional capacity".

"I am all for a greater focus on detecting and managing weeds in the early stages of invasion," Allan says. "As the commissioner notes, nipping new threats in the bud – that is, early detection – minimises the cost of subsequent action and makes eradication or containment more feasible."

It's in the nature of weeds not to respect silos, be they private gardens or territorial local authorities. As National Party conservation spokesperson Jacquie Dean says of pine seedlings, "You take them out, and the wind just blows more seeds back from somewhere else. You have to be strategic."

The trouble is the report makes weed control in this country seem nigh on impossible. It says once a weed has spread more than a hectare in New Zealand, it has never been able to be fully eradicated.

In battle terms, this is like the final scene from the movie Zulu. The country's 2300 native plants are up against 25,000 exotics. Roughly 1800 of those have become naturalised – meaning they thrive without human cultivation – and that number grows by 20 species a year.

Although most New Zealanders are familiar with the "dirty dozen" in the Department of Conservation's official weed-busting programme – old man's beard, wandering willie, ivy and the like – it may be the invaders to come that pose the greatest concern, if only because their impact, given climate change, is unpredictable. The only easy guess is that exotic tropical plants, which survive only in niche climates here today, will probably flourish in higher temperatures.

As in predator ecology, where removal of one menace sometimes simply helps another, the reckoning is confoundingly multifactorial. Temperature change could make some plants more susceptible to disease, including natives. It's hard to anticipate which plants will become dominant and which less viable. Climate change-wrought storms can devastate or spread plants. Wild and plantation pines have already caused catastrophic bush fires, and more are expected given climatologists' projections of warmer, drier conditions for some districts. Greater flooding propensity in other areas will bring different challenges and opportunities for plants.

Upton's report is pessimistic, saying modelling so far suggests all exotic naturalised plants will probably find the coming climate change more to their liking than not.

A further perspective on the uniqueness of New Zealand's weed threat is that although we do have the world's most notorious invasive flora, Japanese knotweed, it's far from our most troublesome pest. So invasive it can annihilate the piling under houses, and so difficult to eradicate that infested properties overseas are often uninsurable, the creeping rhizome is massively outmenaced here by the pine tree.

Each conifer can produce up to 17,000 seeds a year, 10 per cent of which are likely to sprout – and far from the mother plant, thanks to aerodynamic packaging that optimises wind-spread. Dean says she applauds the review's call for tightly co-ordinated weed management. When it comes to pines, anything less than all districts in it together is often a waste of everyone's time.

Since mass planting of 10 species of pine began in the 1960s – often for the laudable goal of stopping erosion – conifers have spread to occupy nearly 7 per cent of New Zealand's land mass. Every year, they spread 5 per cent further – an annual gain of about 90,000ha.

Warmer conditions may mean some cones' seed load is greatly increased. Yet investors are planting more pines for carbon farming at an accelerating rate.

Upton believes the solution, or at least much-needed mitigation, lies in pooling data and expertise, which at present are fragmented, incoherent and in some cases serendipitously accrued rather than methodically recorded.

The same pest plants are treated with different levels of urgency in different parts of the country, with little comparative assessment or orderly prioritisation.

Embarrassingly, the Government's One Billion Trees programme debuted listing several invasive exotics as eligible for funding, including one, Japanese spindle, which is so invasive it's banned from importation. Douglas fir, being intensively eradicated in Marlborough's Awatere Valley among many other areas, is still on the list.

There are even pest plants being accorded council protection as trees of local significance – while the same council is trying to stop their spread. The Auckland Unitary Plan makes it difficult to remove mature bangalow palms, yet birds can spread the 12,000 annual seed crop from a single palm far and wide, as the council's pest management scheme could attest.

Some data on pest-plant location isn't even publicly available; plants can be on one department's list but not another's. Some data isn't updated regularly, or even at all over periods of years.

Neighbours Wellington and Hutt Valley manage old man's beard differently. Wellington has an eradication plan for moth weed, Manawatū-Whanganui has one for woolly nightshade, yet both plants are as rampant in both regions.

Some areas get plaudits for timely efficiency, such as Marlborough, which targets a schedule of invasive weeds by ensuring their flowers are removed before setting seed. But Upton complains that data on what's under way and what's been successful is seldom shared and sometimes kept unnecessarily secret.

The breakdown of who does what weeding is quite a shaggy dog story. The Ministry for Primary Industries (MPI) has a national pest priority programme that targets only nine weeds – a schedule not updated in 16 years. Its main focus is on stopping new plants with pest potential coming into the country.

Regional councils each have their own weed management programmes – some with 200 targets in their sights, others with only a dozen.

As for DoC, Upton concluded it was hard to get a fix on what its weed programme consisted of. It allocated about $14 million last financial year for weed removal, via nearly 50 fulltime staff and about 30 temps. A further $100 million is available over four years for weeding more than 800,000ha under the multi-agency Jobs for Nature scheme.

Upton's researchers also counted some 600 community volunteer anti-weed programmes, all operating under different guidance, targets and constraints in trying to restore degraded land areas back to native habitat. These include more than 200 groups that follow DoC's Weedbusters programme.

A slew of other bodies have weed management obligations, including KiwiRail, Land Information, the Defence Force and Waka Kotahi NZ Transport Agency. Given weed control is not, for obvious reasons, a high priority for such agencies, they tend to work reactively rather than proactively, the report says.

The Ministry for the Environment, the review says, "appears to be surprisingly absent from the conversation on exotic plants". It is involved in drafting the National Policy Statement on Indigenous Biodiversity, but otherwise "remains a spectator of the weed management efforts".

As for how much progress all these efforts have made, who could tell? The review found that local authorities rarely monitor the outcomes of their $15 million-odd weeding spend. Last time anyone asked – a 2008 survey – only 1.4 per cent of councils' spending "input" on weed control had been subject to follow-up "outcome" checks.

As Upton says, we can't weed the entire country. "But we can be smart about which ones we choose to tackle, have a clear outcome in mind and make sure we co-ordinate our actions."

The report has been a rude wake-up call at a time when many other countries are flirting with "rewilding" – letting common plants, including those regarded as weeds, have their head in both large land tracts and in civic spaces, such as public parks. The benefits are obvious: carbon sequestration, reduced use of chemicals and much-needed forage, protection and accommodation for insects, birds and animals.

Rewilding in New Zealand, however, would be less of a romantic idyll. Still, weeding appears to have a parliamentary consensus.

Although the National Party is usually sceptical about forced centralised control of issues, the mobility of weeds makes this different in Dean's view. She says it's important to respect and preserve local agency over land management, and to ensure the load and costs are fairly shared.

The Greens' Eugenie Sage, a former Conservation Minister, agrees the problem needs a much more concerted, co-ordinated attack. She is also concerned that we still allow importation of new exotic plants when we know that, thanks to climate change, some may go rogue in future.

Both MPs believe volunteer workers have much to contribute, and well-directed, ongoing weed eradication projects organised at local level could be a big part of the solution.

Much of the commissioner's advice focuses on a multipronged attack: removing groups of weeds at a time rather than just targeting one or a few; repeat treatment to ensure they don't return – more weeding, mulch, spray or hot-water treatment; and re-establishing appropriate native flora as quickly as possible.

Still, sometimes a single sighting can stem an invasion. A contractor carrying out a vegetation survey in Auckland in 2018 spotted a thorny specimen that he recognised as one of the world's most invasive plants, the Himalayan wineberry. He uploaded a picture of it to iNaturalist, a DoC site where amateur nature scouts can report suspicious or interesting findings. Unfortunately, no one in authority followed it up until a year later, when a pair of botanists stumbled on some more of it by chance. After investigation, it turned out that, unbeknown to weed officials, wineberry and another enthusiastic "new" colonist, great willowherb, had been establishing themselves here for several years.

It's unclear how much the country spends on weed management and research. Councils spend an estimated $15 million a year. There are several big research funds, but wilding pine and myrtle rust have absorbed the lion's share of the $23 million allocated in recent years. MPI has $3.5 million to investigate the potential for imported biological control of six weed species, following some success using the cinnabar moth against ragwort and the buddleia leaf beetle on butterfly bush.

As for culpability, home gardeners have inadvertently caused a lot of the weed-spread problem by dumping garden waste. There are hopes greater local authority green-waste collection will help curb future weed spread, as the waste is composted, which, done to sufficient heat intensity, kills seeds.

The “Space Invaders” report concentrated on scientific rather than anecdotal evidence, but there’s an old German saying that best sums it up: let a weed grow for a year and it’ll take seven more to get rid of it.