Landcare Research - Manaaki Whenua

Landcare-Research -Manaaki Whenua

Weeds a ticking time bomb

Tomorrow’s weeds are already here in New Zealand and experts warn they’re a time bomb waiting to explode.

Many are ‘sleeper weeds’ – those growing quietly waiting for the right conditions, or lulling us into a false sense of security as population growth and spread is initially subtle enough that we don’t notice.

The 500 species currently considered weeds in New Zealand are simply the tip of the iceberg, experts say.

‘We believe there are about 25,000 exotic species present in New Zealand, which are currently only present in peoples’ gardens or being cultivated somewhere,’ says research leader Lynley Hayes.

‘That is 10% of the world’s flora. We know from history that on average every 39 days a species escapes cultivation and establishes in the wild. At least a quarter of those go on to become a weed.’

The economic costs of weeds are immense, although often difficult to quantify. However, the cost of Scotch broom (Cytisus scoparius) to farming in New Zealand was recently estimated to be about $10 million a year.

In Canterbury’s Ashley Forest alone more than $100,000 is spent annually controlling broom. The New Zealand forestry industry produces $3.1 billion worth of timber products annually, and a conservative estimate suggests that 10% of forests have a broom problem that costs around 3% of their total production or $90 million annually.

Meanwhile, the Department of Conservation currently spends more than $530,000 each year on controlling broom.

One of the most cost-effective and environmentally responsible methods of controlling weeds is through biological control, and Landcare Research staff are internationally renowned for their skill in this field.

Biocontrol agents don’t eliminate weeds, because they can’t find or kill every plant, but they are more likely to result in smaller, weaker plants that are less likely to spread and can be more easily outcompeted by other plants. Weed infestations may be reduced to a level that we can live with or eliminate effectively and economically by other means.

However, biocontrols aren’t a quick fix because it takes many years for suitable agents to be found, tested, approved, reared, released and established and then for the agents to spread.

And, there’s always this grey area of how to predict which plants will become problem weeds; although, work has recently started to try and bridge the gap in our knowledge.

‘Public apathy to weeds and their threats is an additional problem to the ticking time bomb,’ says researcher Dr Simon Fowler.

‘Weed invasions progress on a human generation scale so we don’t notice anything week by week or even year by year and it often looks like everything is fine. But, it’s not.

‘There are also a lot of people who are unaware a plant – such as broom – is a pest, simply because it’s been there all their lives.

‘Even our high impact, obvious weeds still haven’t spread as far as they could. Broom, for example, will live above treeline throughout the South Island. One potential scenario could be visitors emerging from the Homer Tunnel in Fiordland and driving down to Milford Sound through a sea of flowering yellow broom plants. The prognosis is dire.’

‘And, if people believe we can reverse the current status of weeds, think again,’ Lynley Hayes adds. ‘Not only do many weeds establish huge seed banks that will continue to persist for many years, some research indicates that even if we stem the tide, there could be some permanent, irreversible changes to the environment.’

Early intervention is crucial because studies have shown that once the area of infestation is over a hectare in size, the chances of successful eradication become very small.

Lynley Hayes says patience and foresight are imperative to managing the weed threat and that failure to act will result in serious consequences for biodiversity and the economy.

‘We need to develop more biological control programmes – we have the skills to do this but not the funds. While the time frames are long, it is the only sustainable approach, and the most cost effective.’

Lynley Hayes

Dr Simon Fowler

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