Conservation Comment: The war against weeds

Woolly nightshade out competes native bush. One of many introduced weeds threatening our ecosystem. Photo / Katikati Advertiser

New Zealand has some of the strictest quarantine laws in the world. But as we pour effort into defending our border from invasive plants and animals, are we doing enough to get rid of those already here?

We are at war with weeds, and Parliamentary Commissioner for the Environment Simon Upton is calling for the Government to lift its game.

The environmental watchdog's newly released report Space invaders: A review of how New Zealand manages weeds that threaten native ecosystems is the first ever review of its kind in our country's history.

The investigation found that halting the "slow-motion botanical conquest" of our land has been hampered by inadequate leadership, limited information and haphazard and unco-ordinated actions. Legislation on what introduced plants should be prioritised as targets for action, is absent.

You could say New Zealand is a poster child for the havoc brought on by weeds. Since early settlers first began transposing exotics from distant, temperate shores – mainly for horticultural reasons - more than 25,000 exotics have made themselves at home in our temperate, South Pacific nation.

Each year about 20 of the existing introduced species "naturalise", meaning they jump the fence and thrive in the wild without human help.

Some of those plants - many escapees from our farms or gardens - go on to spread and conquer. This adds to the already 1800 strong army of weed species. For comparison, New Zealand has just 2300 native vascular plants.

Once an exotic plant starts spreading, it can be difficult to eradicate.

"It is a somewhat chilling fact that there is no record in New Zealand of any terrestrial plant having been successfully eradicated when the extent of spread has been greater than one hectare," the commissioner's report said.

We hold the dubious title of having an unusually large number of exotic plants. A recent study found the North Island and the South Island each had among the highest number of naturalised plant species of any island in the world.

Privet and woolly nightshade out compete native bush, vines like banana passionfruit and climbing asparagus strangle understories, deceptively beautiful lupins choke braided rivers, and gorse and wilding pines blanket large swathes of our countryside.

The Horizons Regional Council Pest Management Plan details a list of nasties that threaten not only our forests, but our livestock and our livelihoods.

Plants like field horsetail – well established in Whanganui and Rangitikei but limited elsewhere – is poisonous to livestock; African feather grass out competes pasture and Senegal tea blocks drainage channels, affecting both irrigation and waterway navigation.

The commissioner is calling for clearer direction from both the minister for biosecurity and the minister for conservation on managing weeds that have already made their way here.

Recommendations include a publicly accessible national weed information database; a team to be set up by MPI and DoC to co-ordinate management with regional councils and nip new threats in the bud; and the development of a national policy direction on how weeds that grow in native ecosystems should be dealt with.

Critically, the report states that defeating weeds will not be achieved by top-down policies alone.

Most rural New Zealanders are already trying to undo more than a century's work in excluding stock from gullies and restoring native vegetation – the very same community that has a crucial role to play in holding the line against this silent invasion.

Weeds may well be a poor cousin to other biosecurity threats, but if left to their own devices, they will transform our ecosystems beyond recognition.

Time to turn back the green invasion.

Anne-Elise Smithson is an environmentalist from Auckland and Whanganui region enthusiast.