Weed all about it
Pernicious, pervasive and a perennial pain. These are the weeds to watch out for this summer. Paul Gorman reports.
The names may be exotic but their effects are less than desirable. Saffron thistle, climbing asparagus, banana passionfruit, African love grass – they roll off the tongue and across the countryside, causing chaos to indigenous plants, ecosystems and the environment.
They also affect our economic livelihood through reduced land productivity and pasture yield, animal health issues, trade restrictions and control costs. Each region of the country has its own particular weed horrors. But some are a nationwide menace.
Environment Canterbury (ECan) principal adviser biosecurity Laurence Smith says pest plant invasions ‘‘don’t register the same level of concern or reaction that disease outbreaks do’’. However, their influence is no less malign. ‘‘Pest plants have the potential to have just as much economic impact on the farming industry as diseases and, once established, they are almost impossible to eradicate.
‘‘One of the keys to preventing spread and protecting production and native biodiversity values is for landowners to treat their boundaries as borders and put protocols in place to ensure whatever they bring on to their land is free of pests and their seeds, and also to ensure people visiting their properties are as clean as possible,’’ Smith says.
Stuff asked councils in Auckland, Wellington and Canterbury which weeds and invasive plants Kiwis should keep an eye out for this summer.
Lincoln University professor Philip Hulme has warned that hotter temperatures will help invasive plants flourish in what is already recognised as one of the world’s weediest countries. He says despite billions of dollars being spent managing pest plants every year, they remain one of the country’s ‘‘biggest and most difficult environmental issues’’. The problem plants can smother native flora, affect soil stability and composition and also groundwater, and heighten fire risk.
Auckland
There are already 25,000 types of exotic plants in New Zealand and, on average, four new species naturalise in the Auckland region each year, an Auckland Council spokeswoman says.
‘‘Much more cost effective for us to stop those early on the conveyer belt than deal with them once they’ve already arrived.’’
The council’s biosecurity team chose these as Auckland’s top five weeds:
■ Climbing asparagus – a shade tolerant scrambler and climber in native forest.
■ Wild ginger – a groundcover that forms dense colonies in forests, gardens, streams and along roads, preventing native seedling regeneration.
■ Evergreen buckthorn (Rhamnus) – can smother young native plants, including po¯ hutukawa, and is a significant issue on Hauraki Gulf islands and cliffs on Auckland’s eastern coast.
■ Chinese knotweed or privet – a rapidly growing scrambling vine that can spread from small fragments. It is tolerant of many different conditions and could establish around New Zealand.
■ Moth plant – can cause the collapse of native ecosystems by smothering plants and stopping their regeneration. The flowers kill butterflies and moths, and the plant’s sap irritates skin.
Wellington
Greater Wellington Regional Council’s biodiversity team picks these six pest plants:
■ Climbing asparagus – spreads by birds and tubers and grows into dense thickets.
■ Old man’s beard – a scrambling, strangling weed.
■ Banana passionfruit – a very vigorous climber and smotherer, with delicious fruit.
■ Japanese honeysuckle – another vigorous climber with sweetsmelling flowers.
■ Privet (Ligustrum) – a common hedge to which many people have a reaction, causing hayfever and asthma.
■ Wandering willie (Tradescantia flumiensis) – smothers the understorey and stops native plants seeding.
Canterbury
ECan is keeping a weather eye on six species that could have ‘‘huge impacts’’ if they spread across rural Canterbury. The council will be relying ‘‘heavily on the community’’ to report sightings of these:
■ African love grass – a clump forming grass up to 1.5-metre tall with bright green to blue green leaves. There are several sites in South Canterbury and a new infestation was found early in 2019 near Greta Valley. It is believed wallabies move the seeds around in South Canterbury, as do mowers, stock and vehicles.
■ Puna grass – Tussock-like and bristly with flower spikes up to 1m tall. There are two known sites, across 100 hectares near Amberley and Bromley.
■ Yellow bristle grass – an annual upright grass with spiky seed heads that occurs on three sites in North Canterbury.
■ Chilean needle grass – the tufted plant grows into dense stands and overshadows other grassland species. The sharp penetrating seeds injure livestock and damage wool or hides, and may cause blindness in lambs. There are six known sites across 23 properties, and climate models show more than half of Canterbury could be susceptible to invasion.
■ White-edged nightshade – a quick-growing perennial that can grow to 5m in height, with woody stems and green oak-shaped leaves covered in sharp spines. Currently confined to five sites on Banks Peninsula. It forms impenetrable thickets and prevents native bush regeneration.
■ Saffron thistle – an upright thistle with sharp-tipped spines, forming dense stands. The spines can injure eyes and mouths of stock. There are 13 sites, mainly north of the Rakaia River.