The scale of the problem

About ten times that number of non-native plants have been introduced into this country in the last 100 years, mostly as ornamental plants for gardens.

Already, some 2,500 of these exotic plants have become naturalised to survive and spread in the wild.

Sometimes, however, even a native species can be considered to be a weed in a particular site if it affects an important natural value on that site.

Not all weeds are ecologically damaging but over 250 are causing major ecological impacts to our native plant communities. These environmental weeds are threatening our precious places.

The old saying, 'one year's seeds, seven years weeds', has been proven through science. If weed control is left until the infestation has spread, it is 40 times more expensive than early weed control. Common sense backs this up. What would you rather do – pull out a seedling or cut down a tree?

Weeds can:

• transform ecosystems and landscapes, altering the availability of essentials such fresh water, eg wilding pine.
• threaten the survival of native plants by smothering or out-competing them eg climbing asparagus.
• threaten the survival of native animals through providing cover for predators, reducing the availability of preferred foods and lessening the availability of desirable habitats eg russell lupin.

Weedy environments

It is easy to think of weeds as being a problem only on land and in places where weeds are obvious. But weeds exist in all types of environments and ecosystems – everywhere from marine areas to wetlands, to tussocklands and forests. 

Each area and every weed presents different challenges in the battle to control these plant invaders.

Marine

Sea weeds are invasive, just like land weeds! Undaria, a large brown kelp native to Asia, was first recorded in Wellington in 1987. Heavy infestations of undaria can clog marine farming machinery, affect the growth of marine life and kaimoana (sea food) and restrict water circulation.

Freshwater

Didymo is an algae that forms a dense mat and smothers the freshwater habitats important for native invertebrates, fish and birds. Didymo is spread from one river or lake to another by the movement of water, equipment, clothing or any other damp item. You can’t always see it (e.g. when it spreads as a microorganism) but it is very ugly when it grows into visible ‘rock snot’.

Wetlands

Wetland native plants are often small, and are especially vulnerable to being out-competed by exotic weeds. Such weeds impede water flow and alter the hydrology as well as the environment for native birds. The grey willow and crack willow are obvious examples of invasive weeds in wetland areas.

Forest

Exotic weeds are good at smothering our native plants. Seedlings can be overwhelmed by wandering willie (Tradescantia fluminensis), while shrubs and trees are smothered by vines such as old man’s beard.

Tussocklands

The presence of woody plants threatens to transform iconic landscapes like the South Island McKenzie country from tussocklands to exotic forest. If wilding pines, heather, gorse and broom take over tussocklands, native plants and animals will be displaced and available water reduced.

Sand dunes

The invasion of sand dunes by exotic plants such as marram grass, can result in the alteration of the dune systems and allow other weeds to take hold. Native and endangered plants may become extinct, while pristine dune environments become increasingly rare.