What is a weed?
A weed is a plant growing somewhere it is not wanted. Whether a plant is considered a weed depends on context. The same species may be welcome in one place and unwanted in another. Many weeds grow fast, adapt easily and spread quickly once they establish. They can compete with other plants, alter ecosystems, and cause environmental, economic or social harm.
From a Māori perspective, Pa McGowan describes a weed as a plant that disrupts the balance that Papatūānuku (mother earth) needs to be well. In this view, a weed dominates an ecosystem so completely that it can no longer sustain the life that belongs there. A weed upsets the natural balance of an ecosystem.
Exotic plants and weeds in New Zealand
New Zealand has a very large number of introduced (exotic) plant species. The Parliamentary Commissioner for the Environment’s Space Invaders report notes that more than 25,000 exotic plant species have been introduced. Around 1,800 species of these have naturalised. This means they can survive and reproduce in the wild without human help.
About 20 new exotic plant species establish in the wild each year. Not all become significant weeds. But some can spread widely and cause long‑term environmental, economic or social harm if they are not detected early.
Different categories of weeds
In New Zealand, weeds are recognised and managed through a range of national and regional categories. These reflect different legal, policy and management frameworks. They include:
- Regional Pest Management Plans (RPMPs) developed by regional councils under the Biosecurity Act
- national biosecurity listings managed by the Ministry for Primary Industries (MPI), including the National Pest Plant Accord (NPPA), the National Interest Pest Response (NIPR) programme, Unwanted Organisms (UO) and Notifiable Organisms (NO)
- the List of Environmental Weeds in New Zealand 2024, published by the Department of Conservation
- Weedbusters species listings
Any species on one or more of these lists is marked as “Currently classified as a weed” on SWAT alerts.
Note: To keep things simple, SWAT uses the term 'weed' throughout the tool and in alerts. This includes exotic plant species that are not currently classified as weeds.