It is not always clear whether a plant is likely to become a serious weed. Weed risk depends on the plant, the places where it grows, and how people interact with it. A species that causes major problems in one area may have little impact in another.
CPG weed hazard rankings - coming soon
SWAT will soon include weed hazard rankings based on the Climate-Publication-Global occurrence (CPG) model. This New Zealand-developed method ranks the weed potential of plants. It also helps identify potential "sleeper weeds" that may become problematic in the future.
The model combines three sources of information:
- Climate suitability (C): how similar future climates are to places where a species already occurs worldwide.
- Publications (P): how many scientific publications describe the species as a weed.
- Global occurrences (G): how many verified records of the species exist worldwide.
These factors produce a hazard ranking. It estimates how likely a plant is to become weedy, relative to other plants. The model focuses on establishment and spread, not the impacts of invasion.
The rankings use projected climates for 2070. This helps identify species that may pose future challenges under climate change. To better reflect climate variation, each region is split into highland and lowland subregions. Each subregion gets its own ranking.
The method gives a clear, repeatable and objective way to prioritise emerging weed threats.
CPG rankings by region will be available in SWAT soon. These will include ranked lists of species with the highest potential to become future weeds in each region. They will also show how rankings are expected to change over time.
For more information, see Validating a rapid algorithmic weed hazard ranking method by Chris Buddenhagen et al. (2025).
Assessing weed risk in the meantime
Until CPG rankings are available, you can weigh a range of factors. These cover both how likely a plant is to become invasive, and the consequences if it does.
When you assess a plant, think about its ability to establish and spread. Also think about the impacts it could have if it becomes widespread.
Can it survive and reproduce in New Zealand conditions?
Plants that suit local climates and environments are more likely to form self-sustaining populations.
How quickly can it spread?
Some species produce large numbers of seeds. Others spread vegetatively from plant parts. Some move easily by wind, water, animals or people. These species can expand quickly beyond where they first arrived.
Does it invade natural areas or productive land?
Plants that establish in native ecosystems, farmland, forests, waterways or towns may have greater potential to cause harm.
What impacts could it have?
Consider whether the species could:
- displace native species
- alter habitats or ecosystem processes
- reduce agricultural productivity
- obstruct access to land or waterways
- affect water quality or water flow
- create risks for people or animals
How difficult is it to control?
Some plants are hard to find, remove, or kill. Others recover quickly after control. These often create greater long-term management challenges.
Has it caused problems elsewhere?
A history of invasiveness overseas can give useful clues about how a species might behave in New Zealand. This is especially true in regions with similar climates and environments.
Using judgement and local knowledge
No weed risk assessment method is perfect. There is no single measure that can accurately predict how every species will behave in every situation.
Because weed risk depends on local conditions, the final assessment should always consider local knowledge, site characteristics and professional judgement.
Even if a species is uncommon or known from only one location, it may still warrant concern if it has traits linked to invasive plants. On the otherhand, some introduced species spread only slowly or have limited impacts even after they establish.
Ultimately, weed risk assessment is about weighing the available evidence. The aim is to judge how likely a species is to establish, spread and cause harm in New Zealand.