Invasive Plants

An invasive non-native species is one that has been transported outside of its natural range and that threatens environmental, agricultural or economic resources.

Non-natives are one of the primary threats to global biodiversity, second only to habitat destruction. Invasive plants are aggressive and have the tendency to rapidly overrun the native flora of a range of different habitats, including ponds, ditches, streams, woodlands, grasslands and road verges.

Because invasive plants have few animals that feed upon them, they can out-compete native flora but provide little or no food for native fauna.

The list below notes the invasive plants of most concern here in Gwynedd:

Terrestrial plants

Japanese knotweed (Fallopia japonica)
Rhododendron (Rhododendron ponticum
Himalayan balsam (Impatiens glandulifera
Giant hogweed (Heracleum mantegazzianum)

Aquatic plants

Floating pennywort (Hydrocotyle ranunculoides)
Australian swamp stonecrop (Crassula helmsii)
Parrot’s feather (Myriophyllum aquaticum)
Water fern (Azolla filiculoides)


Cofnod, the North Wales Local Records Centre, is building a database of invasive plant species locations in Gwynedd. Please report your species sighting through their 'Record an Alien' online form.

They are not responsible for dealing with problems associated with invasive plants, so please do not contact them if you wish to report a problem or need advice.


Factors Causing the Spread of the Plants in Gwynedd

  • New introductions arriving as stowaways on boots, cars and traded goods
  • Sale as ornamental plants in garden centres and / or deliberate planting in gardens. For example, as hedging, attractive pond plants
  • Lack of specific natural enemies such as herbivorous animals and pathogens
  • Dispersal by watercourses
  • Movement of contaminated soil and / or material from affected sites.  Machines transporting even small fragments of an invasive plant may be sufficient to contaminate new sites - Japanese knotweed is capable of regeneration from tiny fragments of the parent plant
  • Movement of contaminated machinery and equipment between aquatic systems.  For example, boats, fishing apparatus
  • Inappropriate and / or illegal disposal of cut material, particularly of Japanese knotweed. For example, fly tipping
  • Activities that disturb vegetation and the underlying soil
  • Possible effects of climate change. Increased temperatures, increased carbon dioxide concentrations and stormier weather provide conditions potentially more favourable to non-native species. For example, warmer winters allow the water fern to persist.

The proposed work to deal with these plants are outlined in the 'Invasive Plants' Action Plan with our Local Biodiversity Action Plan (Natur Gwynedd). If you are aware of the presence of some of these invasive plants, particularly invasive aquatic plants, then please let 'Cofnod' know by completing their online invasive plant recording form

If you are unsure with the identification a plant then please email a digital photograph of the plant to the Biodiversity Team.

Further information: