Invasive Plants and Weeds

A number of invasive plants can be found in the Outer Hebrides, including Japanese Knotweed, Gunnera (Giant Rhubarb), Himalayan Balsam and Rhododendron ponticum.  There are also harmful weeds such as Common Ragwort, which has the potential to kill horses and livestock if eaten.

An invasive plant or weed can upset the balance of the ecosystem.  They are often bigger, faster growing or more aggressive than native species and may take over the habitat where they grow.

The Comhairle can only provide advice on non-native species.  It is the landowner’s responsibility to ensure, as far as possible, that their land remains free of invasive plants and weeds.

Identifying Invasive Plants and Weeds

Japanese Knotweed

Japanese Knotweed

  • Highly invasive, growing up to 3 metres in a single season
  • Root system can grow up to 7 metres away from and 3 metres below parent plant
  • Root system hard to eradicate, less than 1 gram can lead to new plant growth
  • Can lead to structural damage of buildings in close proximity
  • Classed as controlled waste and can only be disposed of at a licensed landfill

Read our useful leaflet: Japanese Knotweed - Identification and Treatment (583.6kB)


Gunnera Giant Rhubarb)

Gunnera (Giant Rhubarb)

  • Can grow up to 2 metres tall
  • Leaves can reach over 2 metres across
  • Flowers ‘cone-like’ structure with reddish bristles and spines, growing over 1 metre tall
  • Summer – large leaves and distinctive flowers (June-August)
  • Winter – thick, creeping rhizomes and dead leaf
  • Invades variety of habitats, e.g. grassland, roadsides, bog
  • Care must be taken not to disperse seeds if touching flowers
  • Spraying with herbicide at end of growing season (Aug-Sept) most effective
  • Ongoing programme of spraying likely required to eradicate


Himalayan  Balsam

Himalayan Balsam

  • Fast growing – up to 2-3 metres tall in a single season
  • Purplish-pink (rarely white) helmet-shaped flowers produced June-October
  • Seed pods open explosively when ripe, dispersing seeds up to 7 metres away
  • Each plant can produce up to 800 seeds
  • Tolerates low light levels
  • Grows particularly well along watercourses
  • Most appropriate method to control is pulling or cutting plants before they flower
  • Control also possible by spraying before flowering (early summer)
  • Ongoing programme of spraying over a few seasons likely required to eradicate


Rhododendron Ponticum

Rhododendron Ponticum

  • Given time, can grow up to 10 metres tall
  • Extremely hardy plant, tolerating low light levels and thriving in acidic soils
  • Dark green waxy, oblong leaves with pinkish-purple/lilac flowers
  • Flowering occurs in spring and summer, each plant producing large quantities of seeds
  • Single plant can cover metres of ground with thick branches and large root system
  • Very small seeds are easily dispersed by wind
  • Contains fungus with potential to attack woody plants
  • Grows vigorously when cut, meaning method of stump killing or removal necessary
  • Several stages to control required over several seasons with repeated treatments


Common Ragwort

Common Ragwort

  • Grows to 90cm, usually only living 2 years
  • Clusters of daisy-like yellow, flattened flowers with much divided, feathery leaves
  • Flowering from June-November
  • Produces large number of seeds, dispersed easily by the wind
  • Cut plants pose serious risk to animals; should be removed from grazing and burnt
  • Apply herbicides late April-May. Grazing not safe for 4-6 weeks after spraying
  • Ongoing routine spraying every autumn or every second spring required, as seeds remain viable in soil for up to 15 years

Read our useful leaflet:  Ragwort - Identification and Treatment (479.6kB)

You can also check the Scottish Government (Opens in a new window or downloads a file) website for information, including the Code of Practice on non-native species.

More information on non-native species can be found on the Scottish Natural Heritage (SNH) (Opens in a new window or downloads a file) website.