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Dealing with invasive plants and harmful weeds

Many non-native plant species occur in Northern Ireland without causing any problems. A few plant species can become invasive due to a range of factors, including the absence of a natural predator or the presence of a more suitable climate or habitat type.

Threat to biodiversity

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.

Prevent the spread

There are several steps you can take to stop the spread of invasive plants:

  • know what is growing in your garden – you can get help identifying invasive plants on the Invasive Species Ireland website
  • manage invasive species where they occur on your land – the Invasive Species Ireland ‘Management toolkit' provides best practice management advice for a wide range of species
  • dispose of all plant waste responsibly – it is illegal to plant or cause the spread of many invasive plant species so be careful when disposing
  • know what you are buying - avoid buying plants or seeds known to be invasive
  • Invasive Species Management Toolkit - Invasive Species Ireland website

Identifying common invasive non-native plants

Japanese knotweed, giant hogweed and Himalayan balsam are three of the most commonly occurring invasive non native plants.

Japanese knotweed

Japanese knotweed

Japanese knotweed spreads by the movement of fragments of its plant material or of its root (rhizome). It is difficult to control due to its extremely deep root system, which can grow up to three metres in depth.

Tiny fragments (as little as 0.7 g) of its root can produce a viable plant. The plant can grow as much as two centimetres a day, in any type of soil, and can grow through hard structures such as walls and concrete.

It forms dense clumps which can be up to three metres in height. It has a hollow stem, similar to bamboo but often flecked with dark purple.

Giant hogweed

Giant hogweed

Giant hogweed is easily distinguishable from the similar native hogweed and cow parsley due to its sheer height which can reach three to five metres.

It can be found most commonly in areas of damp soils, such as river banks. Each flower head can produce up to 50,000 seeds, which are easily dispersed by flood water enabling it to quickly take over an area it occurs in.

Its seeds can stay viable for several years, so treatment must be continued until the soil seed bank is fully exhausted and no further growth is observed.

Giant hogweed can cause skin irritation following contact. It contains a sap which irritates skin when it is exposed to sunlight. Symptoms are usually noticeable within 24 hours, and include blistering and swelling on the skin, which may be made worse by over-exposure to the sun.

If you come into contact with Giant hogweed, cover the affected area immediately to avoid exposure to the sun. Wash the area with cold water. If blistering occurs or the contact was with the eyes you should seek medical advice.

Himalayan balsam

Himalayan balsam

Himalayan balsam has pinky red stems with dark green leaves.

The plant can produce large quantities of seeds in exploding capsules that can throw seeds several metres away from the parent plant. This enables it to quickly take over the habitat it occurs in. It is commonly found in areas of damp soil such as river banks and nearby woodlands.

Further help on how to identify and treat Japanese knotweed, Giant hogweed and Himalayan balsam can be found on the Invasive Species Ireland website.

Identifying harmful weeds

Seven particular plants are classified as 'injurious' (harmful) weeds. These plants are:

  • common ragwort
  • spear thistle
  • creeping or field thistle
  • curled dock
  • wild oat (two species)
  • broad-leaved dock

Ragwort can potentially kill horses and livestock if eaten. All parts of the plant are poisonous.

The thistles and docks are economically harmful if allowed to spread, as they can stop pasture and crops growing properly.

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