Dealing with invasive plants and harmful weeds

Non-native plant species exist in Northern Ireland without causing a problem, but a few become invasive. They upset the balance of the ecosystem, as they are bigger, faster-growing or more aggressive than the native species. And, they can often take over gardens.

What you can do to help prevent the spread of invasive plants

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

know what you are growing - take care when swapping plants
dispose of all plant waste responsibly
know what you are buying - avoid buying plants or seeds known to be invasive

More information about invasive non-native plants is available on the Northern Ireland Environment Agency website.

Information on invasive alien species - NIEA website

Identifying common invasive non-native plants

Three of the most invasive non-native plants are Japanese Knotweed, Giant Hogweed and Himalayan Balsam.

Japanese Knotweed

Japanese Knotweed appears to have no natural enemies in Northern Ireland and is difficult to control - tiny fragments (as little as 0.7 g of its rhizome, or root) can produce a viable plant. The plant can grow as much as two centimetres per day, will grow in any type of soil, no matter how poor, and can grow through walls and concrete.

It forms dense clumps up to three metres in height with large, triangular green leaves. It has a hollow stem, similar to bamboo but often flecked with dark red. Japanese Knotweed produces fleshy red-tinged shoots that can reach a height of one and a half metres by May and three metres by June.

Giant Hogweed

Giant Hogweed has a green stem with dark red or purple blotches and spiky dark green leaves. The plant can grow up to 5 m tall and grows mainly in areas of damp soils, such as river banks. Each flowerhead can produce up to 50,000 seeds, which are easily dispersed by water.

Seeds can remain viable for up to 15 years, so disposal has to be carried out very carefully. Giant Hogweed can have adverse effects on skin following contact. Symptoms are usually noticeable within 24 hours, including blisters and swelling on the skin, which may be made worse by exposure to the sun. If you feel unwell after contact with Giant Hogweed speak to your doctor.

Himalayan Balsam

Himalayan Balsam was introduced into these parts in the 19th century. It has pinky red stems with dark green leaves. It grows fast and can reach two to three metres in height.

The plant can produce large quantities of seeds in exploding capsules that can throw seeds several metres.