‘Alien’ Weeds – how non-native plants can damage our property and harm our environment


Throughout history, explorers have brought strange and exotic plants back to the UK from foreign lands. Most of these grow happily and disturb no one. However, some grow so vigorously that they force back other plants, i.e. our native ones, and cause damage to buildings. If these invasive plants are allowed to proliferate, our native eco-systems will be – and in a few cases already are – under threat, so they must be controlled. Developers and builders have strict regulations to follow to ensure that these plants are not present when building properties.

Among the best known for vigorous growth on land are Japanese Knotweed, Himalayan Balsam and Giant Hogweed, and in water there is Floating Pennywort and Azolla Weed.

Japanese Knotweed is listed in the Wildlife and Countryside Act 1981, and grows at a phenomenal rate – up to 4cm per day have been recorded. Even in poor soil, it’s strong enough to break through concrete and therefore can seriously damage buildings. It only takes a very small fragment (less than a gram) of root to start a new plant, and a clump can grow up to 3m tall in a few months. Its leaves are large and triangular, and the stems are dense and hollow like bamboo, but with a speckled appearance. Further information on identification and how to eradicate it can be found on the Environment Agency’s website or by downloading their leaflet (click here).

Himalayan Balsam grows tall and fast, and spreads seeds as far as 7m from the parent plant. When it dies back in winter, it leaves a large bare patch which doesn’t readily re-grow with other plants. It has attractive looking flowers which bees love, but it smothers smaller plants in its vicinity.

Giant Hogweed is also listed in the Wildlife and Countryside Act 1981, which means it is an offence to plant it in the wild. Any contact with any part of the plant can cause severe skin irritation. The plant can grow to 5m tall, and the seed heads contain up to 50,000 seeds which can be easily spread by water. As the seeds can stay dormant for up to 15 years, it is very difficult to eradicate completely.

Floating Pennywort grows very rapidly, and can completely cover the surface of any still or slow-flowing water. In this way, it can de-oxygenate the water and kill other wildlife in it.  Floating pennywort is not always rooted to the bank or bed, because, as the name suggests, it can float.

Azolla Weed is a densely growing free-floating fern, which forms dense mats of plants. Like Pennywort, it de-oxygenates the water by killing off algae and other water plant life. It can look like solid ground in the winter when it turns brown, thus causing a hazard to people and animals, and it can block water pumps and outlets.

Other plants listed by Natural England and named in the Weeds Act 1959 are: Common Ragwort, Spear Thistle, Creeping or Field Thistle, Curled Dock and Broad-leaved Dock. It is not an offence to have them in your garden or on your land, but you must not allow them to spread onto agricultural land, because they are poisonous to animals.

Defra have a useful website with information sheets on non-native flora and fauna. Click here to visit www.nonnativespecies.org, and click on ID Sheets.