Invasive plants in the Borders

Giant Hogweed at Fogo.
Giant Hogweed at Fogo.

What is an invasive species? It is a plant or animal which has arrived from another part of the world and found a new home where it has not only become established but has sometimes taken over, and become a very expensive pest.

Twenty years ago the banks of the River Tweed and its tributaries were infested with a forest of Giant Hogweed (Heracleum mantegazzianum) which had, over several decades, escaped into the wild from gardens and spread along rivers throughout the country.

A large and striking plant (some thought attractive!), it was first introduced into British gardens in the 19th century from the Caucasus mountains in South-west Asia. Like many other garden plants, it is not a problem until people throw it out and it spreads into the countryside.

Apart from choking out the native flora of our riversides, and blocking access for fishermen and walkers, Giant Hogweed has very irritating sap which can cause very unpleasant blistering of the skin. Fortunately the plants are short-lived and die after flowering, usually in their second year, so can be killed with herbicide before they flower and set seed. If allowed to set seed, each plant can produce thousands of seeds and these can survive as a ‘seed bank’ in the soil for many years, and also be washed downstream when rivers are in spate.

Since 2003 the ‘Tweed Invasives Project’ run by Tweed Forum has made a huge effort to eradicate Giant Hogweed from the Tweed catchment, which has been a great success story, though every year a few more spring up without warning and must be dealt with.

Another less obvious invasive plant in our area is Pirri-pirri Burr (Acaena novae-zelandiae) which is very familiar to visitors to Lindisfarne as its seed heads have tiny hooks which annoyingly attach to clothing and have enabled it to become common all over the island. It is believed to have been introduced from Australia or New Zealand from wool imported to the former mills in Galashiels. The first record was from the Tweed at Leaderfoot in 1911, from where it must have being washed downstream eventually reaching Holy Island.

The story of these ‘Wool Aliens’ was described in a remarkable book ‘The Adventive Flora of Tweedside’ published in 1919. This also gives the first local mention of Japanese Knotweed (Fallopia japonica) from the Tweed at Melrose in 1914, though not introduced with wool. Like Giant Hogweed, this is another unwelcome intruder misguidedly brought in as a garden plant.

If you see examples of Giant Hogweed or other invasives in the wild please contact Tweed Forum at 01896 849723.

There is a meeting tonight, December 3, of the SWT Berwickshire Group. “Habitat Creation for Wildlife in the Tweed Catchment” is the subject by Dr Derek Robeson from Tweed Forum. Meeting in Duns Parish Church at 7.30pm. Friends and visitors welcome.