You can hardly broach fishing topics these days without coming across reference to alien species affecting our rivers. Mink, American Signal Crayfish, Chinese Mitten Crabs, Giant Hogweed, Himalayan Balsam, and Japanese Knotweed are just some of the animals and invasive plants that are burgeoning and threatening native species. We have two invasive plants on the banks of the Avon which we could do without – balsam and knotweed. They poke their noses out of the ground in May, and by July are reaching for the sky.

Himalayan Balsam is an annual (re-grows each year from seed) which grows up to 2metres high. Bees love its attractive purple flowers. When unchecked it spreads rapidly and can crowd out normal vegetation, then when it dies back in the autumn its shallow roots provide no protection against bank erosion. It is rife along the Avon and little can be done about it beyond cutting off the flower heads or pulling the plants out before the seed pods mature. Everyone must be familiar with the seed heads which pop when ripe, catapulting countless seeds far and wide. New colonies can be created by seeds being carried downstream on the current, and by people and animals pushing through the plants.

Japanese Knotweed is potentially worse, and is slowly spreading on the Avon. Its hollow, bamboo-like stems can grow even taller than the balsam, but its small white flower clusters are less significant, and the plant supports no insect life. It is a perennial, and its rootstock system, reaching down up to three metres into the ground and spreading several metres in all directions, can penetrate building and road foundations. It is not spread to new places by seed like the balsam, but by pieces being broken off, often by unwitting anglers. The smallest fragment can re-grow to form a new plant, so folk should take care to avoid breaking the stems or cutting them down.

One of the largest patches of it on beat 4 is on the left bank immediately upstream of the Apple Tree Pool, and there is a smaller one at the base of one of the aqueduct's pillars where the Union Canal crosses the river. The chap at Manuel Mill has been applying chemical to a patch on the left bank in front of his steading for two or three years, and some is still growing, which illustrates how difficult it is to get rid of.

SEPA is creating a database of areas of alien plants in Scotland, AA is logging sites on beats 4 and 5, and Slammanan AC reps attended an invasive plant training session in April 2009.

See http://www.netregs.gov.uk/netregs/63095.aspx for more information.