Spot the Monster

Could your pond be harbouring a monster? Lurking in two thirds of all garden ponds is at least one of the invasive species capable of taking over wild ponds and waterways. And in the wider countryside there are other threats from aliens.

Britain became an island when the land bridge to Europe was flooded as the ice melted about 10,000 years ago. Plants and animals in Britain before then are native; those introduced after that by man are non-native. Of these some are termed invasive, if they are capable of spreading and causing damage to the environment. And the damage can be fierce and swift.

The top ten

The Environment Agency came up with the ten most damaging aliens, in order of nastiness:

Japanese Knotweed: can penetrate concrete; its stem resembles bamboo, can grow 10cm per day and reach three metres in height.
American Signal Crayfish: an omnivorous cannibal which digs a one metre long tunnel into river banks to lay its 250 eggs; brings crayfish plague.
American Mink: small but fearless; eats birds, fish and small mammals and is a serious threat to water voles.
Giant Hogweed: grows to four metres and has sap which blisters your skin.
Floating Pennywort: can grow 20cm per day and quickly clog up waterways.
Himalayan Balsam: pretty and it's fun to ping the seeds but has rampant growth and is over-attractive to pollinators, distracting them from native plants.
Australian Swamp Stonecrop: chokes ponds; tiny bits regrow.
Chinese Mitten Crab: uses its woolly claws to undermine river banks; multiplies rapidly and crawls across-country even into swimming pools.
Parrot's Feather: can regrow from tiny pieces - 5mm or less and can continue to grow if there’s no water left.
Topmouth Gudgeon: only 10 cm of it but it eats native fish eggs and carries a nasty parasite.

What to do

The Environment Agency has published a useful guide on managing invasive non-native plants, which you can download here.

Chinese Mitten Crab

Himalayan Balsam

American Signal Crayfish

Photographs courtesy of Wikipedia.