Introduction to invasive aliens

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These are plants introduced from abroad that pose a threat to native wildlife. All of them used to be available for sale but have now been banned. The worst of all is japanese knotweed, very difficult to kill and quick to spread, and capable of growing just about anywhere.

Himalayan balsam is an attractive riverside plant but eventually crowds out everything else, forming dense stands that can stretch for miles. Floating pennywort does a similar job in the river, forming dense mats that exclude light and kill off most aquatic life.

New Zealand pygmyweed and parrot's feather are aquatic plants that are equally invasive and almost as damaging.