Japanese knotweed is one of the most invasive plants growing in the UK. It grows so powerfully, at up to a metre a month, that it can force its way through concrete and tarmac. It is very hard to kill -- if you leave even a single tiny piece of it in the soil it can regenerate itself. And it is one of the few plants whose cultivation is banned by law!

Introduced as an ornamental plant in 1840, it was several years before its invasive nature and threat to our environment was understood. But by then it was too late -- it was already out of control.

Now there are plans to try to bring it under control by introducing a tiny insect, Aphalara itadori, that sucks its sap and helps to keep the plant under control in its native Japan. It is estimated that it would cost £1.5 billion to eradicate it using chemicals and other conventional means. Against this, though, must be balanced the potential threat that could arise from the introduction of an alien species in the law of unintentional consequences. I'm thinking here of grey squirrels, cane toads and Signal crayfish.

The next step will be an assessment by The Department for Environment, Food and Rural Affairs (Defra) followed by a period of public consultation.