what are the dangers of japanese knotweed?

My nephew has just discovered japanese knotweed on land he just recently purchased. It has a fishing lake. What will he have to do? Close the lake before eradication of the weed? Carry on as normal being careful not to dissturb the weed?
Please send surjestions.

Answers:
This is the plant that takes over wherever it grows in the wild. The resource below might be helpful. It does say that you have to be careful disposing of any soil in which it has grown. The plant can grow wherever it is dumped.

Other answers:
Here is a website
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