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This plant is taking over our waterways. It is a v.vigorous grower, allowing nothing else below its canopy. It then dies back leaving banks vulnerable to erosion.
Roots poisonous to domestic livestock but, strangely, wild animals (e.g. water vole) and sheep seem to show no ill effect from eating the leaves.
Now that you have said it is taken over our waterways i felt like i had to ask if it is a species which needs managed such as Knotweed? Obviously not to the extent that Knotweed does but i couldn't really think of a better example.
Lat/Lng: 55.136271, -1.579064
OS grid ref: NZ269825