<I picked up a rather attractive plant on the weekend which I believe is
<Water Milfoil or Myriophyllum aquaticum. Isn't this the same plant which
<is considered such a virulent pest that is spreading in North America?

Myriophyllum aquaticum (also known as brasiliense) is subtropical but does 
overwinter up here in SW British Columbia where it is not a pest and not 
likely to be one. The milfoil problem species in North America, including
here (I paid my mortgage for 10 years working on the problem) is M.
spicatum L. a Eurasian species. We have 7 more species here in BC and there
are many more in the rest of Canada and the USA.