Hydrocotyle ranunculoides is commonly known as floating pennywort, not pennyroyal, I think. I need to check that out but to my knowledge Pennyroyal is an herb in the mint family, Lamiaceae.
I might be mistaken about this, but I have never heard of floating pennyroyal, which only means I'm in the dark, not you.
It sounds like a real problem, Neil, and we have too many water problems already. And just as Lynn said, kudzu is taking over where natives used to be here in my state too.
