Yes, it's an annual weed. The "seed is spread by water" statement means
that when seeds fall into a stream or river, they will still remain viable
for some time, and germinate where they are eventually washed ashore.
I doubt you will ever be completely rid of it, as there's bound to some
nearby that will be spread by birds. But I have had success in keeping it
at bay. The roots are very unsubstantial, and extremely easy to pull. I
put on sturdy clothing and heavy gloves (those prickles are small but
nasty), grab a bunch, and start rolling it up into large clumps or bales for
removal.. The work goes quickly. A huge amount of it is often traced back
to one or two roots. If you get it out now (or earlier in the season)
before the seeds drop, you will significantly reduce the amount you will
have next year.
Next spring you can go out and spot it and remove it while it is still
small. Again, the roots are puny, and it's easy to remove. It's a pioneer
plant, preferring disturbed areas, so you can also help keep it under
control by establishing a more desirable ground cover.
I've never used herbicides so I can't help you there. Good luck,
Zone 6, South-central PA
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